Lepidodendron.

i. General.

The genus Lepidodendron included species comparable in size with existing forest trees. A tapered trunk rose vertically to a height of 100 feet or upwards from a dichotomously branched subterranean axis of which the spreading branches, clothed with numerous rootlets, grew in a horizontal direction probably in a swampy soil or possibly under water. A description by Mr Rodway[237] of Lycopods on the border of a savannah in Guiana forming a miniature forest of Pine-like Lycopodiums might, with the omission of the qualifying adjective, be applied with equal force to a grove of Lepidodendra. The equal dichotomy of many of the branches gave to the tree a habit in striking contrast to that of our modern forest trees, but, on the other hand, in close agreement with that of such recent species of Lycopodium as L. cernuum ([fig. 123]), L. obscurum ([fig. 124]) and other types. Linear or oval cones terminated some of the more slender branches ([fig. 188]) agreeing in size and form with the cones of the Spruce Fir and other conifers or with the male flowers of species of Araucaria, e.g. A. imbricata. Needle-like leaves, varying considerably in length in different species, covered the surface of young shoots in crowded spirals and their decurrent bases or leaf-cushions formed an encasing cylinder continuous with the outer cortex. The fact that leaves are usually found attached only to branches of comparatively small diameter would seem to show that Lepidodendron, though an evergreen, did not retain its foliage even for so long a period as do some recent conifers.

By the activity of a zone of growing tissue encircling the cylinder of wood the main trunk and branches grew in thickness year by year: the general uniformity in size of the secondary conducting elements affords no indication of changing seasons. As the branches grew stouter and shed their leaves the surface of the bark resembled in some degree that of a Spruce Fir and other species of Picea, in which the leaf-scars form the upper limit of prominent peg-like projections, which, at first contiguous and regular in contour, afterwards become less regular and separated by grooves ([fig. 140]) and at a later stage lose their outline as the bark is stretched to the tearing point ([fig. 140], C). The leafless branches of Lepidodendron were covered with spirally disposed oval cushions less peg-like and larger than the decurrent leaf-bases of Picea, which show in the upper third of their length a clean-cut triangular area and swell out below into two prominent cheeks separated by a median groove and tapering with decreasing thickness to a pointed base, which in some forms (e.g. Lepidodendron Veltheimianum, [fig. 185], C, D), is prolonged as a curved ridge to the summit of a lower leaf-cushion.

Fig. 140. Picea excelsa. Shoots of different ages showing changes in the appearance of the leaf-cushions: a leaf attached to a cushion in fig. A. (Slightly enlarged.)

A portion of the cushion below the triangular leaf-scar often shows transverse gaping cracks or depressions ([fig. 185], C) such as occur on a smaller scale on the older cushions of a Fir twig ([fig. 140]). Secondary thickening, as in recent trees, is not confined to the vascular cylinder but at an early stage, frequently before there are any signs of secondary wood, the outer region of the broad cortex becomes the seat of active cell-formation which results in the addition of a considerable thickness to the bark. At a later stage of increase in girth, the leaf-cushions are stretched apart and the original surface-features become obliterated by vertical cracks and by the exfoliation of the superficial tissues[238].

Some species of Lepidodendron produced branches characterised by spiral or vertical series of scars; these in older shoots were replaced by depressions having a diameter of several inches and comparable in appearance, as also perhaps in manner of formation, with the scars left on the stem of a Kauri Pine (Agathis australis)[239] on the abscission of lateral branches by a natural process. These shoots, known as Ulodendron, are described in a subsequent section. ([page 128].)

A fully-grown Lepidodendron must have been an impressive tree, probably of sombre colour, relieved by the encircling felt of green needles on the young pendulous twigs. The leaves of some species were similar to those of a fir while in others they resembled the filiform needles of the Himalayan Pine (Pinus longifolia). The occasional presence of delicate hyphae in the tissues of Lepidodendron demonstrates susceptibility to fungal pests.

Architecturally, if one may use the term, Lepidodendron owed its power of resistance to the bending force of the wind to its stout outer bark formed of thick-walled elements produced by the activity of a cylinder of cortical meristem (figs. [148], [172], etc.). The vascular axis, of insignificant diameter in proportion to the size of the stem (figs. [152], [153], [172], [181], A), must have played a subordinate part, from a mechanical point of view, as compared with the solid mass of wood of a Pine or an Oak.

•••••

Within the compass of a text-book it is impossible, even if it were desirable, to include an account of the majority of the species of the widely distributed Palaeozoic genus Lepidodendron. In spite of the great number of known species of this common member of Carboniferous floras, our knowledge of the type as a whole is deficient in many points, and such information as we possess needs systematising and extending by comparative treatment based on a re-examination of available data.

In order to appreciate the meaning of certain external features characteristic of Lepidodendron stems it is essential to have some knowledge of the internal structure.

A dual system of terminology has been unavoidably adopted for species of Lepidodendron: the majority of specific names have been assigned to fossils known only in the form of casts or impressions, while petrified fragments, which unfortunately seldom show the surface-features, have received another set of names. A glance at the older palaeobotanical literature reveals the existence of several generic designations, which fuller information has shown to have been applied to lepidodendroid shoots deprived of some of their superficial tissues before fossilisation and differing considerably in appearance from the more complete branches of the same species[240]. It has in some instances been possible to correlate the two sets of specimens, casts or impressions, showing external features, and petrified fragments. We may reasonably expect that future discoveries will enable us to piece together as definite specific types specimens at present labelled with different names.

A well-preserved leaf-cushion of a Lepidodendron—the most obvious distinguishing feature of the genus—is rhomboidal or fusiform and vertically elongated ([fig. 146], C, E; [fig. 185], C, D): in exceptional cases it may reach a length of 8 cm. and a breadth of 2 cm. The cushion as a whole represents a prominent portion of the stem or branch comparable with the elevation on the twig of a Spruce Fir and the leaf-base of a Lycopodium (cf. [fig. 121], A, lower portion) which appears in a transverse section of a branch as a rounded prominence (cf. Lycopodium, [fig. 125], A and H). Disregarding differences in detail, a typical Lepidodendron leaf-cushion is characterised by a clearly defined smooth area often situated in the middle region ([fig. 146], C, s). This is the leaf-scar or place of attachment of the base of the leaf which was cut off by an absciss-layer while the branch was comparatively young, as in recent forest trees and in some species of Ferns. On the leaf-scar are three smaller scars or cicatricules, the central one is circular or more or less triangular in outline, the two lateral scars being usually oval or circular. The central pit marks the position of the single vascular bundle which constituted the conducting tissue connecting the leaf with the main vascular system of the stem. The two lateral scars (figs. [145], A, p; [146], C, s; [147], p) represent the exposed ends of two strands of tissue, the forked branches of a strand which pass from the middle cortex of the stem into the leaf; this is known as the parichnos, a name proposed by Professor Bertrand in 1891[241].

The specimen shown in [fig. 141] shows the linear leaves attached to their respective cushions.

Fig. 141. Lepidodendron Sternbergii. From a specimen in the British Museum (No. v. 1235) from the Coal-Measures of Shropshire. (Nat. size.)

The lamina has a well-defined median keel on the lower surface and on either side a groove in which sections of petrified leaves have demonstrated the occurrence of stomata (cf. [fig. 142]).

ii. Leaves and Leaf-cushions.

All Lepidodendron leaves, so far as we know, possessed a single median vein only. In some species, as for example in Lepidodendron longifolium Brongn., they have the form of long and slender acicular needles very similar to those of Pinus longifolium; in L. Sternbergii ([fig. 141]) they are much broader and shorter. In external form as in internal structure it is often impossible to distinguish between the leaves of Lepidodendron and Sigillaria. The distinguishing features enumerated by the late M. Renault cannot be employed, with any great degree of confidence, as diagnostic characters. In transverse section the lamina of a Lepidodendron leaf presents the same appearance as that of the Sigillarian leaves represented in [fig. 142]. Near the base the free part of the leaf is usually sub-rhomboidal in section with short lateral wings, a ventral keel and two stomatal grooves ([fig. 142], A, B, g). The form and arrangement of stomata are shown in [fig. 143], A, which was drawn from a piece of a leaf shown in surface-view in a section lent to me by Professor Weiss. It should, however, be pointed out that the leaf cannot be certainly identified with Lepidodendron rather than with Sigillaria, but as the leaves of these two genera are constructed on the same plan the identification is of secondary importance.

Fig. 142. Leaves of Sigillaria in transverse section.

The single xylem bundle consists of primary tracheae only, at least in such laminae as have been identified as Lepidodendroid. Surrounding the xylem strand occur delicate parenchymatous cells in some cases accompanied by darker and thicker-walled elements. As in Sigillaria, the leaves of which are more fully described on [page 210], a fairly broad sheath of wider and shorter scalariform or spiral transfusion tracheids surrounds the conducting strand (figs. [142], t; [143], B, C, t). As Renault shows in the case of Lepidodendron esnostense[242], the small leaves of which are 1·5–2 mm. broad at the base and several centimetres long, the stomatal grooves and keel die out towards the apex when the lamina assumes a more nearly circular form ([fig. 143], C).

Fig. 143.

The area of the cushion excluding the leaf-scar is spoken of by some writers as the field. Below the leaf-scar the kite-shaped cushion tapers to a gradually narrowing basal position: in Lepidodendron Veltheimianum, a species characteristic of Lower Carboniferous strata, it is seen to be continuous, as a ridge with sloping sides, with a lower cushion ([fig. 185]).

Below a leaf-scar the cushion frequently shows a pair of oval areas on which a fine pitting may be detected in well-preserved impressions, these oval scars, as seen in [fig. 185], D, are practically continuous at the upper end with the parichnos scars on the leaf-scar area; this is explained by the fact that these infra-foliar scars also owe their existence to patches of lacunar, aerenchymatous tissue in close connexion with the parichnos[243].

Shortly before entering the base of the leaf-lamina the parichnos divides into two arms which diverge in the outer cortical region right and left of the vascular bundle, and passing obliquely upwards they come close to the surface of the leaf-cushion just below the leaf-scar. The diagram—[fig. 144], B—shows a leaf-trace, lt, in the leaf-cushion, as seen in a diagrammatic drawing of a vertical radial section of a stem, the dotted lines, p, p′, show the two parichnos arms which are represented as impinging on the surface of the leaf-cushion at p′, and then bending upwards to pass into the leaf-base right and left of the vascular bundle or leaf-trace. For convenience the arms of the parichnos are represented in one plane though actually in different vertical planes.

Fig. 144, A, shows the difference between a view of the original surface of a Lepidodendron, as at a, where a leaf-cushion with a leaf-scar is seen, and a view of an impression representing the outer cortex, b, a short distance below the surface. The surface b, in [fig. 144], A, corresponds to the face d-e in the diagrammatic longitudinal section [fig. 144], B: the outline of each cushion is clearly visible and in the centre is seen the leaf-trace, lt, with its parichnos.

The surface-features, a ([fig. 144], A), have been impressed on the rock, c, ([fig. 144], B) in which the specimen was entombed and by the removal of the cast of the stem, that is the thickness b to e in [fig. 144], B, the form of the leaf-cushion is revealed. The presence of the two infra-foliar parichnos scars at p′ ([fig. 144], A) is explained by the diagram, [fig. 144], B, p′.

The relation of the parichnos to the oval scars below a Lepidodendron leaf-cushion has been worked out in detail by Weiss who shows that, at least in some species, the two arms do not bend downwards as shown in the diagram, [fig. 144], B, but pursue a straight gradually ascending course as seen in [fig. 145], A. Just below the leaf-scar region of the cushion each arm comes into association with a group of lacunar, aerenchymatous tissue, such as occurs in the roots of certain Mangrove plants, and it is this aerenchyma which is exposed on the two oval depressions below the leaf-scar. The structure of this aerenchyma is shown in [fig. 145], B; it consists in this species (L. Hickii Wats.) of stellate cells which would constitute an efficient aerating system. Probably, as Weiss suggests, these patches of aerenchyma were originally covered by an epidermis provided with stomata, and it is owing to the destruction of this superficial layer that the two oval scars often form a prominent feature on Lepidodendron leaf-bases[244]. The diagram reproduced in [fig. 144], B, may be taken as practically correct, as the patches of aerenchyma described by Weiss do not differ essentially from the parichnos tissue.

Fig. 144. Lepidodendron Veltheimianum Sternb.

  1. Leaf-cushion and leaf-scar seen in surface-view at a; on the rest of the specimen a slightly lower surface is exposed. (After Stur.)
  2. Diagrammatic longitudinal section to explain the differences between its two surfaces a and b shown in fig. A.
  3. The shaded portion c represents the rock matrix, the surfaces ab, ed, mark the outer and inner edge of the outer portion of the bark of the Lepidodendron stem.
  4. lt, leaf-trace; p, p′, parichnos.

Fig. 145.

  1. Diagrammatic surface-view and longitudinal section of a Lepidodendron leaf-cushion.
  2. Aerenchyma below the leaf-scar. (After F. E. Weiss.)

The parichnos scars are shown on the leaf-scar and cushion in [fig. 146], C. In the lower leaf-cushion shown in [fig. 146], E, the infra-foliar parichnos scars, p, are clearly seen, but the preservation of the leaf-scar is not sufficiently good to show them on that part of the fossil. In the upper cushion ([fig. 146], E) the position of the parichnos arms is shown on the leaf-scar, but the infra-foliar parichnos scars are hidden by two small spiral shells. The genus Spirorbis, to which these shells are referred, appears to have persisted from the Silurian epoch to the present day. The comparatively frequent occurrence of Spirorbis shells on the leaves and other parts of Palaeozoic plants, has recently been dealt with in a paper by Barrois[245] who discusses in detail the habitats of these small animals from the point of view of the conditions under which the plants were preserved. In a note by Malaquin appended to Barrois’ paper the belief is expressed that Spirorbis lived on pieces of Palaeozoic plants which lay under water.

The fact that with one exception all the Spirorbis shells on the specimen of Lepidodendron, of which two leaf-cushions are shown in [fig. 146], E, occur on the large parichnos scars on the cheeks of the cushions, suggests the possibility that the escape of gases from the parichnos tissue may have rendered the position attractive to the Spirorbis. It can hardly be accidental that the shells occur on the parichnos strands. This fact recalls the view held by Binney[246] and accepted with favour by Darwin[247] that Lepidodendron and other coal-forest trees may have lived with the lower parts of the stems in sea water.

Above the leaf-scar is a fairly deep triangular or crescentic pit ([fig. 146], C, l) known as the ligular pit from the occurrence on younger shoots of a delicate organ like the ligule of Isoetes ([fig. 132]) embedded in a depression in the upper part of the leaf-cushion. The ligule was first figured in Lepidodendron by Solms-Laubach[248] and described in English material by Williamson under the name of the adenoid organ[249].

In some Lepidodendron stems a second triangular depression may occur above the ligular pit, the meaning of which is not clear: this has been called the triangulum by Potonié[250]. Stur[251] suggested that it may represent the position occupied by a sporangium in Lepidodendron cones.

It is important to remember that as a branch increases in girth the leaf-cushions are capable of only a certain amount of growth: when the limit is reached they are stretched farther apart and thus the narrow groove which separates them is converted in older stems into a comparatively broad and flat channel, thus altering the surface characters.

Fig. 146. Lepidophloios and Lepidodendron leaf-cushions.

Another feature worthy of notice in reference to the leaf-cushions of Lepidodendron is the occurrence in rare instances of alternate zones of larger and smaller cushions. This variation in the size of the leaf-cushions is by no means uncommon in the closely allied genus Sigillaria; in Lepidodendron it has been described by Potonié[252] in L. volkmannianum and more recently by Mr Leslie and myself[253] in a South African species L. vereenigense.

Owing to the natural exfoliation of the superficial layers of the outer bark at a certain stage in the growth of the plant, or in some instances no doubt as the result of post-mortem decay, which destroys the delicate cells of the meristematic zone in the outer cortex, isolated leaf-cushions and strips of the external surface are occasionally met with as carbonised impressions.

The appearance presented by a Lepidodendron stem which has been deprived of its superficial tissues may be dealt with more intelligibly after we have become familiar with the anatomical characters.

iii. Lepidophloios.

Before proceeding further with the genus Lepidodendron a short account may be intercalated of the external features of a lepidodendroid type of stem which it is customary to describe under a distinct generic title Lepidophloios. This name is convenient for diagnostic purposes though it seems clear that apart from the form of the leaf-cushion ([fig. 146], A) we are at present unable to recognise any well-defined differences between the two forms Lepidodendron and Lepidophloios. For general purposes the name Lepidodendron will be used as including plants possessing leaf-cushions of the type already described as well as those with the Lepidophloios form of cushion.

The generic name Lepidophloios was first used by Sternberg[254] for a Carboniferous species which he had previously described as Lepidodendron laricinum. In 1845 Corda[255] instituted the name Lomatophloios for specimens possessing the same external characters as those for which Sternberg had chosen the name Lepidophloios. The leaf-cushions of Lepidophloios differ from those of the true Lepidodendron in their relatively greater lateral extension (cf. [fig. 146], A and C), in their imbricate arrangement and in bearing the leaf, or leaf-scar, at the summit. In some species referred to Lepidophloios the cushions are however vertically elongated and in this respect similar to those of Lepidodendron: an example of this type is afforded by Lepidophloios Dessorti a French species described by Zeiller[256]. In younger branches the cushions may be directed upwards having the leaf-scar at the top; but in the majority of specimens the cushions are deflexed as in figs. [146], D; [160], A. The shoot of Lycopodium dichotomum shown in [fig. 121], B, with the leaves in the reversed position bears a close resemblance to a branch of Lepidophloios.

The photograph of Lepidophloios scoticus Kidst.[257] reproduced in [fig. 160], A, illustrates the dichotomous branching of the stem and the form of the cushions with the leaf-scars pointing downwards. In the fertile branch of the same species shown in [fig. 160], B, the leaf-scars face upwards.

In most species the cushions are simply convex without a median keel, but in some cases a median ridge divides the cushion into two cheeks as in the genus Lepidodendron. The leaf-scar bears three small scars, the larger median scar marking the position of the leaf-trace, while the lateral scars are formed by the two arms of the parichnos: in some examples of deflexed cushions, though not in all, a ligular pit occurs on the cushion a short distance above the leaf-scar.

The drawing reproduced in [fig. 146], A, showing the leaf-scar on the upper edge of the cushion should have been reversed with the leaf-scars pointing downwards. This figure represents part of the surface of a specimen consisting of the outer cortex of a stem with leaf-cushions 3 cm. broad. The thickness of this specimen is 4 cm.: a section through the line ab is represented in [fig. 146], D (reproduced in the correct position, with the leaf-scars, sc, pointing downwards): internal to the cushions is a band of secondary cortex (the shaded strip on the outer edge of the section) which was formed on the outside of the phellogen. The phellogen is a cylinder of actively dividing cells in the outer part of the cortex of the stem, often spoken of as the cork-cambium or cortical meristem, which produces a considerable amount of secondary cortical tissue on its inner face and a much smaller amount towards the stem surface. This delicate cylinder frequently forms a natural line of separation between the outer shell of bark and the rest of the stem. In the specimen before us, the thin-walled cells of the phellogen were ruptured before petrification and the outer shell of bark was thus separated as a hollow cylinder from the rest of the stem: this cylinder was then flattened, the two inner surfaces coming into contact. Fig. 146, D, represents a section of one half of the thickness of the flattened shell.

This separation of the outer cortex, and its preservation apart from the rest of the stem, is of frequent occurrence in fossil lycopodiaceous stems. The flattened outer cortical shell of a Lepidophloios, specifically identical with that shown in [fig. 146], A and D, was erroneously described by Dr C. E. Weiss in 1881 as a large lepidodendroid cone[258].

Fig. 146, B, affords a view of the inner face of the specimen of which the outer surface is seen in [fig. 146], A: the surface shown in the lower part of the drawing, on which the boundaries of the cushions are represented by a reticulum, corresponds to the inner edge of the strip of secondary cortical tissue represented by the vertically shaded band in [fig. 146], D.

The shaded surface in [fig. 146], B, represents a slightly deeper level in the stem which corresponds to the outer edge of the vertically shaded band of [fig. 146], D: the narrow tapered ridges ([fig. 146], B) represent the leaf-traces passing through the secondary cortex, and the fine vertical shading indicates the elongated elements of which this strip of secondary cortex is composed.

In the longitudinal section diagrammatically reproduced in [fig. 146], D, cut along the line ab of [fig. 146], A, the parenchymatous tissue of the stout cushions has been partially destroyed, as at a; at s is seen the section of a Stigmarian rootlet which has found its way into the interior of a cushion. Each leaf-trace is accompanied by a parichnos strand as in the true Lepidodendron; at the base of the leaf-cushion the parichnos branches into two arms which diverge slightly right and left of the leaf-trace, finally entering the base of the leaf lamina as two lateral strands ([fig. 147], p). At one point in [fig. 146], D the section has shaved a leaf-trace represented by a black patch resting on the parichnos just above the line ef, but it passes through one of the parichnos arms p′ which debouches on to the leaf-scar sc at p. Had the section been cut along the line cd of [fig. 146], A the leaf-trace would have been seen in a position similar to that occupied by the parichnos p′ in [fig. 146], D.

Fig. 147. Lepidophloios leaf-cushion in tangential section. (From a section in the Williamson Collection, British Museum, No. 1973.)

Fig. 147, A, affords a good example of a tangential section through a Lepidophloios leaf-cushion, 1 cm. broad, like that represented in [fig. 146], A, showing the vascular bundle lt, the two parichnos strands, p, composed of large thin-walled cells (cf. Isoetes, [fig. 133], H, I), and the ligular pit near the upper edge of the section enclosing the shrunken remains of the ligule ([fig. 147], B, l).

LEPIDODENDRON

[Fig. 147], B, shows the form of the tangentially elongated leaf-cushions of Lepidophloios and their spiral disposition.

[Fig. 146], F, represents a section similar to that shown in [figs. 147], A and B, but in this case the leaf-trace, lt, and the parichnos strands, p, lie in a cavity formed by the destruction of some of the leaf-cushion tissue. It is worthy of notice that the parichnos cells have resisted decay more successfully than the adjacent tissue of the cushion.

The diagrammatic sketches reproduced in [fig. 146], H and I, were made from a transverse section similar to one originally figured by Williamson[259]: [fig. 146], H, corresponding in position to the line gh in [fig. 146], A, passes through the ligular pit, l, and cuts across the parichnos in the act of branching; the leaf-trace passes outwards beyond the Y-shaped parichnos strand. In the other section, [fig. 146], I, the parichnos is shown in a horizontal plane and the leaf-trace, lt, appears in oblique transverse section. In both sections and in [fig. 146], G the shaded band at the base represents the secondary cortical tissue external to the phellogen.

The transverse section represented in [fig. 146], G, shows in the left-hand cushion, a, the exit of the two parichnos arms and the leaf-trace between them: it illustrates also the various forms assumed by lepidodendroid leaf-cushions when cut across at different levels.

iv. The Anatomy of Lepidodendron vasculare Binney[260].
[Figs. 148–155], [168], A.

In the earlier literature dealing with the anatomy of Lepidodendron and Sigillaria the presence or absence of secondary vascular tissue was made the criterion of generic distinction and the distinguishing feature between the classes Pteridophytes and Gymnosperms, Lepidodendron being relegated to the former class because it was supposed to have no power of forming secondary wood, while Sigillaria, characterised by a considerable development of such tissue, was classed by Brongniart and afterwards by Renault as a Gymnosperm. Binney[261] in 1865 recognised that the two types of stem pass into one another, but it was Williamson[262] who provided complete demonstration of the fallacy of the Brongniartian view.

These two undoubted Pteridophytes agree very closely in anatomical structure and both are now recognised as arborescent genera of Lycopodiaceous plants. In a paper published by Lomax and Weiss in 1905[263] a specimen is described from the Coal-Measures of Huddersfield, in which a decorticated stem with the anatomical characters of Binney’s Sigillaria vascularis gives off a branch having the anatomical structure which it has been customary to associate with the species Lepidodendron selaginoides, so-called by Sternberg and founded by him on impressions showing well-preserved external characters.

In 1862 Binney[264] described petrified specimens of vegetative shoots from the Lower Coal-Measures of Lancashire under the names Sigillaria vascularis and Lepidodendron vasculare. These were afterwards recognised as different states of the same species. A few years after the publication of Binney’s paper Carruthers[265] identified Binney’s species Lepidodendron vasculare with Sternberg’s L. selaginoides. The evidence on which this identification rests has not been stated, but many writers have retained this specific designation for the well-defined type of anatomical structure first described by Binney as L. vasculare. The use of the specific name selaginoides is, however, open to objection. The species Lepidodendron selaginoides, as pointed out by Kidston[266], is probably identical with the plant which Brongniart had named L. Sternbergii before the institution of Sternberg’s species, and we are not in possession of convincing evidence as to the connection of L. Sternbergii (= L. selaginoides) with specimens possessing the anatomy of Binney’s type. Binney’s designation is therefore retained for the anatomical type described in the following pages[267].

The most detailed account hitherto published of the anatomy of Lepidodendron vasculare is that by the late M. Hovelacque[268], based on material from the Lower Coal-Measures of England.

Fig. 148. Lepidodendron vasculare Binney.

  1. Transverse section. (Based on a section 2·5 cm. in diameter, in the Cambridge Botany School Collection.)
  2. Longitudinal section. (Drawn from a section in Dr Kidston’s Collection.)

The small shoot, represented somewhat diagrammatically in [fig. 148], A, illustrates the anatomical features of a typical example of the species: the shoot has a diameter of 2·5 cm. and its central cylinder (x-sc) is 2·5 mm. in width.

Noticeable features are (i) the small size of the central cylinder (or stele) in proportion to the diameter of the branch, (ii) the production at a comparatively early stage of growth of a zone of secondary wood, x2, which gradually assumes the form of a complete cylinder of unequal breadth, surrounding the primary xylem, x, (iii) the formation of a secondary cortical tissue by a meristematic cylinder (phellogen, pl) situated close to the leaf-cushion region of the outer cortex. On the outer edge the stele consists of narrow tracheae some of which show in longitudinal section the spiral form of thickening characteristic of most protoxylem elements: towards the centre of the stele the diameter of the tracheae gradually increases and parenchymatous cells become associated with the elongated scalariform elements. In the central region the stele is composed of parenchymatous tissue arranged in vertical series of short cells, interspersed with short tracheae distinguished by the greater thickness of their walls and by their scalariform and reticulate thickening bands. Some of these short tracheae are shown in vertical section in [fig. 149], B: the fine and broken lines connecting adjacent thickening bands probably represent the remains of the original wall. These delicate bands, which have been figured in various species of lepidodendroid plants[269], are worthy of notice in connexion with the recent work of Mr Gwynne-Vaughan[270] who has shown that in many recent ferns the scalariform bands in the xylem elements are not connected by a thin pit-closing membrane, but are separated from one another by open spaces. In the Lepidodendron tracheae we seem to have a stage in which the intervening membrane is in process of absorption. It is, however, possible that the threads may be the result of contraction and splitting of the membrane during drying or decay.

Fig. 149. Lepidodendron vasculare. a, immature tracheae; m, meristem; mr, medullary ray; x, xylem.

  1. Longitudinal section through the edge of the secondary wood.
  2. Short tracheae in the centre of the stele. (From a specimen from the Halifax Hard bed in Dr Kidston’s Collection.)

The stele of Lepidodendron vasculare, before the addition of any secondary xylem, may be described as a protostele, a term originally proposed by Professor Jeffrey[271], in which the central part of the conducting strand of xylem elements has been converted into rows of parenchyma and short tracheids, the latter being better adapted to storage than to conduction. It is probable that this type of stelar anatomy, which distinguishes L. vasculare from other species, represents a comparatively primitive arrangement forming a transition between the stele of L. esnostense, which consists of a solid rod of tracheids, and the stele of L. Harcourtii ([fig. 179], A) and other species in which the xylem forms a cylinder enclosing a large parenchymatous pith.

Parenchymatous cells occur in contact with the outer edge of the xylem-cylinder some of which are distinguished by an irregular reticulate pitting. The tangential section represented in [fig. 148], B, illustrates the appearance of a shoot of L. vasculare in which no secondary xylem is present: the central strand of tissue consists of the parenchyma abutting on the xylem with several leaf-traces (lt) passing upwards in an almost vertical course from the outer edge of the stele.

The secondary xylem ([fig. 148], A, x2) consists of radially arranged scalariform tracheae with associated rows of parenchymatous cells which form medullary rays ([fig. 149], mr). Leaf-traces pass through the medullary rays in the secondary xylem cylinder in a direction at right angles to the primary xylem stele from which they are given off, but at the outer edge of the secondary xylem they bend suddenly upwards and for a time follow a steep and almost vertical course.

In well-preserved longitudinal sections the outermost secondary xylem tracheae are seen to be succeeded by a few narrow and vertically elongated elements ([fig. 149], A, a), which represent young unlignified tracheae: these are followed by shorter parenchymatous cells (m) forming part of a meristematic zone from which the secondary xylem receives additions.

Returning to [fig. 148], A; the zone of secondary wood, x2, composed of scalariform tracheids and medullary rays, is succeeded by a few layers of parenchymatous cells and beyond this is a broader zone, sc, to which the term secretory zone has been applied[272]; this is made up of small parenchymatous cells varying in size and of larger spaces which appear to have been formed by the disorganisation of thin-walled elements. The whole zone presents a characteristic appearance due to the association of small cells, large clear spaces, and a certain amount of dark-coloured material suggestive of tissue disorganisation and secreted products. The anatomical characters of the secretory zone are shown in the photograph, [fig. 168], A, sc. Several leaf-traces are seen in transverse section in the secretory zone (black dots in [fig. 148], A, sc; [fig. 154], C, lt): each trace consists of a strand of narrow tracheae accompanied by a few encircling layers of small parenchymatous cells. As a trace continues its steeply ascending course through the secretory zone, it becomes associated with a strand of that tissue and assumes the form of a collateral vascular bundle, the outer part of which does not consist of typical phloem but of shorter elements derived from the secretory zone. Beyond the secretory zone we find a more homogeneous tissue composed of parenchymatous elements slightly extended tangentially ([figs. 148], A, c1; [fig. 168], A, c); this is spoken of as the inner cortical region. In the great majority of sections of L. vasculare as of other species of the genus, the broader middle cortex ([fig. 148], c2) is occupied by mineral matter, introduced subsequent to decay of the tissue; or it is represented by patches of delicate tissue composed of loosely arranged parenchymatous cells varying considerably in size and shape, some being small, oval or polygonal elements while others have the form of sinuous hypha-like tubes.

In this middle cortical region may be seen leaf-traces passing outwards in an almost horizontal course ([fig. 148], A, lt): after leaving the inner cortex the leaf-traces bend somewhat abruptly outwards to follow a more direct path through the middle and outer cortex. The ring of tissue, s, seen in the middle cortex of [fig. 148], A, belongs to a Stigmarian rootlet.

The outer cortex ([fig. 148], A and B, c3) consists of homogeneous parenchyma which is stronger and more resistant to decay than the looser middle cortex. The leaf-traces, as shown in [fig. 148], B, pass through this region in a rather steeply ascending direction: each is seen to be enclosed by a space originally occupied by a strand of middle cortical tissue which accompanies lepidodendroid leaf-traces on their under side and has already been described as the parichnos, ([pp. 97], [100–103]; [figs. 146, 147]).

The surface of the stem shown in section in [fig. 148], A, is composed of broad leaf-cushions. A single leaf-trace with its parichnos passes into each cushion, but in the neighbourhood of the base of a cushion the parichnos bifurcates (cf. [fig. 146], H, I) and the arms diverge slightly to the right and left finally passing beyond the cushion into the lamina of the leaf, their position being shown, as already explained, by the two small lateral scars on the leaf-scar area.

The diagrammatic sketch of a radial longitudinal section through a leaf-cushion represented in [fig. 150] illustrates the relation of the leaf-trace to the leaf-cushion. The trace consists of xylem, x, above and a strand of the secretory zone, st, below; the parichnos tissue was originally present on the under side of the leaf-trace at a. The external surface, bc, marks the limit of the leaf-scar through the middle of which passes the vascular strand lt.

The lower gap a has been formed by the tearing of thin-walled cells of the phellogen, the meristematic tissue from which a considerable amount of secondary cortical tissue or phelloderm has been produced at pd. On the outside of the cushion, c, the cells are somewhat crushed and distinguished by their darker colour from the bulk of the parenchymatous tissue d.

This section also illustrates another characteristic feature of Lepidodendron, namely the presence of a ligule and a ligular pit: the former is represented by a carbonised patch of tissue and the latter extends from the surface of the cushion at b, just above the leaf-scar, almost to the level of the leaf-trace, lt. A comparison of this section with [figs. 146 and 147] will make clear the relation of the several parts of the cushion and leaf-scar.

The gaps gg, seen in [fig. 148], A and B, mark the position of the delicate meristematic zone or phellogen which arises close to the bases of the leaf-cushions; the phellogen has already produced a few rows of radially disposed elements, represented by short radial lines in the drawing, which constitute secondary cortical tissue.

Fig. 150. Lepidodendron vasculare. Leaf-cushions in longitudinal section. (From a specimen in Dr Kidston’s Collection.)

In older shoots the amount of the secondary cortical tissue developed on the inner side of the phellogen is considerable (cf. figs. [152], [153]).

The structure of the cortex of a shoot in which secondary growth, both in the stele and in the outer cortex, has progressed further than in the specimen shown in [fig. 148] is represented in [fig. 151].

Fig. 151. Lepidodendron vasculare. An older stem than that shown in [fig. 148]. (From a section in the Manchester Museum. No. 351.)

The section ([fig. 151], A) measures 7 × 3·8 cm. in diameter; the primary xylem is surrounded by a fairly broad cylinder of secondary wood ([fig. 151], E, x and x2). The almost smooth surface of the primary wood ([fig. 151], E, x) is succeeded by the secondary xylem, x2, characterised at its inner edge by the tapered ends of the radial rows of scalariform tracheids between which occur several delicate parenchymatous cells ([fig. 151], E, a). The occurrence of such isodiametric elements, often exhibiting a delicate spiral thickening band, is a characteristic feature of the boundary between primary and secondary wood in lepidodendroid stems. The secondary wood is penetrated by numerous medullary rays and in some of them are seen strands of narrow spirally thickened tracheae—the leaf-traces—which are in organic continuity with the exarch protoxylem of the primary wood. The leaf-traces are oval and mesarch. The space, c2, ([fig. 151], A) originally occupied by the delicate middle cortex, is succeeded by a shell of outer cortex composed chiefly of secondary tissue (phelloderm, pd) passing towards the inner boundary of this region into the primary outer cortex g ([fig. 151], A and C). The radially disposed elements which make up the bulk of the phelloderm are associated with concentric rows of secretory strands, represented by tangentially arranged dots in [fig. 151], A: on the outer edge of the phelloderm a few patches of primary cortex are still preserved, as at c, fig. A. One of these is shown on a larger scale in fig. B; at m the phelloderm is interrupted by a gap beyond which the cells have thinner walls and show signs of recent division; this is probably the position of the phellogen. The tissue b, [fig. 151], B, consists of secondary cortex succeeded beyond d by the parenchymatous tissue of the leaf-cushion, in which the remains of a ligule, l, are seen in the ligular pit. This section corresponds in position to a line drawn across [fig. 150] at the level of b. In this specimen we have two kinds of secondary cortical tissue: that formed external to the phellogen, from m to d in [fig. 151], B, is less in amount than that produced internal to the phellogen. We cannot make any satisfactory statement as to the nature of this secondary tissue, whether or not any of it agreed in composition with the cork which is usually formed external to the phellogen in recent plants. As the stem of a Lepidodendron grew in girth the leaf-cushions became separated by intervening depressions composed of the secondary cortex formed external to the phellogen, but at a later stage the cushions were thrown off, leaving the outer edge of the phelloderm as the superficial tissue. This exposed tissue became fissured as growth and consequent stretching continued, producing the appearance seen on the surface of the still older stem represented in [fig. 153].

The inner edge of the phelloderm seen at e in [fig. 151], C, passes suddenly into the inner primary region of the outer cortex ([fig. 151], A and C, g) which comprises two types of parenchymatous tissue, patches of isodiametric cells, g, g, alternating with radially arranged areas consisting of tangentially elongated elements (fig. C, f, f; fig. D) which extend as wedges into the phelloderm.

The longitudinal section represented in [fig. 152], B, shows an equal bifurcation of a stem in which no secondary xylem is present; in the lower part of the section the xylem and the outgoing leaf-traces are seen in radial section and at the upper end of each arm the leaf-traces alone, lt, are exposed, as in [fig. 148], B. It is interesting to notice the large amount of phelloderm which has been produced in the fork of the branch, at pd, where greater strength is required.

Fig. 152. Lepidodendron vasculare. Sections of dichotomously branched shoot.

  1. From a section (10·5 × 9 cm.) in the Cambridge Botany School Collection.
  2. From a section (8 cm. long) in the Cambridge Collection.

The section represented diagrammatically in [fig. 152], A, has lost the outermost part of the cortex together with the leaf-cushions; it consists largely of secondary cortex composed of radially disposed phelloderm cells and tangentially placed secretory strands (represented by the discontinuous black lines in the drawing): the dotted region in the central part of the axis is composed of primary cortical parenchyma, and the two spaces surrounding the steles contain portions of the lacunar middle cortex. Each stele possesses a narrow crescentic zone of secondary xylem; the amount is greater in the case of the right-hand stele, of which a small piece is shown on a larger scale; the striking contrast in size between the outer and more internal secondary tracheae is no doubt the expression of some unfavourable condition of growth. The position of the secretory zone beyond the secondary xylem is shown at sc, [fig. 152], A.

Fig. 153. Lepidodendron vasculare.
(From a specimen (16 × 7·5 cm.) in the Manchester Museum.)

An example of a large and partially decorticated stem is afforded by the specimen (16 × 7·5 cm.) shown in [fig. 153]. The irregularly ribbed surface is formed of rather thick-walled phelloderm, in which occur tangentially arranged rows of secretory strands. The tapered form of the secondary cortex as it abuts internally on the primary cortex is shown very clearly in the drawing (cf. [fig. 151], C). The stele in this much older stem consists mainly of secondary wood.

Fig. 154. Lepidodendron vasculare. Shoot (2·8 cm. diam.) with two steles. (From a specimen from Halifax in the Williamson Collection, British Museum, No. 340.)

An interesting example of a small shoot, the largest diameter of which is 2·8 cm., is shown in [fig. 154], A: the section was cut a short distance above the bifurcation of the stele into two approximately equal branches. The outer part of the cortex consists of phelloderm, pd, with the usual rows of secretory tracts, and primary outer cortex g; the middle cortex is represented by patches of parenchyma with a few leaf-traces. To one of the steles, s′ ([fig. 154], A), a crescent-shaped band of secondary xylem has been added; the other stele, S, possesses no fully developed secondary elements.

Fig. 154, B and C, illustrates the anatomical features immediately external to the primary xylem of the smaller stele, s. The comparatively broad band of radially disposed parenchyma, m, is connected with the outermost elements of the xylem by a few rather dark and small crushed parenchymatous cells. The band m, which we may speak of as the meristematic zone, clearly consists of cells in a state of division; it is in this region that the secondary xylem is produced. Beyond the leaf-trace, ([fig. 154], C, lt), occurs a portion of the secretory zone, some of the smaller cells of which show signs of disorganisation; but most of this tissue has been destroyed ([fig. 154], B, sc). The outer edge of the secretory zone is shown in [fig. 154], D abutting on the cells of the inner cortex, c′. The leaf-trace shown in the inner cortex in [fig. 154], B illustrates the more oval or tangentially extended form of the xylem in this region, in contrast to the more circular outline which it exhibits on the inner side of the secretory zone.

Fig. 155. Lepidodendron vasculare. Outer edge of secondary xylem: m, meristematic zone; mr, medullary ray. (Drawn from the section shown in [fig. 168], A).

The transverse section, part of which is reproduced in [fig. 168], A, illustrates a characteristic feature, namely the juxtaposition of the outermost tracheae of the secondary xylem and much smaller cells of the meristematic zone. This is seen in [fig. 155], which shows a small piece of [fig. 168], A, on a larger scale. In plants with a normal cambium the segments cut off from the initial layer fit on to the elements of the xylem or phloem to which they are to form additions, but in Lepidodendron it seems to be a general rule to find each of the most external lignified elements abutting on a group of two or three much smaller cells. It is difficult to believe that the meristem shown in [fig. 155], m, could produce secondary xylem elements equal in size to those already formed: in all probability had growth continued there would have been a marked difference between the size of the secondary tracheids, as in [fig. 152], A, x2, where there was no doubt some cause which interfered with normal cambial activity. This disparity in size between the secondary xylem elements and the adjacent parenchymatous tissue of the meristematic zone is by no means exceptional and may be described as the general rule. It is at least certain that in Lepidodendron vasculare, as in other species, the secondary xylem was succeeded by a broad band of parenchymatous tissue, from which new tracheae and medullary-ray elements were produced, and not by a narrow cambium such as occurs in recent plants.

v. Lepidodendron stems as represented by casts and impressions of partially decorticated specimens.

The differentiation of the outer cortex of a Lepidodendron into comparatively thin-walled and more resistant tissue has been the cause of unequal decay and the consequent formation of shrinkage cavities. In addition to the unequal resisting power of contiguous tissues, another important factor in determining the nature of casts and impressions is the existence of the cylinder of delicate cells in the outer cortex of stems and branches. As already pointed out, this meristematic cylinder or phellogen constitutes a natural line of separation, as in the case of the cambium layer between the wood and the external tissues in a fresh Sycamore twig. The result of the separation of an outer shell of bark from the rest of the stem and the results of unequal decay in the more superficial tissues, have necessarily led to the preservation of the same specific type under a variety of forms.

Our knowledge of the anatomy of Lepidodendron stems enables us to recognise in fossils of very different appearance specimens in various conditions of preservation of one and the same type. Such names as Knorria, Bergeria and Aspidiaria are examples of generic titles instituted before any adequate knowledge of Lepidodendron anatomy was available.

Differences in age as well as different degrees of decortication have contributed in no small measure to the institution of generic and specific names which more recently acquired knowledge has shown to be superfluous.

a. Knorria.

The designation Knorria, after a certain G. W. Knorr of Nürnberg, was proposed by Sternberg in 1826[273] for casts of Palaeozoic stems of a type figured more than a century earlier by Volkmann[274]. Goeppert, in his earlier works, published drawings of fossil stems which he referred to Sternberg’s genus: one species he at first called Didymophyllum Schollini. He afterwards[275] described some specimens which showed that the features characteristic of Knorria may occur on partially decorticated stems with leaf-cushions of the true Lepidodendron type. His specimens, preserved in the Breslau Museum, demonstrate the accuracy of his drawings and conclusions. Goeppert, and after him Balfour[276], drew attention to the different appearances presented by branches of Araucaria imbricata when preserved with the surface intact and after partial decortication, as illustrating possible sources of error in the determination of fossil stems.

Although it is now a well-established fact that fossils bearing the name Knorria are imperfect lepidodendroid stems, the use of the term may be conveniently retained for descriptive purposes. The specimen from the Commentry coal-field of France, shown in [fig. 156], affords some excuse for the institution of several generic names for different states of preservation or decortication of one species. The cortical level exposed at e is characterised by spirally disposed peg-like ridges with truncated apices: it is this form of cast which is usually designated Knorria. The ridges vary in size and shape in different types of stem; they may be narrow as shown at e, [fig. 156], or short and broad with rounded distal ends. In some cases they are forked at the apex, as in the partially decorticated specimen of Lepidodendron Veltheimianum represented in [fig. 185], A.

Fig. 156. A dichotomously branched Lepidodendroid stem (Knorria mirabilis Ren. and Zeill.). (After Renault and Zeiller.) (¼ nat. size.) The original specimen is in the Natural History Museum, Paris.
a–g, surface features exposed as the result of different degrees of decortication. (See vol. I. p. 102, fig. 23).

The Knorria state represents the impression or cast of the outer cortical region too deep below the leaf-cushion region to retain any indications of the cushion-form; the ridges are the casts of the spaces produced in the cortex by the decay of the sheath of delicate cells surrounding each leaf-trace and by the decay of the thin-walled cells of the parichnos. The occasional forked apex of a ridge is the expression of the fact that the cast was made at the region where the parichnos divides into two arms (cf. p. 100). In certain specimens it is possible to connect the Knorria casts with associated lepidodendroid stems which may be determined specifically; but when we have no evidence as to surface-features the fossils may be designated casts of lepidodendroid stems in the Knorria condition. Such casts are illustrated by numerous drawings in palaeobotanical literature[277].

b. Bergeria.

This is another name first used by Sternberg in his classic work, Die Flora der Vorwelt, for casts of lepidodendroid plants such as Steinhauer[278] had previously figured as Phytolithus cancellatus. Brongniart[279] recognised that the application of the generic title Lepidodendron should be extended to include specimens referred by Sternberg to Bergeria, and a few years later Goldenberg[280] realised that this name does not stand for well-defined generic characters. The correctness of these views was, however, first satisfactorily demonstrated by Carruthers[281] and by Feistmantel[282].

If a Lepidodendron stem loses its superficial layers of outer cortex and in this condition is embedded in sand or mud, the cast is distinguished from that of a perfect stem by the absence of the leaf-scars and by other features. It may, however, still show spirally disposed areas, corresponding approximately to the original leaf-cushions, which are characterised by a small depression or pit either at the apex or near the centre of each oval area: the pit marks the position of the leaf-trace and its parichnos strand. In some cases the exposed surface may be smooth without any indication of leaf-cushions, while narrow spirally arranged grooves represent the obliquely ascending vascular bundles passing through the cortex to the leaves.

Fig. 185, B, shows the Bergeria state of Lepidodendron Veltheimianum, which differs from the Knorria condition in the fact that decortication had not extended below the level at which the form of the leaf-cushions could be recognised. It is clear that no sharp line can be drawn in all cases between the different degrees of decortication as expressed by the terms Knorria and Bergeria.

A list of synonyms of Knorria, Bergeria, and Aspidiaria forms of stem and a detailed treatment of their characteristic features may be found in a recent work by Potonié[283].

c. Aspidiaria.

In one of the earliest English books on fossil plants, the Antediluvian Phytology by Artis[284], a specimen from the Carboniferous sandstone of Yorkshire is figured as Aphyllum cristatum, and a similar fossil is described as A. asperum. These are impressions of Lepidodendron stems in which the characteristic leaf-cushions are replaced by smooth and slightly convex areas with a narrow central ridge. To this type of specimen Presl gave the name Aspidiaria[285], under the impression, shared by subsequent writers, that the supposed external features were entitled to generic recognition.

It is to Stur[286] that we owe the first satisfactory interpretation of fossils included under the name Aspidiaria: he showed that on the removal of the projecting convex areas from some of his specimens a typical Lepidodendron leaf-cushion was exposed ([fig. 144], A, a). The Aspidiaria condition ([fig. 144], A, b) represents the inner face of the detached shell of outer bark of a Lepidodendron stem, while in the Bergeria casts we have a view of the external face of a stem deprived of its superficial tissues.

In a Lepidodendron stem embedded in sediment the more delicate portions of the leaf-cushions would tend to shrink away from the internal and more resistant tissues of the outer cortex, thus producing spaces between each cushion; further decay would cause rupture of the leaf-traces and the superficial tissues would thus be separated from the rest of the stem. The tendency of Lepidodendron stems to split along the line of phellogen in the outer cortex is seen in [fig. 148], A, g. The deposition of sediment on the exposed inner face of this cortical shell would result in the production of a specimen of the Aspidiaria type: the reticulum enclosing the spirally disposed convex areas is formed by the impression of the firmer tissue between the leaf-cushions.

vi. Lepidodendroid axes known as Ulodendron and Halonia.

a. Ulodendron.

This generic name was suggested by Lindley and Hutton[287] for two specimens from the English Coal-measures characterised by leaf-cushions like those of a Lepidodendron, but distinguished by the presence of two vertical rows of large and more or less circular cup-shaped scars. These authors, while recognising the possibility that the fossils might be identical with Lepidodendron, regarded them as generically distinct. The generic title Ulodendron, though no longer denoting generic rank, is still applied to certain shoots of lycopodiaceous plants which may belong to the genera Lepidodendron, Bothrodendron, and according to some authors[288], also to Sigillaria.

The large specimen from the Belgian coal-measures, represented in [fig. 211], affords a good example of the Ulodendron form of shoot of the genus Bothrodendron, which is described on [page 249]. The specimen shown in [fig. 157] shows the Ulodendron shoot of Lepidodendron Veltheimianum.

Casts of large Ulodendron scars are occasionally met with as separate fossils bearing a resemblance to an oval shell.

In Steinhauer’s paper on Fossil Reliquiae[289] a drawing is given of a Ulodendron stem under the name Phytolithus parmatus and a similar stem specifically identical with that shown in [fig. 157] was figured by Rhode[290], one of the earliest writers on fossil plants, under the comprehensive designation “Schuppenpflanze.”

Fig. 157. Lepidodendron Veltheimianum. Ulodendron condition. (From a photograph by Dr Kidston of a specimen from the Calciferous Sandstone series, Midlothian; ⅖ nat. size.) [Kidston (02) Pl. LVII.

There has been no lack of ingenuity on the part of authors in offering suggestions as to the meaning of these large cup-like depressions, and there is still difference of opinion as to their significance. Lindley and Hutton[291] described them as the scars of branches or masses of inflorescence. Sir Joseph Hooker[292] speaks of a specimen of Ulodendron, shown to him by Mr Dawes, on which a large organ, supposed to be a cone, was inserted in one of the depressions, but he was unable to arrive at any conclusion as to the real nature of the fossil. While most authors have seen in the scars pressure-areas formed by the pressure of sessile cones against the surface of a growing branch, others, as for example Geinitz[293], have described the depressions as branch-scars. Carruthers[294] regarded the scars as those of adventitious roots and Williamson referred to them as the scars of reproductive shoots. The depressions vary considerably in size. The Belgian example shown in [fig. 211] possesses scars 9 cm. in diameter. A specimen of Bothrodendron in the Manchester Museum from the Lancashire Coal-Measures, to which Williamson[295] has referred, bears two rows of scars 11–12 cm. in diameter on a stem 112 cm. in girth and 233 cm. long. The scars occur in two alternate series, on opposite faces of the axis, the distance between the successive scars in the same row being 29 cm. The surface-features of this large stem are not preserved.

Before considering the nature and origin of the scars it is important to remember the considerable size to which they may attain; other points of importance are the occurrence, either in the centre of each depression or in an excentric position, of an umbilicus or slightly projecting boss, in the centre of which is a pit formed by the decay of an outgoing vascular strand. The sloping sides of the scars sometimes bear elevations resembling leaf-cushions like those on the rest of the stem surface. In the specimen shown in [fig. 157] the lower margin of each cup shows indistinctly the outlines of what appear to be leaf-cushions, while the rest of the sloping face is characterised by radial ridges, which may be due to bracts or leaves.

It is obvious that in these cups we have the scars of some lateral organ, but the evidence afforded by specimens of which the depressions contain the remains of such organs is by no means conclusive. A Ulodendron has been figured by D’Arcy Thompson[296], in which the lower part of a lateral organ is attached by a narrow base to one of the scars, but the preservation is not sufficiently good to enable us to decide whether the organ is a cone or a vegetative shoot. Kidston[297] has described other examples showing portions of organs in connexion with the scars, but an examination of the specimens in his collection failed to convince me that his interpretation of them as strobili is correct.

The phenomenon known as cladoptosis, as shown on a stem of the Conifer Agathis[298] and certain Dicotyledonous trees such as Castilloa, suggests a possible explanation of the Ulodendron scars. This comparison was made by Shattock[299] in 1888, but he did not accept the resemblance as a real one. An objection may be urged to the cladoptosis hypothesis that in Ulodendron the branch, whether vegetative or reproductive, was not attached to the whole of the depressed area. On the other hand, a lateral branch originally attached by a narrow base may have continued to increase in diameter until its base became slightly sunk in the bark of the stem, thus producing a cup-like depression which, on the fall of the branch, would retain traces of the original surface-features of the stem.

Mr Watson[300] of Manchester recently published a paper on Ulodendron scars, in which he adduces fresh and, as it seems to me, satisfactory arguments in favour of the branch-scar hypothesis. Fig. 158, from one of Mr Watson’s blocks, illustrates the nature of his evidence. He points out that in the obverse half of a large specimen of Bothrodendron in the Manchester Museum, the umbilicus consists of a cylindrical hole, 18 mm. deep and 8 mm. in diameter, surrounded by a projecting ring of mineral material which doubtless represents some portion of the original plant: on the reverse half of the specimen the continuation of the ring is seen as a prominent cone fitting into the cup-like depression in the obverse half: the conical cast shows that numerous small vascular strands were given off from this ring of tissue, and these strands have the same arrangement and size as the dots which are found on typical Ulodendron scars. He interprets the ring surrounding the umbilicus as the remains of the primary wood and the small strands as leaf-traces supplying the branch.

Fig. 158. Diagrammatic section through the base of a branch to illustrate the Branch theory of the Ulodendroid scar. (After Watson.)

In the diagrammatic section shown in [fig. 158] the outer cortex of the main stem is represented by oc 1; this consists of secondary tissue. The corresponding tissue in the branch is seen at oc 2. The stele of the stem is shown at Tr. St. and that of the branch at Br. St.; lt, lt, mark the position of the leaf-traces. If we assume the branch to be detached along the line LS, the depression would show numerous spirally arranged dots representing the points of exit of leaf-traces and the vascular axis would be exposed in the umbilicus. This explanation appears to me to be in harmony with the surface-features of Ulodendron scars on both Bothrodendron and Lepidodendron stems. The occasional occurrence of leaf-cushions on a portion of a Ulodendron scar is a difficulty on the cladoptosis hypothesis. Assuming that true leaf-cushions occur, their presence may, as Watson suggests, be due to the folding back of a piece of the outer cortex of the branch which has been “crushed down on to the area of the scar[301].”

Since this account was written a note has been published by M. Renier[302] in which he describes a specimen of Bothrodendron from Liège, one face of which shows a projecting Ulodendroid scar with an excentric umbilicus. On the other face is a dichotomously branched shoot with surface-features corresponding to those on the scar; the evidence that the scar represents the base of the branch is described as indisputable.

Stur[303] held the view that the depressions on Ulodendron stems represent the places of attachment of special shoots comparable with the bulbils of Lycopodium Selago, or, it may be added, with the short branches occasionally produced on Cycas stems. If the depressions were formed by the pressure of the bases of cones, it is clear that the size of the cavity must be an index of the diameter of the cone. The larger Ulodendron scars exceed in diameter the base of any known lepidodendroid strobilus. Another obvious difficulty, which has not been overlooked by Kidston who holds that the scars were produced by sessile cones, is that in Lepidodendron Veltheimianum strobili were borne at the tips of slender branches; the same difficulty is presented by Bothrodendron (Fig. 213). It is unlikely that two types of strobili were produced on the same plant, particularly as the cone of L. Veltheimianum was heterosporous.

The cones of certain species of Pinus remain attached to the tree for many years and their bases become embedded in the stem; this is particularly well shown in the drawing of a cone of Pinus clausa ([fig. 159]), for which I am indebted to Mr Sudworth, Dendrologist in the United States Forest Service. Mr Sudworth has drawn my attention to P. attenuata and P. muricata in illustration of the same phenomenon[304]. The example shown in [fig. 159] cannot, however, be matched by any known specimen of Ulodendron; in the case of the depressions on the stem of a Pine the cone-base fits the circular scar, but in the fossil stems it is practically certain that this was not the case.

Fig. 159. Pinus clausa. ½ nat. size.

There can be little doubt that certain Palaeozoic Lycopods shed their branches by a method similar to that employed by the Kauri Pine of New Zealand and by some species of Dicotyledons. The evidence adduced in the case of Bothrodendron punctatum is a strong argument in favour of extending the same explanation to other Ulodendron shoots.

Fig. 160.

  1. Lepidophloios scoticus Kidst. From a specimen from the Calciferous Sandstone, Midlothian, in Dr Kidston’s Collection; rather less than ⅓ nat. size.
  2. L. scoticus cone. From a specimen from the Calciferous Sandstone of Midlothian in Dr Kidston’s Collection; slightly reduced.
b. Halonia.

The branched axis with Lepidophloios leaf-cushions, represented in [fig. 160], A, illustrates a special form of shoot described by Lindley and Hutton[305] under the generic name Halonia. The original specimens referred to this genus are decorticated axes showing remains of Lepidodendroid leaf-cushions. The spirally disposed circular scars in the specimen of Halonia (Lepidophloios scoticus[306]) shown in [fig. 160] constitute the characteristic feature of the genus; they may have the form, as in [fig. 160], A, of circular discs with a central umbilicus marking the position of a vascular strand, or, as in the sandstone cast of Halonia tortuosa shown in [fig. 161][307], they may appear as prominent tubercles. The latter example illustrates the condition characteristic of partially decorticated stems.

Fig. 161. Halonia tortuosa L. and H. From a specimen in Dr Kidston’s Collection, from the Lower Coal-measures of Ayrshire (No. 1561); ⅔ nat. size.

In 1883 Williamson[308] described a specimen, now in the Leeds Museum, which convinced him that Halonia is merely a special form of Lepidodendron concerned with the production of fertile shoots or strobili. Feistmantel[309] also recognised that Halonia regularis is identical in the form of the cushions with the type known as Lepidophloios laricinus. It is worthy of note that under the name Halonia, Feistmantel[310] figured a piece of decorticated axis characterised by two rows instead of the usual spiral series of large cup-shaped scars. Recent researches have, however, tended to break down the distinction between Ulodendron and Halonia founded respectively on the biseriate and spiral arrangement of the scars or tubercles.

The interpretation of Halonial branches as cone-bearing members of Lepidodendroid plants has passed into a generally accepted statement of fact, but, so far as I know, only one specimen has been figured in which strobili are seen attached to an Halonia axis. This specimen, described by Grand’Eury[311] from the coal-field of Gard, is hardly sufficiently well-preserved to constitute a demonstration of the correctness of the generally received view, which, as is not unusual, has been repeated by one writer after another without due regard being paid to the nature of the evidence on which the statement is based. It may, indeed, be correct to describe Halonial branches as cone-bearing, but there are certain considerations which make one pause before unhesitatingly accepting this explanation. The vascular strand which passes from the central cylinder of the shoot to the tubercle or scar is composed of a solid rod of xylem distinguished from the main stele by the absence of a pith. In such petrified peduncles as have been discovered the stele is of the medullated type. The common occurrence of strobili terminating slender branches of lepidodendroid plants, though not a fatal objection to their attachment to Halonial shoots, shows that in many cases the cones were borne at the tip of leafy shoots. It may be that some of the Halonial scars are in origin like those of the Ulodendron axes of Bothrodendron and mark the position of deciduous vegetative branches.

The first account of the anatomy of Halonia we owe to Dawes[312]; this was followed by a fuller description by Binney[313]. The history of our knowledge of this type of branch has been given by Carruthers[314], who expressed the opinion that Halonia is merely a fertile condition of Lepidophloios and possibly of other lepidodendroid plants. He was also inclined to regard the Halonial tubercles as younger stages of the larger scars characteristic of the genus Ulodendron. Williamson’s contributions to our knowledge of Halonia are of primary importance; he supplied further proof of the Lepidodendroid nature of these branches and advanced our knowledge of their anatomy. In an early paper[315] he expressed the view that the differences on which Halonia and Ulodendron are separated are such as result from a difference in age and are not of generic importance. In the last memoir, of which he was sole author, published by the Royal Society[316], Williamson brought forward further evidence in support of this well-founded opinion.

That the fossils known as Halonia are branches of a lepidodendroid plant is at least certain, and it is probable that the lateral branches which they bore were fertile, though satisfactory proof of this is lacking. We know also that Halonia branches are characterised by the Lepidophloios form of leaf-cushion; there is, however, no sufficient reason to assume that such branches were never attached to stems with the cushions of the Lepidodendron form. The further question, namely whether Williamson was correct in his contention as to the absence of any essential distinction between Ulodendron and Halonia, does not admit of an unchallenged answer. In 1903 Weiss[317] described the anatomy of a specimen of a biseriate Halonia branch of Lepidophloios. The form of the leaf-cushions is unfortunately not very well preserved, but Weiss figures other specimens with two rows of tubercles on which the leaf-cushions are sufficiently distinct to justify a comparison with those of Lepidophloios. He believes with Williamson that it is the presence of tubercles in place of scars which distinguishes Halonia from Ulodendron, and that the arrangement of the tubercles or scars is a matter of little importance. He expresses the opinion justified by the evidence available that the absence or presence of tubercles is merely due to accidents of preservation or, one may add, to difference in age. Kidston[318] dissents from Weiss’s description of his specimen as a biseriate Halonia; he regards it as a Ulodendron branch of Sigillaria discophora (König). Until specimens with more clearly preserved external features are forthcoming it is impossible to settle the point in dispute, but on the facts before us there would seem to be a prima facie case in favour of Weiss’s contention.

The designation Halonia may be retained as a descriptive term for Lepidodendroid shoots characterised by spirally disposed scars or tubercles and bearing leaf-cushions of the Lepidophloios type. In the case of specimens showing prominent tubercles, the superficial tissues are usually absent and, as in the fossil represented in [fig. 161], the name Halonia does not necessarily imply the presence of leaf-cushions of a particular type.

vii. Anatomical characters of Vegetative Lepidodendron shoots (Lepidodendron and Lepidophloios).

The type already described under the name Lepidodendron vasculare differs from those dealt with in the following pages chiefly in the anatomy of the stele. The simplest and probably most primitive type of Lepidodendron stem is that in which the xylem forms a solid rod; the type of stele most frequently represented is that of L. Harcourtii, L. fuliginosum, and other species in which the diameter of the stele is greater and a cylinder of primary xylem encloses a comparatively large parenchymatous pith.

1. Lepidodendron esnostense, Renault[319].

This species was founded by Renault on petrified specimens from the Culm beds of Esnost in France. The surface of a young twig bears prominent leaf-cushions of elongated rhomboidal form similar to those of Lepidodendron obovatum ([fig. 173]) and other species. In older branches the primary cortex is replaced by a considerable thickness of radially disposed secondary cortical tissue which, as shown in tangential section, consists of a reticulum of elongated pointed elements with comparatively thick walls enclosing meshes filled with large-celled parenchyma. It is worthy of note that if such a branch were exposed to decay, the earlier destruction of the more delicate tissue in the meshes of the secondary cortex would produce a series of oval depressions, corresponding to the parenchymatous areas, separated by a projecting reticulum of the more resistant elements: a cast of this partially decayed surface would be indistinguishable from that of some types of Sigillaria or of a Lyginodendron. The inner regions of the cortex of the type-specimens have not been preserved. The xylem, which is the only part of the stele represented, has the form of a protostele or solid cylinder of scalariform tracheids with peripheral groups of narrower protoxylem elements which mark the points of exit of the leaf-traces: in a branch 1–2 cm. wide the xylem column has a diameter of 3 mm. The small leaves ([fig. 143], B, C), similar to those of a Sigillaria, are sub-rhomboidal in section near the base and approximately circular near the apex[320]. The mesophyll consists of palisade cells having the appearance of typical chlorophyll-tissue. The heterosporous strobili attributed to this species bore microsporangia on the upper and megasporangia on the lower sporophylls; the megaspores, of which a considerable number occur in each megasporangium, are identical in size with those of another Culm form, Lepidodendron rhodumnense. Some of these have retained traces of prothallus tissue, and in one spore Renault figures what he regards as an archegonium: the drawing is by no means convincing.

2. Lepidodendron rhodumnense, Renault[321].

The species from the Culm of Combres (Loire) agrees in its solid xylem cylinder and in the differentiation of the secondary cortex, as also in the association of two kinds of spore, with Lepidodendron esnostense. A comparison of the leaves of the two types reveals certain differences which may be of specific rank, but, apart from minor differences, these Culm species may be classed under one anatomical type.

3. Lepidodendron saalfeldense, Solms-Laubach[322].

This Devonian species was founded on a specimen 3 × 2·5 cm. broad at the base, which shows the stumps of four branches recalling the dichotomously branched arms of Stigmaria and Pleuromeia. If these are in reality the remains of Stigmaria-like horizontal branches the species affords an interesting example of a Lepidodendron axis with a subterranean rhizome of the type which has been found in several Sigillarian stems. In the upper end of the axis the stele consists of a solid strand of xylem which is not sufficiently well preserved to show the position of the protoxylem groups. A transverse section taken near the base reveals a type of stele differing from that at the upper end in being composed of radially disposed tracheids and in its resemblance to the stele of Stigmaria.

4. Lepidodendron fuliginosum, Williamson. Figs. [162–172], [179], E.

The name Lepidodendron fuliginosum was proposed by Williamson in 1887 for petrified stems previously included by him in Witham’s species L. Harcourtii, but subsequently recognised as a distinct type characterised by “the greater uniformity in the composition of the entire cortex” and by other features some of which do not constitute distinctive characters. The species agrees with L. Harcourtii and with L. Veltheimianum in having a medullated stele; it is distinguished not only by the more frequent preservation of the middle cortex, a fact due to a difference in minute structure, but chiefly by the peculiar structure of the secondary tissue added to the stele; this is in part composed of radial series of parenchymatous cells and of a varying amount of tracheal tissue the elements of which are narrower than in other species and are characterised also by their sinuous vertical course. As is pointed out in the sequel, the anatomical features of L. fuliginosum, as at present understood, are not confined to one type of Lepidodendron stem. Specimens have been described with leaf-cushions of the form characteristic of L. aculeatum, L. obovatum and Lepidophloios combined with the anatomical features of Williamson’s species: it is possible that the two species L. obovatum and L. aculeatum are not really distinct[323], but it is certain that shoots with both the Lepidodendron and Lepidophloios cushions may have the same type of anatomical structure.

A more detailed knowledge of the structural features of Lepidodendron shoots may enable us to define anatomical species with more exactness than is possible at present. There can, however, be little doubt that well-marked anatomical features may be associated with more than one specific form of shoot as defined by the form of the leaf-cushions.

Solms-Laubach proposed the name Lepidodendron Williamsoni for the anatomical type L. fuliginosum of Williamson, but the latter name has been generally adopted.

In the following account special attention is directed to the nature and origin of the secondary stelar tissue and to the secretory zone, as difference of opinion exists as to the interpretation of these features. Among the best examples of shoots of Lepidodendron fuliginosum without secondary tissue or in which it is feebly developed are those originally described by Binney. The stele includes a large parenchymatous pith, the cells of which frequently show signs of recent division, a feature observed also in the pith of the large stem of L. Wünschianum, represented in figs. [181], [182]. The primary xylem cylinder has an irregularly crenulate outer edge like that of L. Wünschianum and L. Harcourtii and the protoxylem elements occupy an exarch position. Isodiametric reticulately-pitted elements are met with both on the inner and outer edge of the xylem.

Fig. 162. Lepidodendron fuliginosum. Part of the stele in transverse section. (Binney Collection, Sedgwick Museum, Cambridge.)

Fig. 163. Lepidodendron fuliginosum. Longitudinal section. (Binney Collection, Cambridge.)

Figs. [162] and [163] illustrate the structure of the outer portion of the xylem and adjacent tissues in a section of a shoot 3·8 cm. × 2·5 cm. in diameter, which is in the act of branching, as shown by the occurrence of two steles of equal size. A figure of the complete section will be found in Binney’s memoir[324], and additional illustrations were published in 1899[325].

Fig. 164. Lepidodendron fuliginosum. Leaf-trace. (Binney Collection, Cambridge.)

The primary xylem (figs. [162], [163], x) is succeeded by 2–3 rows of polygonal cells with dark contents and associated with isodiametric tracheae: these pass into clearer parenchymatous tissue, a, characterised by the arrangement of the cells in vertical series, to which the term meristematic zone has been applied. The secretory zone, s, abutting on the meristematic zone, consists of more or less disorganised parenchymatous cells and broader and more elongated spaces; it is interrupted here and there by an outgoing leaf-trace, as at lt 1 and lt 2 in [fig. 162]. The secretory zone is succeeded by a homogeneous inner cortex like that described in L. vasculare; part of this region is seen at the upper edge of [fig. 162]. The broad middle cortex, which is separated from the inner cortex by a sharply defined boundary, is composed of rather small lacunar parenchymatous tissue consisting of sinuous tubular elements interspersed among isodiametric cells of various sizes ([fig. 166], p). In the middle cortical region the leaf-traces pursue an almost horizontal course; one is shown in [fig. 164], in oblique longitudinal section, in a reversed position; the xylem, x, should be on the inner side of the secretory tissue, s. The clear space between the two parts of the vascular bundle was originally occupied by a few layers of parenchymatous cells, as seen in the transverse sections, figs. [165] and [166]. In some specimens the leaf-traces pass through the middle cortex in a much more vertical course, as shown by the section represented in [fig. 165]. This section illustrates the structure of a typical leaf-trace with unusual clearness; it shows the tangentially elongated group of xylem, the strand of tissue which occupies the position of phloem, s (to which the term secretory zone is applied), the compact parenchyma between the two parts of the bundle, and surrounding the whole a narrow sheath sharply contrasted by the smaller and more uniform size of the cells from the middle cortex, a few cells of which are seen in the photograph. The middle cortex shows a well-defined junction with the more compact outer cortical region, which consists of primary parenchyma passing externally into a zone of phelloderm composed of thick-walled and more elongated cells. A noticeable feature in many Lepidodendron shoots is the occurrence of a circle of strands of secretory cells often surrounding fairly large ducts just internal to the edge of phelloderm: similar strands form irregularly concentric circles, as was pointed out in the case of L. vasculare, in the phelloderm itself.

Fig. 165. Lepidodendron fuliginosum. Leaf-trace: x, xylem; s, secretory zone. (Binney Collection, Cambridge.)

Fig. 166. Lepidodendron fuliginosum. Leaf-trace: p, parichnos. (Binney Collection, Cambridge.)

Fig. 166 shows a leaf-trace in the outer cortex accompanied by its crescent-shaped parichnos, p, derived from the middle cortex and by means of which the outer cortex and the lamina of the leaves are connected with the inner region of the shoot. This lacunar middle cortex and parichnos doubtless constitute an aerating tissue-system which after leaf-fall is exposed directly to the air at the ends of the parichnos arms on the leaf-scars.

Some of the sections in the Binney Collection (Sedgwick Museum, Cambridge) show early stages in the production of secondary xylem: in the section represented in [fig. 167] the secretory zone is succeeded on its inner face by a zone of radially elongated cells, m, which are clearly in a meristematic condition. The same section shows also the more radially extended form of the xylem of a leaf-trace with its internal protoxylem, px, in contrast to the tangentially elongated form which is assumed during its passage through the cortex (cf. figs. 165, [166]).

Fig. 167. Lepidodendron fuliginosum. (Binney Collection, Cambridge.)

Some sections of Lepidodendron fuliginosum in the Manchester University Collection are of special interest from the point of view of the method of secondary thickening. In the section reproduced in [fig. 168], B, the meristematic zone is seen to consist in part of radially elongated elements, m, with parallel cross-walls evidently of recent origin. The same tissue is shown also in [fig. 168], C, a, D, a, and in [fig. 169], A, a This band of meristem, which we may speak of as the cambium, occurs in the outer region of the meristematic zone immediately internal to the secretory zone, sc.

Fig. 168.

  1. Lepidodendron vasculare. (Botany School, Cambridge.)
  2. Lepidodendron fuliginosum. (From a specimen from Shore, Lancashire, in the Cambridge Botany School Collection).
  3. L. fuliginosum. (“Biseriate Halonia” of Weiss No. 257, Manchester University Museum.)
  4. L. fuliginosum. (Manchester Univ. Museum.)

The result of the activity of this cambium band is the production of secondary parenchyma and tracheal tissue. In [fig. 179], E, drawn from a portion of the section represented in [fig. 168], B, a projecting arm of primary xylem is seen at x; this is followed by 2–3 layers of parenchymatous cells, some of which have dark contents, and beyond this is seen a group of secondary elements, tr, cut across somewhat obliquely, which are evidently products of the cambial cells on the inner margin of the secretory zone, sc. The longitudinal section ([fig. 169], D) shows the cambial cells, a, next the secretory zone, sc, passing internally into crushed and imperfectly preserved elongated elements which are presumably miniature tracheae, and these are succeeded by older and more completely lignified xylem elements, x. In larger shoots the amount of secondary tissue is considerably greater; it may consist almost entirely of short-celled parenchyma ([fig. 168], C, from x to sc), or it may include a large proportion of radially disposed and vertically elongated tracheae ([fig. 168], D, x2, and [fig. 170], A, x2), or it may consist of parenchyma containing scattered groups of tracheae (fig. 169, A, x2)[326].

Fig. 169. Lepidodendron fuliginosum.

Fig. 169, A, is a diagrammatic sketch of the tissues—1 mm. wide—between the primary xylem, x, and the inner cortex. The primary xylem is succeeded by short parenchymatous cells followed by a zone of radially elongated elements passing occasionally into rows of narrow scalariform tracheae, some of which, owing to their sinuous longitudinal course ([fig. 171], C), are seen in oblique section, as at C, [fig. 169], A. At its outer edge this secondary tissue, x2, consisting of parenchyma and tracheae, passes into the cambial band ([fig. 169], B, a).

Fig. 170. Lepidodendron fuliginosum. (From sections in the Manchester Museum.)

The radial longitudinal section represented in [fig. 168], C, is taken from the fossil described by Weiss as a biseriate Halonia; it agrees sufficiently closely in structure with others referred to Lepidodendron fuliginosum to be classed as an example of this anatomical type. A complete transverse section of the stem measures 9 × 6·3 cm.; the breadth of the tissues between the edge of the primary xylem and the outer edge of the secretory zone is 2·5 mm. The middle cortical region, characterised by the sooty appearance, which led Williamson to choose the specific name fuliginosum, is traversed by the leaf-traces and is sharply differentiated from both the inner and outer cortex. The longitudinal section ([fig. 168], C) shows the outer edge of the primary xylem, x, abutting on a band of dark and small-celled parenchyma which passes into the broad zone of secondary tissue, m, the inner region of which consists of fairly thick-walled elements in radial series passing externally into the thin-walled cells of the cambial region, a, on the inner edge of the secretory zone, sc. This section shows also the interruption of the secretory zone by an outgoing leaf-trace, lt, the lower part of which, sc, is continued downwards into the secretory zone. The exit of a leaf-trace produces a gap in the secretory zone of the stem, but not in the xylem. If we applied the term phloem to the secretory zone—a course adopted by Prof. F. E. Weiss and some other authors, but which I do not propose to follow—we should speak of a phloem foliar-gap as a characteristic feature of a Lepidodendron shoot. This applies to other species of the genus as well as to L. fuliginosum.

Fig. 171. Lepidodendron fuliginosum. (From sections in the Manchester Museum.)

Fig. 171, A, shows more clearly the broad zone of secondary parenchyma with the thinner-walled cambial region, a; the latter is represented on a larger scale in [fig. 171], B. The section shown in [fig. 168], D, and in [fig. 170], A, affords an example of a stem in which the secondary tissue consists largely of narrow scalariform tracheae, x2; the primary stele has a diameter of 1 cm.; the secondary xylem, x2, forms a fairly broad zone of parenchyma and tracheal elements through which leaf-traces pass vertically, a fact of some interest in comparison with the horizontal course which they pursue through the medullary rays in the normal secondary wood of L. vasculare and L. Wünschianum. The secondary tracheae pass gradually into thin-walled cambial cells (a, [fig. 168], D; 170, A) with parallel tangential walls. Fig. 171, C, shows the sinuous course of the secondary tracheae as seen in longitudinal section, and a few small groups of parenchymatous cells, mr, which may be of the nature of medullary rays, enclosed between the winding scalariform tracheae.

Fig. 172. Lepidodendron fuliginosum. From a section (4 × 3·4 cm.) in the Williamson Collection, British Museum (No. 379), figured by Williamson, Phil. Trans. R. Soc. 1881, Pl. 52.

The secretory zone of Lepidodendron fuliginosum agrees essentially with that of other species; it usually presents the appearance shown in [fig. 168], B, sc; [fig. 169], B and C; fig. 170, B (longitudinal section); [fig. 171], D, sc. The comparatively large clear spaces which characterise this tissue, as seen in [fig. 168], B, appear to owe their origin to groups of small cells which gradually break down and give rise to spaces containing remnants of the disorganised elements, as in [fig. 171], D, and [fig. 169], B, b. The secretory tissue seen in [fig. 170], B, consists of large and small parenchymatous cells without any of the broad sacs or spaces such as are shown in [fig. 169], C.

Fig. 172 represents a diagrammatic sketch of a transverse section (4 × 3·4 cm. in diameter) of a young shoot from the Lower Coal-Measures of Lancashire figured by Williamson[327] in 1881 as Lepidodendron Harcourtii. It shows the features characteristic of L. fuliginosum and is of importance as affording an example of a shoot giving off a branch from the stele to supply a lateral axis of the type characteristic of Halonia. The exit of the branch-stele forms a gap in the main stele; a ramular gap as distinguished from a foliar gap. The outgoing vascular strand is at first crescentic, but becomes gradually converted into a solid stele. The primary xylem of the main stele (black in the figure) consists of a ring six tracheae in breadth; this is succeeded by a few layers of dark parenchymatous cells and a band of radially elongated elements, a, which abuts on the secretory zone. The middle lacunar cortex, c2, with Stigmaria rootlets, s, is fairly well preserved. In the outer cortex occur several leaf-traces, lt, accompanied by spaces originally occupied by the parichnos strand, p. A band of secondary cortex, consisting chiefly of phelloderm, is seen at pd. The prominent leaf-cushions, some of which show the parichnos, p, appear to be of the Lepidophloios type.

•••••

It remains to consider the external characters of Lepidodendroid shoots possessing the anatomical features represented by the comprehensive species Lepidodendron fuliginosum.

Certain sections exhibiting this type of structure were described by Binney in 1872 as Halonia regularis[328] on evidence supplied by Mr Dawes, who stated that they were cut from a specimen bearing Halonia tubercles. The section represented in [fig. 172] is no doubt from an Halonia axis. In 1890 Cash and Lomax[329] stated that they had in their possession a stem of the L. fuliginosum type with the external features of Lepidophloios; this identification has been confirmed by Kidston[330] and Weiss[331]. It is, however, equally clear that certain species with the elongated leaf-cushions of Lepidodendron must be included among examples of shoots with the anatomical characters of L. fuliginosum.

Fig. 173. Lepidodendron obovatum. (From a specimen lent by Dr D. H. Scott.)

Dr Scott[332] published in 1906 a short account of the structure of a specimen from the Lower Coal-Measures of Lancashire, the external features of which were identified by Kidston with those of Lepidodendron obovatum Sternb. Dr Scott generously allowed me to have drawings made from his specimen; these are reproduced in [fig. 173]. The form of the leaf-cushion is by no means perfect; there is a well-marked median ridge, and the small circular scar near the upper end of some of the cushions may represent the ligular cavity. At the base of the leaf-cushions a cortical meristem has produced a zone of secondary cortex; at c a second meristem is seen in the outer cortex: the dark dots in the cortex mark the positions of leaf-trace bundles. The inner cortex, d, is a more compact tissue surrounding the imperfectly preserved secretory zone. From the medullated stele a lateral branch, b, is being given off; its crescentic form becoming changed to circular as it passes nearer to the surface.

Fig. 174. Lepidodendron aculeatum. (Cambridge Botany School.)

Fig. 175. Lepidodendron aculeatum. (Cambridge Botany School.)

A type of Lepidodendron, L. Hickii, founded on anatomical characters by Mr Watson[333], is believed by him to possess leaf-cushions like those of L. obovatum; if this is so, it is interesting, as he points out, to find two distinct anatomical types associated with one species. Watson thinks it probable that the “species” L. obovatum includes at least two widely different species. This merely emphasizes the importance of correlating structure and external characters as far as available data permit.

Fig. 176. Lepidodendron aculeatum. (Cambridge Botany School.)

The specimen, of which part of the surface is shown in [fig. 174], is in all probability L. aculeatum Sternb. This was described by me in detail in The Annals of Botany (1906) as another example of the co-existence of the Lepidodendron fuliginosum type of anatomy with a true Lepidodendron. The locality of the specimen is not known. The leaf-cushions are 1·5 cm. long with tapered upper and lower ends; a ligular cavity may be recognised on some parts of the fossil, also faint indications of leaf-trace scars. The tubercles ([fig. 174], A–C, t) probably represent leaf-traces which the shrinkage of the superficial tissues has rendered visible in the lower part of their course. The circular scar, s (fig. B), on the partially decorticated surface is apparently a wound. The stele is sufficiently well preserved to justify its reference to L. fuliginosum. The irregularly crenulated edge of the primary xylem, x ([fig. 175]), is succeeded by a broad band of parenchyma (the meristematic zone), m, and beyond this are remnants of the secretory zone, s. The structure of the leaf-traces corresponds with that of other specimens of the type, but the much steeper course of these vascular strands, lt, lt′ ([fig. 176]), is a feature in which this example differs from most of those referred to L. fuliginosum. Such evidence as is available would seem to point to the absence of trustworthy criteria enabling us to separate, on anatomical grounds, Lepidophloios and Lepidodendron[334].

Fig. 177. Stigmaria radiculosa (Hick). (From sections in the Manchester University Collection.)

Stigmaria radiculosa (Hick).

We have no proof of the nature of the subterranean organs of Lepidodendron fuliginosum, though it is not improbable that the specimens described below may be correctly assigned by Weiss to that species. Prof. Weiss[335] has made an interesting contribution to our knowledge of a type first described by Hick[336] under the name Tylophora radiculosa, a designation which he afterwards altered to Xenophyton radiculosum[337] and for which we may now substitute Stigmaria radiculosa (Hick). Prof. Williamson expressed the opinion that Xenophyton exhibited considerable affinity with Stigmaria ficoides and Weiss’s further study of the species leads him to regard Hick’s plant as probably the Stigmarian organ of Lepidodendron fuliginosum. The diagrammatic transverse section represented in [fig. 177], A (4·5 cm. in diameter), shows an outer cortex of parenchyma, c3, consisting in part of radial rows of secondary tissue and of a band of compact parenchyma bounded by the wavy line a; at sc is a series of secretory strands exactly like those in a corresponding position in Lepidodendron fuliginosum and other species of the genus. The greater part of the organ is occupied by a lacunar and hyphal middle cortex identical in structure with that shown in [fig. 178], B, drawn from a rootlet. At d, [fig. 177], A, the middle cortex has been invaded by a narrow tongue of outer cortical tissue. The stele is characterised by a large pith filled with parenchyma; in Stigmaria ficoides[338] the general absence of pith-tissue has led to the inference that the stele was hollow. The xylem is represented by a ring of bundles separated by broad medullary rays; each bundle contains a few small, apparently primary, elements on its inner edge but is mainly composed of radial rows of secondary tracheae x2, [fig. 177], B. On the outer face of the secondary xylem occur a few smaller and thinner walled cells, c, having the appearance of meristematic tissue; from these additional tracheae were added to the xylem. This meristematic zone occurs, as in the stems of Lepidodendron, immediately internal to the secretory tissue, sc; at c1, [fig. 177], B, is seen the inner cortical tissue.

Fig. 178. Rootlet of Stigmaria. (From a section in the Manchester Collection.)

In surface-view a specimen figured by Hick[339] shows a number of circular scars agreeing in shape and arrangement with the rootlet scars of Stigmaria ficoides. At b in [fig. 177], A, the basal portion of a rootlet is shown in organic connexion with the outer cortex. The rootlet-bundles are given off from the stele as in other examples of Stigmaria; each bundle consists of a triangular strand of xylem with an endarch protoxylem at the narrow end accompanied by a portion of the secretory tissue as in the leaf-traces. As in Stigmaria ficoides the rootlets are attached to the outer cortex above a cushion of small cells. It is interesting to find that rootlet-bundles, as seen in tangential section of the main axis, are associated with a parichnos strand, but this is on the xylem side of the vascular strand, whereas in the case of leaf-traces the parichnos is on the other side of the bundle.

Fig. 178, A, represents a transverse section of a rootlet (6 mm. in diameter) associated with Stigmaria radiculosa and probably belonging to this species. The xylem strand x is composed of a group of tracheae with a single protoxylem strand, px, at the pointed end and with small metaxylem elements at the broad end next the space originally occupied by the so-called phloem. A parenchymatous sheath, c′, surrounds the bundle, and beyond this is the broad middle cortex, a small portion of which is shown on a larger scale in [fig. 178], B; as Weiss points out, some of the outermost cells of the lacunar cortex (m) are clearly in a state of meristematic activity.

The preservation of the middle cortex and the small quantity of secondary xylem are characters which this Stigmaria shares with Lepidodendron fuliginosum, and although decisive evidence is still to seek, we may express the opinion that Weiss’s surmise of a connexion between Stigmaria radiculosa and Lepidodendron fuliginosum is probably correct.

5. Lepidodendron Harcourtii. Fig. 179, A–D.

In 1831 Mr Witham[340] published an anatomical description of a fragment of a Lepidodendron which he named Lepidodendron Harcourtii after Mr C. G. V. Vernon Harcourt from whom the specimen was originally obtained. The fossil was found in rocks belonging to the Calciferous series in Northumberland. Witham reproduced the account of this species in his classic work on Fossil Vegetables[341], and Lindley and Hutton[342], who examined Mr Harcourt’s material, published a description of it in their Fossil Flora in which they expressed the view that Lepidodendron is intermediate between Conifers and Lycopods. Adolphe Brongniart[343] included in his memoir on Sigillaria elegans an account of Witham’s species based on material presented to the Paris Museum by Mr Hutton and Robert Brown. Dr Kidston[344] has shown that the actual transverse section figured by Witham is now in the York Museum; a piece of stem in the same Museum, which is not the specimen from which Witham’s section was cut, supplied the transverse section figured by Brongniart. The figures given by Lindley and Hutton do not appear to have been made from the York specimens. In 1887 Williamson[345] published a note in which he pointed out that some of the specimens described by him as L. Harcourtii should be transferred to a distinct species, which he named L. fuliginosum. Subsequently in 1893 he gave a fuller account of Witham’s species; it has, however, been shown by Dr Kidston and by Mr Watson[346] that certain specimens identified by Williamson as L. Harcourtii differ sufficiently from that type to be placed in another species, for which Watson proposes the name L. Hickii.

A paper on L. Harcourtii published by Bertrand[347] in 1891 extends our knowledge of this type in regard to several anatomical details. It was recognised by Williamson that the absence of secondary wood in shoots possessing the anatomical characters of L. Harcourtii is a feature to which no great importance should be attached. It is possible that the large stems from the Isle of Arran described by Williamson[348] as Lepidodendron Wünschianum, in which the secondary wood is well developed, may be specifically identical with the smaller specimens from Northumberland and elsewhere which are recognised as examples of Witham’s type.

The diagrammatic sketch shown in [fig. 179], A, was made from a section figured by Williamson in 1893[349]; it has a diameter of 9 × 8·5 cm. The stele is of the medullated type like that of L. Wünschianum, and the outer edge of the primary xylem is characterised by sharp and prominent projecting ridges similar to those of L. fuliginosum but rather more prominent. Parenchymatous cells succeed the xylem, as in other species, but in this case there is no indication of meristematic activity; beyond this region occur occasional patches of a partially destroyed secretory zone. Remains of a lacunar tissue are seen in the middle cortical region; also numerous leaf-traces, lt, consisting of a tangentially elongated xylem strand accompanied by a strand of secretory zone tissue enclosed in a sheath of delicate parenchyma. In the inner part of the outer cortex, c3, the leaf-traces lie in a space originally occupied by the parichnos; in the outer portion of the same region a band of secondary cortex, pd, has been formed; immediately internal to this occur numerous patches of secretory tissue, represented by small dots in the drawing close to pd; one is shown on a larger scale in fig. B.

The position of the phellogen is seen at a; external to this are radial rows of rather large cells with dark contents.

Fig. 179.

Fig. 179, C, x, shows the characteristic form of the primary xylem edge, beyond which are seen oval or circular leaf-traces with a mesarch protoxylem, lt, px. It is possible that this specimen may not be specifically identical with Witham’s species, but it represents a very similar if not identical type; it may on the other hand be referable to L. fuliginosum. The importance of the specimen, apart from its precise specific position, is that it serves to illustrate the general appearance of the xylem surface met with in both species, L. Harcourtii and L. fuliginosum. A tangential longitudinal section, taken through the line ab in fig. C, is represented in [fig. 179], D. The xylem of the leaf-traces lt, consisting chiefly of scalariform tracheae, alternates with patches of crushed and delicate parenchyma which immediately abut on the primary xylem; at p, p, the section passes through some of the projecting arms of the xylem cylinder; at m is seen a patch of meristematic zone tissue. This section together with the similar section of Lepidodendron vasculare described on a previous page demonstrates that the projecting ridges of the primary xylem form apparently vertical bands: they are not characterised by a lattice-work arrangement as described by Bertrand and by other authors who have accepted his conclusions. If a reticulum of intersecting ridges were present on the face of the xylem cylinder its existence would be revealed by such a section as that represented in [fig. 179], D.

Fig. 180. Lepidodendron Wünschianum. From Arran. (⅕ nat. size.) (Sedgwick Museum, Cambridge.)

6. Lepidodendron Wünschianum (Williamson). [Figs. 180–184].

Reference was made in Volume I. to the occurrence of large stems of a Lepidodendron in volcanic beds of Calciferous sandstone age in the island of Arran[350]. These were discovered and briefly described by Mr Wünsch in 1867[351] and afterwards named by Carruthers Lomatophloyos Wünschianus[352]. Mr Carruthers visited the locality and published an account of the peculiar method of preservation of the plant remains[353]. It is, however, to Williamson[354] that we owe the more complete description of these Arran stems. Portions of large stems from the Arran beds are preserved in the British Museum, the Sedgwick Museum, Cambridge, and in the Manchester Museum. The section of one of these is shown in [fig. 180]; an outer shell of bark encloses a mass of volcanic ash in which are embedded several woody cylinders originally described as “internal piths[355],” and by Carruthers as young stems produced from spores which had germinated in the hollow trunk of a large tree. The true interpretation was supplied by Williamson who showed that a stem of the dimensions of that represented by the outer cortex, e, [fig. 180], must have possessed a single stele of the size of those seen in the interior of the hollow trunk. The additional woody cylinders, or steles, were derived from other stems, and carried, probably by water, into the partially decayed trunk. In addition to large Lepidodendron stems Williamson described smaller shoots as well as an Halonial branch and made brief reference to some cones described by Binney[356] in 1871 from the same locality.

The following account of Lepidodendron Wünschianum is based on an exceptionally fine specimen discovered by Mr T. Kerr of Edinburgh in Calciferous sandstone volcanic ashes at Dalmeny in Linlithgowshire. The material from this locality described by Mr Hill and myself[357] was generously placed in my hands by Dr Kidston of Stirling. Fig. 181, A, shows a transverse section, 33 cm. in diameter, consisting of a shell of outer cortical tissue enclosing a core of light-coloured volcanic ash; on the decay of the more delicate middle cortex the cylindrical stele dropped to one side of the hollow trunk. The stele, [fig. 182], has a diameter of 6·5 cm.; the centre is occupied by concentric layers of silica, s, surrounded externally by the remains of a parenchymatous pith, p, made up of isodiametric and sinuous hypha-like elements like those in the middle cortex of Lepidodendron shoots. On the inner edge of the primary xylem, x′, occur several isodiametric tracheae with fine scalariform and reticulate thickening bands like those in the central region of the stele of Lepidodendron vasculare: it is probable that these elements are vestiges of conducting tissue which in ancestral forms formed a solid and not a medullated stele.

Fig. 181. Lepidodendron Wünschianum. Calciferous Sandstone, Dalmeny. (A, Sedgwick Museum, Cambridge. B–F, Botany School, Cambridge.)

Fig. 182. Lepidodendron Wünschianum. The stele of the stem shown in [fig. 181], A. (Cambridge Botany School.)

The primary xylem is limited externally by an unequally fluted surface with exarch protoxylem elements; it is, however, noteworthy that there is not always a very clearly defined difference between the small protoxylem and the large centripetally developed tracheae. Immediately beyond the primary xylem occur numerous thin-walled parenchymatous cells with spiral and reticulate pitting; beyond these is the broad zone of secondary xylem, x2, composed of scalariform tracheae and numerous medullary rays consisting of one, two, or several rows of radially elongated elements with spiral and reticulate pitting. In tangential sections the rays are seen to vary considerably in size, some being made up of a single row of cells while others are longer and broader; through the latter leaf-traces pass horizontally. Portions of medullary rays are seen at mr in [fig. 181], C and E.

The leaf-traces given off from projecting ridges on the outer edge of the primary xylem pass upwards for a short distance and then bend outwards through a broad medullary ray; on reaching the limit of the secondary xylem they again bend sharply upwards, appearing in transverse section at lt [fig. 181], B. Each leaf-trace consists at first of long tracheae accompanied by numerous thin-walled spiral and reticulate parenchymatous cells derived from the tissue in contact with the outer edge of the primary wood. Fig. 181, B, shows a leaf-trace near the edge of the secondary xylem; it consists of a group of primary tracheae, with narrower protoxylem elements, px, near the outer margin, almost completely enclosed by radially disposed series of smaller and more delicate tracheae. These secondary elements of the leaf-trace are apparently added during its passage through the medullary ray, but additions are also made to this tissue by the meristematic zone, m, [fig. 181], B and E. In contact with the outermost tracheae of normal size at the edge of the secondary xylem there are some smaller lignified elements, as at a, [fig. 181], E, and at T, [fig. 183]; this juxtaposition of large and small tracheae has been referred to in the description of L. vasculare.

Prof. Williamson[358], in his account of the Arran specimens of this species, expressed the opinion that the trees probably perished “in consequence of the mephitic vapours which filled the atmosphere”; it maybe that in the striking difference in the diameter of the conducting elements on the margin of the wood we have evidence of approaching death.

Beyond the most recently formed tracheae we have a band of delicate parenchymatous cells (m, [figs. B and E, 181]; C, figs. 183, 184) forming the meristematic zone[359]. The longitudinal section represented in [fig. 184] shows some recently formed narrow tracheae, T, and beyond these the meristematic zone composed of thin-walled short cells, C, arranged in horizontal rows. It is this small-celled tissue to which the name phloem has been applied by some authors[360], a term which seems to me to be misleading and inappropriate. In passing through this zone of dividing cells the leaf-traces become surrounded by an arc of meristem from which elements are added to the radially placed rows of secondary tracheae. Beyond the meristematic region portions of the secretory zone are preserved, consisting of large sacs or spaces and small dark cells as seen in figs. [181], B, E, sc, F; [183], [184]. This tissue has the same structure as in L. vasculare and in L. fuliginosum: it is a striking fact that there are no indications of any additions to the secretory zone even in stems with such a large amount of secondary xylem as in the Dalmeny specimen ([fig. 182], x2). If the secretory zone were of the nature of phloem we should expect to see signs of additions made to it in the course of growth. In this connexion it is worth mentioning that in the recent fern Botrychium (Ophioglossaceae) secondary xylem is formed in the stem, but apparently no additions are made to the phloem. The structure of the secretory zone tissue as seen in the longitudinal section [fig. 184], S, is also a serious difficulty in the way of accepting the designation phloem as employed by Scott and Weiss. Between the secretory zone and the outer cortical region, no tissues have been preserved. The shell of bark consists chiefly of radial rows of elongated cells with rather thick walls characterised by the occurrence of small intercellular spaces and by tangentially placed bands of secretory cells and sacs ([fig. 181], D, s). Immediately internal to the secondary cortex or phelloderm occur groups of secretory tissue as shown in the section of L. Harcourtii ([fig. 179], B).

Fig. 183. Lepidodendron Wünschianum. From the specimen shown in [fig. 181], S, secretory zone; C, meristem; T, immature tracheae.

Fig. 184. Lepidodendron Wünschianum. Longitudinal section of the specimen represented in transverse section in [fig. 183].

The large tree shown in transverse section in [fig. 181], A, has lost its leaf-cushions; the bark, as seen in the lower part of the photograph, presents a fissured appearance like that with which we are familiar on an old Oak or Elm stem. A radial longitudinal section through the phelloderm revealed the existence of a crushed leaf-trace passing outwards in an approximately horizontal course accompanied by a strand of parenchymatous tissue[361] having the characteristic structure of a parichnos. It is probable that the surface of this partially decorticated stem differed in appearance from that of an old Sigillaria (cf. [fig. 198]) in the much smaller and less conspicuous parichnos strands.

In addition to the large stems of L. Wünschianum from Arran and Dalmeny numerous examples of smaller axes from the former locality are represented in the Williamson collection (British Museum). Some of the twigs are characterised by a solid stele (protostele) giving off numerous leaf-traces accompanied by short spirally thickened tracheids like those which occur at the outer edge of the primary xylem in the larger stem: these extend into the leaf where they are arranged round the vascular bundle like the transfusion tracheids[362] in many recent conifers. The surface of these smaller shoots bears large leaf-cushions which are seen in longitudinal section to have the form characteristic of Lepidophloios. It is worthy of note that a section of a bifurcating axis of this species from the Calciferous Sandstone of Craigleith (British Museum Collection[363]), although its diameter is 19 × 14 cm., shows no signs of secondary wood. This late appearance of secondary xylem and other anatomical features suggest the possibility of the specific identity of L. Wünschianum and L. Harcourtii[364].

In 1871 Binney[365] described a specimen of a heterosporous cone, Lepidostrobus Wünschianus, from Arran exhibiting the ordinary features of lepidodendroid strobili; this was probably borne by Lepidodendron Wünschianum.

7. Lepidodendron macrophyllum (Williamson). Fig. 186, C.

The diagrammatic sketch reproduced in [fig. 186], C, was made from the transverse section of a small twig, slightly less than 2 cm. in its longest diameter, originally figured by Williamson[366] in 1872. Earlier in the same year Carruthers[367] published a short account of the same form based on specimens collected by Mr Butterworth from the Coal-Measures of Lancashire near Oldham, but both authors refrained from instituting a new specific name. In a later publication Williamson spoke of the type as Lepidodendron macrophyllum[368]. Williamson’s species has nothing to do with Lycopodites macrophyllus of Goldenberg[369]. The most striking feature of this rare form is the large size of the leaf-cushions, which are of the Lepidophloios type, in proportion to the diameter of the shoot. The stele consists of a ring of xylem, all of which is primary in the sections so far described, enclosing a parenchymatous pith: a Stigmarian rootlet is shown at s.

8. Lepidodendron Veltheimianum Sternb. (General account). Figs. [157], [185], [186], A, B.

The above list may serve to call attention to a few synonyms[370] of this plant, and to a selection of sources from which full information may be obtained as to the history of our knowledge of this characteristic and widely spread Lower Carboniferous type.

Lepidodendron Veltheimianum is represented by casts of stems, the largest of which hitherto described reaches a length of 5·22 metres with a maximum diameter of 63 cm.; this specimen, figured by Stur[371], consists of a tapered main axis giving off smaller lateral shoots, some of which exhibit dichotomous branching. Fig. 185, C and D, represent the external features of a well-preserved cast and impression respectively. Oblique rows of prominent cushions wind round the surface of the stem and branches: each cushion is prolonged upwards and downwards in the form of a narrow ridge with sloping sides which connects adjacent cushions by an ogee curve. At the upper limit of the broader kite-shaped portion of the cushion the ligular pit forms a conspicuous feature; immediately below this is the leaf-scar with its three small scars,—the lateral parichnos strands and the central leaf-trace. The two oval areas shown in [fig. 185], D, just below the lower edge of the leaf-scars, represent the parichnos arms which impinge on the surface of the cushions on their way to the leaves, as explained on a previous page. It is possible that these areas were visible on the living stem as strands of loose parenchyma comparable with the lenticel-like pits on the stipules of Angiopteris[372] and the leaf-bases of Cyatheaceous ferns, or it may be that their prominence in the specimen before us is the result of the decay of a thin layer of superficial cortex which hid them on the living tree. Fig. 185, B, illustrates the appearance of a stem in a partially decorticated condition (Bergeria state). A further degree of decortication is seen in [fig. 185], A, which represents the Knorria condition.

Fig. 185. Lepidodendron Veltheimianum. From specimens in Dr Kidston’s Collection. (Approximately nat. size.)

[Fig. 157] shows a Ulodendron axis of this species; in the lower part the specimen illustrates the partial obliteration of the surface features as the result of the splitting of the outer bark consequent on growth in thickness of the tree. By an extension of the cracks, shown in an early stage in [fig. 157], the leaf-cushions would be entirely destroyed and the surface of the bark would be characterised by longitudinal fissures simulating the vertical grooves and ridges of a Sigillarian stem. The large stumps of trees shown in the frontispiece to Volume I. are probably, as Kidston[373] suggests, trunks of L. Veltheimianum in which the leaf-cushions have been replaced by irregular longitudinal fissures. In old stems of Sigillaria the enlarged parichnos areas constitute a characteristic feature (p. 205), but it does not follow that the absence of large parichnos scars is a distinguishing feature of all Lepidodendra.

In this species, as in others, the form of the leaf-cushion exhibits a considerable range of variation dependent on the thickness of the shoot; the contiguous cushions of young branches become stretched apart as the result of increasing girth of the whole organ, and casts of still older branches may exhibit very different surface-features[374]. The leaves as seen on impressions of slender branches are comparatively short, reaching a length of 1–2 cm. It is important to notice that leafy twigs of this species may bear terminal cones[375] resembling in form those of Picea excelsa and other recent conifers, though differing essentially in their morphological features.

The fossil stumps of trees represented in the frontispiece to Volume I. bear horizontally spreading and dichotomously branched root-like organs having the characters of Stigmaria ficoides[376]. Geinitz has suggested that Stigmaria inaequalis Göpp. may be the underground portion of Lepidodendron Veltheimianum.

It is unfortunately seldom possible to connect petrified Lepidodendron cones with particular species of the genus based on purely vegetative characters, but it is practically certain that we are justified in recognising certain strobili described by Williamson[377] from the Calciferous Sandstone series of Burntisland on the Firth of Forth as those of Lepidodendron Veltheimianum. Williamson believed that the cone which he described belonged to the plant with shoots characterised by the anatomical features of his species Lepidodendron brevifolium (= L. Veltheimianum), a conclusion which is confirmed by Kidston[378]. The cone of L. Veltheimianum, which reached a diameter of at least 1 cm. and a length of 4 cm., agrees in essentials with other species of Lepidostrobus; the axis has a single medullated stele of the same general type as that of the vegetative shoots of Lepidodendron fuliginosum and L. Harcourtii. The sporophylls are described by Williamson as spirally disposed, and Scott notices that in some specimens they are arranged in alternate whorls; as in recent Lycopods both forms of phyllotaxis may occur in the same species. The heterosporous nature of this strobilus, to which Scott first applied the name Lepidostrobus Veltheimianus, is clearly demonstrated by the two longitudinal sections contributed by Mr Carruthers and figured by Williamson in 1893[379].

Each sporophyll, attached almost at right angles to the cone-axis, bears a radially elongated sporangium seated on the median line of its upper face; its margins are laterally expanded as a thin lamina; from the middle of the lower face a narrow keel extends downwards between two sporangia belonging to a lower series. From the base of a sporangium a mass of sterile tissue penetrates into the spore-producing region as in the large sporangia of Isoetes (cf. [fig. 191], H, a, and [fig. 133], H). The distal and free portion of the sporophylls is bent upwards as a protecting bract. Some of the sporangia in the upper part of the cone produced numerous microspores, while 8–16 megaspores occur in the lower sporangia. The megaspores, having a mean diameter of 0·8 mm. “quite 40 times the size of the microspores[380],” are characterised by tubular capitate appendages, and by a conspicuous three-lobed projection ([fig. 191], E)[381] which, as Scott suggests, may represent the outer spore-wall which has split as the result of germination. It is not improbable, as shown in [fig. 191], I, that this cap was present before germination. The megaspores represented in [fig. 191], I, illustrate their characteristic form as seen in a section of a megasporangium, Sm; the open beak-like portion of the larger spore is probably the apical region which has split along the three-rayed lines. These lines form a characteristic feature of both recent and extinct spores and denote their origin in tetrads. The spore shown in [fig. 191], E[382], illustrates the external features. The apical region of the prothallus of a megaspore of Lepidodendron Veltheimianum described by Mr Gordon[383] consists of smaller cells than those occupying the greater part of the spore-cavity, a differentiation which he compares with that of the prothallus of Selaginella.

Fig. 186.

There can be little doubt that the petrified shoots described by Williamson[384] from the Calciferous Sandstone beds of Burntisland as Lepidodendron brevifolium are identical with specimens possessing the external features of L. Veltheimianum. In 1872 Dawson expressed the opinion that Williamson’s species should be referred to L. Veltheimianum, and evidence subsequently obtained confirms this view. The stele of this species is of the medullated type, differing from that of L. fuliginosum and L. Harcourtii in the absence of prominent ridges on the external surface of the primary xylem, and from L. vasculare in the possession of a parenchymatous pith. In younger twigs the cortex consists of fairly homogeneous tissue, but in older branches there is a greater distinction between a delicate middle cortex and a stronger outer cortex. Fig. 186, A, represents a stem in which the vascular cylinder is composed of a primary xylem ring, x, 1·5 mm. broad, succeeded by a zone of secondary wood 1·2 cm. in breadth. The junction between the primary and secondary xylem is shown on a larger scale in [fig. 186], B. The tissues abutting on the secondary xylem have not been preserved; the outer cortex, which consists chiefly of secondary elements, is divided superficially into unequal ridges corresponding to the leaf-cushions which have been more or less obliterated as the result of growth in thickness of the stem.

9. Lepidodendron Pedroanum (Carruthers).

In 1869 Mr Carruthers described some specimens of vegetative stems and isolated sporangia, collected by Mr Plant in Brazil, as Flemingites Pedroanus[385]. From a more recent account published by Zeiller[386] it is clear that Carruthers’ species is a true Lepidodendron; an examination of the type-specimens in the British Museum confirms this determination. The contiguous leaf-cushions have rounded angles similar in form to those of Lepidodendron Veltheimianum and L. dichotomum, but it is not unlikely that the Brazilian plant is specifically distinct from European species. A figure of one of the specimens on which Carruthers founded the species is given by Arber[387] in his Glossopteris Flora. The Brazilian plant is chiefly interesting as affording proof of the existence of Lepidodendron in the southern hemisphere; the species has also been recognised in South Africa from material collected by Mr Leslie at Vereeniging[388].

As Zeiller[389] has suggested, it is not improbable that the fossils described by Renault[390] from Brazil as Lycopodiopsis Derbyi may be the petrified stems of Lepidodendron Pedroanum. The structure of the central cylinder of Renault’s species is of the type represented by L. Harcourtii; the xylem forms a continuous ring and does not consist of separate strands of tracheae as Renault believed.

10. Lepidodendron australe (M’Coy). [Figs. 187], A–C.

Specimens described under this name are interesting rather on account of their extended geographical range and geological antiquity than on botanical grounds. The drawings reproduced in [fig. 187] illustrate the characteristic appearance of this Lower Carboniferous and Upper Devonian type, as represented by a specimen recently described[391] from the Lower Karroo (Dwyka) series, which is probably of Carboniferous age, near Orange River Station, South Africa. The surface is divided into polygonal or rhomboidal areas (figs. A and B) 8–9 mm. long and 7–8 mm. broad, arranged in regular series and representing leaf-scars, comparable with those of Sigillaria Brardi and other species, or possibly partially decorticated leaf-cushions. A short distance below the apex of each area there is a more or less circular prominence or depression ([fig. 187], B) and on a few of the areas there are indications of a groove (fig. A, g) extending from the raised scar to the pointed base, as at g, g.

Fig. 187. Lepidodendron australe. Fig. A, nat. size.

In examining the graphitic layer on the surface of the South African specimen shown in [fig. 187], A, use was made of a method recently described by Professor Nathorst[392]. A few drops of collodion were placed on the surface, and after a short interval the film was removed and mounted on a slide. The addition of a stain facilitated the microscopic examination and the drawing of the collodion film. The cell-outlines ([fig. 187], C) on the surface of the polygonal areas may be those of the epidermis, but they were more probably formed by a subepidermal tissue; the scar, which interrupts the continuity of the flat surface, may mark the position of a leaf-base, or, assuming a partial decortication to have occurred prior to fossilisation, it may represent a gap in the cortical tissue caused by the decay of delicate tissue which surrounded the vascular bundle of each leaf in its course through the cortex of the stem. If the impression were that of the actual surface of a Lepidodendron or a Sigillaria, we should expect to find traces of the parichnos appearing on the leaf-scar as two small scars, one on each side of the leaf-bundle. In specimens from Vereeniging described in 1897[393] as Sigillaria Brardi, which bear a superficial resemblance to that shown in fig. A, the parichnos is clearly shown. On the other hand, an impression of a partially decorticated Lepidodendroid stem need not necessarily show the parichnos as a distinct feature: owing to its close association with the leaf-trace in the outer cortex, before its separation in the form of two diverging arms, it would not appear as a distinct gap apart from that representing the leaf-bundle. The absence of the parichnos may be regarded as a point in favour of the view that the impression is that of a partially decorticated stem. Similarly, the absence of any demarcation between a leaf-cushion and a true leaf-scar such as characterises the stems of Lepidodendra and many Sigillariae is also favourable to the same interpretation.

In 1872 Mr Carruthers[394] described some fossils from Queensland, some of which appear to be identical with that shown in [fig. 187] under the name Lepidodendron nothum, Unger[395], a species founded on Upper Devonian specimens from Thuringia. The Queensland plant is probably identical with Dawson’s Canadian species, Leptophloeum rhombicum[396]. In 1874 M’Coy[397] instituted the name Lepidodendron australe for some Lower Carboniferous specimens from Victoria, Australia: these are in all probability identical with the Queensland fossils referred by Carruthers to Unger’s species, but as the identity of the German and Australian plants is very doubtful[398] it is better to adopt M’Coy’s specific designation.

Krasser[399] has described a similar, but probably not specifically identical, type from China; from Devonian rocks of Spitzbergen Nathorst[400] has figured, under the name Bergeria, an example of this form of stem, and Szajnocha[401] has described other specimens from Lower Carboniferous strata in the Argentine.

Lepidodendron australe has been recorded from several Australian localities[402] from strata below those containing the genus Glossopteris and other members of the Glossopteris, or, as it has recently been re-christened, the Gangamopteris[403] Flora.

viii. Fertile shoots of Lepidodendron.

A. Lepidostrobus.

The generic name Lepidostrobus was first used by Brongniart[404] for the cones of Lepidodendron, the type-species of the genus being Lepidostrobus ornatus, the designation given by the author of the genus to a Lepidostrobus previously figured by Parkinson[405] in his Organic Remains of a Former World. The generic name Flemingites proposed by Carruthers[406] in 1865, under a misapprehension as to the nature of spores which he identified as sporangia, was applied to specimens of true Lepidostrobi. Brongniart also instituted the generic name Lepidophyllum for detached leaves of Lepidodendron, both vegetative and fertile; the specimen figured by him in 1822 as Filicites (Glossopteris) dubius[407], and which was afterwards made the type-species of the genus, was recognised as being a portion of the lanceolate limb of a large single-veined sporophyll belonging to a species of Lepidostrobus.

In an unusually large Lepidophyllum, or detached sporophyll of Lepidostrobus, in the Manchester University Museum, the free laminar portion reaches a length of 8 cm.

It is not uncommon to find Lepidodendron preserved in the form of a shell of outer cortex, which has become separated along the phellogen from the rest of the stem; as the result of compression the cylinder of bark may assume the appearance of a flattened stem covered with leaf-cushions. A specimen preserved in this way was described by E. Weiss as a cone of Lomatophloios macrolepidotus Gold., and is quoted by Solms-Laubach and other authors[408] as an example of an unusually large Lepidostrobus. An examination of the type-specimen in the Bergakademie of Berlin convinced me that Weiss had mistaken the partially destroyed leaf-cushions for sporophylls, and Stigmarian rootlets, which had invaded the empty space, for sporangia[409].

In external appearance some species of Lepidostrobus bear a superficial resemblance to the cone of a Spruce Fir (Picea excelsa), but the surface of a lycopodiaceous strobilus is usually covered by the overlapping and upturned laminae which terminate the more or less horizontal sporangium-bearing portion of the sporophyll.

Fig. 188 affords a good example of a long and narrow Lepidostrobus. This specimen from the Middle Coal-Measures of Lancashire has a length of 23 cm.; like other Lepidostrobi it is borne at the tip of a slender shoot. The fossil is sufficiently well preserved to show the characteristic radially elongated form of the large sporangia and the long and upturned distal portions of the sporophylls.

We may briefly describe Lepidostrobus as follows:—Cylindrical strobili consisting of an axis containing a single cylindrical stele which agrees generally with that of the vegetative shoots of L. Harcourtii and other species. The amount of parenchymatous pith varies in different forms; in some the primary xylem is almost solid. The middle cortical region, which has usually been destroyed before fossilisation, possesses the loose lacunar structure characteristic of this region in the vegetative branches. The thicker walled outer cortex is continued at the periphery into crowded, usually spirally disposed sporophylls, each of which consists of a more or less horizontal pedicel, which may be characterised by a keel-like median ridge on its lower surface, while to the central region of the upper face is attached a large radially elongated sporangium. One of the chief differences between a Lepidodendron cone and those of the recent genus Lycopodium is the greater radial elongation of the sporangia in the former. Some species of Lepidostrobus may have been homosporous; some are known to be heterosporous. In the latter the megasporangia borne on the lower sporophylls usually contain several megaspores as in Isoetes (cf. [fig. 133], E). Beyond the distal end of the sporangium the sporophyll becomes broader in a horizontal plane and is bent upwards as a lanceolate limb; it may also be prolonged a short distance downwards as a bluntly triangular expansion.

Fig. 188. Lepidostrobus. Middle Coal-Measures, Bardsley, Lancashire. From a specimen in the Manchester Museum. (½ nat. size.)

There can be little doubt that the Palaeozoic Lepidodendra, like Lycopodium cernuum ([fig. 123]) and other recent Lycopods, usually bore their cones at the tips of slender shoots. The fertile shoot of Lepidophloios scoticus shown in [fig. 160], B, affords one of several instances supporting this statement; similar examples are figured by Brongniart[410], Morris[411], and by more recent writers. The apparently sessile cone figured by Williamson[412] from a specimen in the Manchester Museum is certainly not in situ, but is accidentally associated with the stem.

The general absence of secondary wood in the steles of Lepidostrobi is, as Dr Kidston[413] points out, consistent with the view that the cones were shed on maturity and that fertilisation probably took place on the ground, or perhaps on the surface of the water where the slender hairs of the megaspores ([fig. 191], F, I) may have served to catch the microspores.

Fig. 189. Lepidostrobus. Section through the apical region of a cone above the axis. (Manchester University Collection.)

Fig. 189 is an accurate representation of a transverse section, 6 mm. in diameter, of what is no doubt the apical portion of a Lepidostrobus from the Coal-Measures of Shore, Lancashire. The section cuts across the upturned free laminae above the level of the apex of the cone-axis. Each lamina contains a small vascular bundle composed of a few tracheae and some thin-walled cells surrounded by delicate mesophyll tissue. Immediately in front of the distal end of a sporangium a small ligule is borne on the upper face of the sporophyll ([fig. 191], A, B, l) occupying the same position as in Selaginella (cf. [fig. 131], F). Strands of vascular tissue pass in a steeply ascending course from the xylem to the pedicels of sporophylls, finally curving upwards and ending in the upper limb. Each vascular bundle consists of a strand of xylem, apparently of mesarch structure, accompanied by a few layers of parenchyma on its outer face and by a group of cambiform elements, the whole being enclosed in a sheath of parenchyma continuous with the inner cortex of the cone axis. The vascular bundle is accompanied by a parichnos in the outer cortex and in the sporophyll.

Reference has already been made to the belief on the part of some palaeobotanists that the large scars of Ulodendron represent attachment-surfaces of sessile cones, and reasons have been given against the acceptance of this view.

There is considerable range in the size of Lepidostrobi. An incomplete specimen, 33 cm. long and 6 cm. broad, which may have been 50 cm. in length, is described by Renault and Zeiller[414] from the Commentry Coal-field. The larger cones afford a striking demonstration of the enormous spore-output of some species of Lepidodendron.

Among the earliest accounts of the anatomy of Lepidostrobus are those by Hooker[415] and Binney[416]. One of the specimens described by the former author ([fig. 190]) affords an interesting example of an unusual manner of fossilisation; a hollow stem or Lepidodendron is filled with sedimentary material containing several pieces of Lepidostrobi in an approximately vertical position.

Fig. 190. Lepidodendron stem with Lepidostrobi. (After Hooker.)

  1. Side-view showing leaf-cushions on the left-hand side and the Knorria condition on the right.
  2. View of transverse section; s, sections of Lepidostrobi.

The fact that Lepidostrobi usually occur as isolated specimens renders it impossible in most cases to refer them to particular species of Lepidodendron. Neither external features nor anatomical characters afford satisfactory criteria by which to correlate vegetative and fertile shoots; in some measure this is due to the imperfection of our knowledge as regards the range of structure within the limits of species; it is also due to lack of information as to the extent to which the transition from sterile to fertile portions of a shoot is accompanied by anatomical differences. Prof. Williamson wrote: “I have for many years endeavoured to discover some specific characters by which different Lepidostrobi can be distinguished and identified, but thus far my efforts have been unsuccessful[417].” In a few cases, such as those mentioned in the description of Lepidodendron Veltheimianum and L. Wünschianum, it has been possible to correlate cones and vegetative shoots.

The most complete account we possess of the anatomy of Lepidodendron cones is that by Mr Maslen[418], who first demonstrated the occurrence of a ligule on the sporophylls, and thus supplied a missing piece of evidence in support of the generally accepted view as to the homology of the sporangium-bearing members and foliage leaves.

i. Lepidostrobus variabilis (Lindley and Hutton).

Under this specific name are included strobili from Upper Carboniferous rocks which, in spite of minor differences, may be considered as one type. The cylindrical cones vary considerably in size, some reaching a length of 50 cm. or more. The sporophylls are attached by a pedicel, 4–8 mm. long, at right angles to the axis, while the distal portion forms an oval lanceolate limb 10–20 mm. in length. The sporangia are 4–8 mm. long.

The branched example figured by Lindley and Hutton[419] as a variety (L. ornatus var. didymus) illustrates a phenomenon not uncommon in both Palaeozoic and recent lycopodiaceous strobili.

Fig. 191. Lepidostrobus.

ii. Lepidostrobus oldhamius Williamson[420]. Fig. 191, A–D.

Williamson[421] instituted this term for strobili previously described by Binney[422], without adequate evidence, as the cones of Lepidodendron Harcourtii. In shape and in the main morphological features this type resembles L. variabilis, which is however known only in the form of casts and impressions. A cone of L. oldhamius, 2–3 cm. in diameter, possesses a medullated stele consisting of a ring of primary xylem (fig. 191, D, x) with exarch protoxylem and no secondary elements. Maslen found several short tracheae at the periphery of the xylem and states that these led him to compare the cone with the vegetative shoots of Lepidodendron vasculare, but the common occurrence of such elements in different types of shoot renders them of little or no specific value. The inner cortex is like that of vegetative shoots of Lepidodendron and the middle cortex, which was no doubt of the type described in Lepidostrobus Brownii, is represented by a gap in the sections, beyond which is the stronger outer cortex ([fig. 191], D) passing into the horizontal pedicels of the sporophylls. The section of the axis reproduced in [fig. 191], D, was figured by Binney[423] as Lepidodendron vasculare. The leaf-traces, several of which are seen in the middle cortical region in fig. D, lt, consist of a strand of scalariform tracheae, with a mesarch protoxylem, succeeded by a few parenchymatous cells; beyond these there is usually a small gap which was originally occupied by a strand of thin-walled cells. It is important to note that in one sporophyll-trace figured by Maslen[424] there is a strand of thin-walled elongated elements abutting on the xylem, which he describes as phloem. This tissue is certainly more like true phloem than any which has hitherto been described in the leaf-traces of vegetative shoots. The state of preservation is not, however, sufficiently good to enable us to recognise undoubted phloem features.

In such cones as I have examined no tissue has been seen which shows the histological features characteristic of the secretory zone of vegetative shoots: the “phloem” (Maslen) occupies the position in the sporophyll bundle which in the vascular bundles of foliage leaves is occupied by a dark-celled and partially disorganised tissue in continuity with the secretory zone of the main stele. It may be that in the strobili this tissue occurred in a modified form, but even assuming that the section figured by Maslen shows true phloem, an assumption based on slender evidence, this is not sufficient justification for the application of the term phloem to a tissue occupying a corresponding position in vegetative shoots and distinguished by well-marked histological features.

The sporophyll-traces, as seen in the outer cortex in fig. 191, D, are partially surrounded by a large crescentic space, p, which was originally occupied by the parichnos. The sporangia are attached along the middle line of the sporophyll and, as in Lepidostrobus Brownii, a cushion of parenchyma projects into the lower part of the sporangial cavity ([fig. 191], A, a; C, a).

The diagrammatic sketch of part of a section in the Binney Collection reproduced in [fig. 191], B, shows the position of the ligule, l. No megaspores have been discovered in any specimens of this type; the microspores, which occur both singly and in tetrads, have a length of 0·02–0·03 mm.

The drawing shown in [fig. 191], A, based on a section in the Binney Collection, illustrates the general arrangement of the parts of a typical Lepidostrobus. I have made use of this sketch instead of that given by Maslen, as his figure conveys the idea that the sporophylls are superposed, whereas, whether they are verticillate or spiral, a radial longitudinal section would not cut successive sporangia in the same plane.

iii. Lepidostrobus Brownii (Brongn.).

In 1843 a specimen of a portion of a petrified cone was purchased by the British Museum, assisted by the Marquis of Northampton and Robert Brown, for £30 from a French dealer. This fossil, from an unknown locality, was briefly described by Brown in 1851[425] and named by him Triplosporites, but in a note added to his paper he expressed the opinion that the generic designation Lepidostrobus would be more appropriate. Brongniart afterwards named the cone Triplosporites Brownii[426], and Schimper[427] described it in his Traité as Lepidostrobus Brownii. The type-specimen is preserved in the British Museum and the Paris Museum possesses a piece of the same fossil.

The central axis of the cone has a stele of the type characteristic of Lepidodendron fuliginosum and L. Harcourtii, and the xylem is surrounded by a thin-walled tissue described by Bower[428] as possibly phloem; but in the absence of longitudinal sections it is impossible to say how far the tissue external to the xylem agrees with that in Lepidodendron stems. The sporophylls consist of a horizontal portion, to the upper face of which the radially elongated sporangia are attached, one to each sporophyll; beyond the distal end of the sporangium the sporophyll bends sharply upwards as a fairly stout lamina. The wall of the sporangium is composed of several layers of cells, as shown in a drawing published by Bower[429]; in the interior occur groups of microspores, and from a ridge of tissue which extends along the whole length of the sporangium irregular trabeculae of sterile tissue project into the sporangial cavity, as in Isoetes ([fig. 191], H: cf. [fig. 133], H).

Further information in regard to Lepidostrobus Brownii has recently been supplied by Prof. Zeiller[430], who recognises the existence of a ligule, and draws attention to some interesting histological features in the tissue of the sporophylls[431].

Spores of Palaeozoic Lycopodiales.

The calcareous nodules from the Coal seams of Yorkshire and Lancashire are rich in isolated spores, many of which are undoubtedly those of Lepidostrobi. Examples of spores were figured by Morris[432] in 1840, and their occurrence in coal has been described by several authors, one of the earliest accounts being by Balfour[433]. The drawings of Palaeozoic and recent spores published by Kidston and Bennie[434] demonstrate a striking similarity between the megaspores of existing and extinct Lycopods, the chief difference being the larger size of the fossils.

The general generic name Triletes, originally used by Reinsch[435], is a convenient term by which to designate Pteridophytic spores which cannot be referred to definite types.

It is usual to find more than four megaspores in each megasporangium in Palaeozoic and not infrequently, as we have seen, in Mesozoic lycopodiaceous strobili, but in some Palaeozoic cones, e.g. Bothrostrobus ([fig. 216]) and Lepidostrobus foliaceus[436], a single tetrad only appears to have reached maturity.

The occurrence of long simple or branched and sometimes capitate hairs is a common feature of Carboniferous megaspores ([fig. 191], E, F, I). It is possible that these appendages served to catch the microspores, thus facilitating fertilisation. A peculiar form of megaspore has been described by Mrs Scott[437], and assigned by her to Lepidostrobus foliaceus, the megasporangium of which apparently contained only four spores. As shown in [fig. 191], G, a large bladder-like appendage characterised by radiating veins is attached to the thick spore-coat; it is suggested that this excrescence may be compared with the “swimming” apparatus of the recent water-fern Azolla. The epithet swimming which it is customary to apply to the appendages of Azolla megaspores would seem to be inappropriate if Campbell[438] is correct in stating that spores of Azolla are incapable of floating.

B. Spencerites.

Spencerites insignis (Williamson). Fig. 192.

Another type of lycopodiaceous strobilus, differing sufficiently from Lepidostrobus to deserve a special generic designation, is that originally described by Williamson[439], from the Lower Coal-Measures of Yorkshire, as a type of Lepidostrobus, L. insignis, but afterwards[440] more fully investigated and assigned to a new genus by Scott[441]. It should be pointed out that in a later publication Williamson spoke of the lycopodiaceous axis, which he suspected might belong to his L. insignis, as possibly worthy of recognition as a distinct generic type.

Fig. 192. Spencerites insignis (Williamson). (After Miss Berridge.)

Of the two species included by Scott in his genus Spencerites only one, S. insignis, need be considered. Since the publication of Scott’s paper our knowledge of this type has been extended by Miss Berridge[442] and by Prof. Lang[443].

The axis of the strobilus has a stele characterised by a pith of elongated elements, most of which have thin walls; the xylem cylinder possesses about twenty protoxylem strands forming more or less prominent exarch ridges. The cortex exhibits a differentiation comparable with that in the shoots of Lepidodendron. The sporophylls are arranged in alternating verticils, each whorl consisting of ten members: the narrow horizontal pedicel of a sporophyll, containing a single vascular bundle, as shown in [fig. 192], is expanded distally into a prominent upper lobe bearing a cushion of small and delicate cells, to which the sporangium is attached, and prolonged obliquely upwards as a free leaf-like lamina. The lower blunt prolongation of the sporophylls appears to form a thick dorsal lobe, but, as Lang has pointed out, it is highly probable that the present form of the dorsal lobe is of secondary origin, and is “due to the disappearance of a mucilage cavity from a large sporophyll base[444].” As Miss Berridge remarks, the vascular bundle of the sporophyll does not give off a branch to the ventral lobe and sporangium. In attachment, in shape, and in the structure of the wall the sporangia differ markedly from those of Lepidostrobi. The spores, which also constitute a characteristic feature of the genus, have a maximum diameter of 0·14 mm.; they are described as oblate spheroids with a broad hollow wing running round the equator ([fig. 192]) comparable with the air-sacs of the pollen of Pinus. Scott points out that the spores of Spencerites are intermediate in size between the microspores of Lepidodendron and the megaspores of Lycopodium; it is difficult therefore to decide to which category they should be referred. Spencerites is clearly distinct from Lepidostrobus; the absence of a ligule, the manner of attachment of the sporangia, and the form and size of the spores, are characteristic features.

A comparison of Spencerites with the strobili of Lycopodium cernuum (figs. [123], [126–129]) has recently been made by Lang, who draws attention to the striking agreement as regards general plan and even detailed structural features between the Palaeozoic and the recent type of strobilus. It is interesting to find, as Lang points out, that in the original account of the fossil cone by Williamson, the view is expressed that the sporangiophores were confluent. An examination of the section figured by Williamson[445] led Lang to confirm this opinion. It would be out of place to enter here into a detailed comparison of Spencerites insignis and the cone of Lycopodium, but the resemblances are considered by Lang to be sufficiently close to suggest that the striking similarity may be indicative of relationship[446].

It is worthy of notice that the radial section of Spencerites ([fig. 192]) presents a fairly close resemblance to a corresponding section through a cone-scale of Agathis (Kauri Pine)[447]. In each case the megasporangium is attached by a narrow pedicel to the sporophyll and the latter has a similar form in the two plants, though the extent of the resemblance is somewhat lessened by Lang’s more complete account of the Palaeozoic type. If the Spencerites sporangia possessed an integument the similarity with the Agathis ovule would of course be much closer: recent palaeobotanical investigations have shown that ovules and sporangia are not separated by impassable barriers.

[Since this Chapter was set up in type a paper has appeared by Dr Bruno Kubart on a new species of Spencerites spore, S. membranaceus, from the Ostrau-Karwiner Coal-basin (Austria). The spores are larger than those of S. insignis and in some the cells of a prothallus are preserved. Kubart figures a section of a spore containing a group of seven cells, a central cell, which he regards as an antheridial mother-cell, surrounded by six wall-cells. Kubart (90).]