Fossil plants, Vol. 1: [A text-book] for students of botany and geology

by A. C. Seward

I. EQUISETALES.

Leaves usually small in proportion to the size of the whole plant, arranged in whorls at the nodes. Sporangia borne on specially modified sporophylls or sporangiophores, which are aggregated to form a definite strobilus or spore-bearing cone.

Equisetaceae. (Recent Species.)

The leaves are in whorls, coherent in the form of a sheath, and traversed by longitudinal veins which do not fork or anastomose. The stem is divided into comparatively long internodes separated by the leaf-bearing nodes, and the branches arise in the leaf-axils at the nodes. The fertile leaves or sporophylls differ from the sterile leaves, and usually occur in definite aggregations or strobili containing spores of one kind (isosporous). In the single living genus Equisetum, the outer coat of the mature spore forms two hygroscopically sensitive filamentous structures or elaters. On the germination of the spore the gametophyte is developed in the form of a small lobed prothallium 1–2 cm. in length. In most cases there are distinct male and female prothallia.

The genus Equisetum L., the common Horse-tail, is the sole living representative of this Family. It occurs as a common native plant in Britain, and has a wide geographical distribution. Species of Equisetum are abundant in the temperate zones of both hemispheres, and occur in arctic as well as tropical latitudes. Wallace[484] speaks of Horse-tails, “very like our own species,” growing at a height of 5000 feet on the Pangerango mountain in Java. In favourable situations the large British Horse-tail, Equisetum maximum Lam. (= E. Telmateia Erhb.), occasionally reaches a height of about six feet, and growing in thick clusters forms miniature forests of trees with slender erect stems and regular circles of long and thin branches. A tropical species, Equisetum giganteum Linn.[2] living in the marshes of Mexico and Cuba[485], and extending southward to Buenos Ayres and Chili, reaches a height of twenty to forty feet, but the stem always remains slender, and does not exceed an inch in diameter. Groves of such tall slender plants on the eastern slopes of the Andes[486] suggest to the palaeobotanist an enfeebled forest-growth recalling the arborescent Calamites of a Palaeozoic vegetation. The twenty-five existing species of Equisetum are remnants of various generic types of former epochs, and possess a special interest from the point of view of the geological history of plants. A brief description of the main characters of the recent genus will enable the student to appreciate the points of difference and agreement between the extinct and present representatives of the Equisetales.

Fig. 52. Equisetum maximum Lam. A. Fertile shoot with strobilus and sterile leaf-sheaths [after Luerssen (89); slightly less than nat. size]. B. Sporophyll bearing open sporangia (after Luerssen; slightly enlarged). C. Part of a transverse section (diagrammatic); v, vallecular canals, e, endodermis, c, carinal canals (after Luerssen; × 20). D. Equisetum arvense L. Part of a transverse section of an internode of a sterile shoot. v, cortex, e, endodermis, x, xylem tracheids, a remains of annular tracheids of the protoxylem, c, carinal canal (after Strasburger; × 90).

Equisetum.

The plant consists of a perennial underground creeping rhizome, branching into secondary rhizomes, divided into well-marked nodes and internodes. From the nodes are given off two sets of buds, which may develope into ascending aerial shoots or descending roots. At each node is a leaf-sheath more or less deeply divided along the upper margin into teeth representing the tips of coherent leaves (fig. 52, A).

In some species one or more internodes of underground branches become considerably swollen and assume the form of ovate or elliptical starch-storing tubers, which are capable of giving rise to new plants by vegetative reproduction. Tubers, either singly or in chains, occur in E. arvense Linn., E. sylvaticum Linn., E. maximum Lam., among British species.

Fig. 53. Rhizome (R) of Equisetum palustre L. with a thin shoot giving off roots and tuberous branches from a node [after Duval-Jouve (64)].

In the example shown in fig. 53 (Equisetum palustre L.[487]) the stout rhizome R gives off from its node, marked by a small and irregular leaf-sheath, two thin roots and a single shoot. The latter has a leaf-sheath at its base, and from the second node, with a larger leaf-sheath, there have been developed branches with tuberous internodes; the constrictions between the tubers and the tips of the terminal tubers bear small leaf-sheaths. Branched roots are also given off from the upper node of the erect shoot.

Near the surface of the ground the buds on the rhizome nodes develope into green erect shoots. The shoot axis is marked out into long internodes separated by nodes bearing the leaf-sheaths. The surface of each internode is traversed by regular and more or less prominent longitudinal ridges and grooves; each ridge marking the position of an internal longitudinal vascular strand. In the axil of each leaf, that is in the axil of each portion of a leaf-sheath corresponding to a marginal uni-nerved tooth, there is produced a lateral bud which may either remain dormant or break through the leaf-sheath and emerge as a lateral branch. At the base of each branch an adventitious root may be formed from a cell immediately below the first leaf-sheath, but in aerial shoots the roots usually remain undeveloped. The lateral branches repeat on a smaller scale the general features of the main axis. In some species, the shoots are unbranched, and in others the slender branches arise in crowded whorls from each node. Leaves, roots and branches are given off in whorls, and the whorls from each node alternate with those from the node next above and next below.

ANATOMY OF EQUISETUM.

In some species of Equisetum the aerial stem terminates in a conical group of sporophylls, while in others the strobilus is formed at the apex of a pale-coloured fertile shoot, which never attains any considerable length and dies down early in the season of growth (fig. 52, A). Below the terminal cone or strobilus there occur one or two modified leaf-sheaths. Such a ring of incompletely developed leaves intervening between the cone of sporangiophores and the normal leaves, is known as the annulus. The annulus is seen in fig. 52, A, immediately below the lowest whorl of sporophylls; it has the form of a low sheath with a ragged margin. In the region of the cone the internodes remain shorter, and the whorls of appendages, known as sporophylls or sporangiophores, have the form of stalked structures terminating distally in a hexagonal peltate disc, which bears on its inner face a ring of five to ten oval sporangia (fig. 52, B). Each sporangium contains numerous spores which eventually escape by the longitudinal dehiscence of the sporangial wall. The opening of the sporangia is probably assisted by the movements of the characteristic elaters formed from the outer wall of each spore.

The spores, which are capable of living only a short time, grow into aerial green prothallia, 1–2 cm. in length; these have the form of irregularly and more or less deeply lobed structures. On the larger and more deeply lobed prothallia the archegonia or female reproductive organs are borne, and the smaller or male prothallia bear the antheridia. On the fertilisation of an egg-cell, the Equisetum plant is gradually developed. For a short time parasitic on the female prothallium or gametophyte, the young plant soon takes root in the ground and becomes completely independent.

As seen in transverse section through a young stem near the apex, the axis consists of a mass of parenchyma, in which may be distinguished a central larger-celled tissue, surrounded by a ring of smaller-celled groups marking the position of a circle of embryonic vascular strands. In each young vascular strand, a few of the cells next the pith may be seen to have thicker walls and to be provided with a ring-like internal thickening; these have passed over into the condition of annular tracheids and represent the protoxylem elements. At a later stage, a transverse section through the stem shows a central hollow pith, formed by the tearing apart and subsequent disappearance of the medullary parenchymatous cells, which were unable to keep pace with the growth in thickness of the stem. The pith cavity is bridged across at each node by a multi-layered plate of parenchyma, which forms the so-called nodal diaphragm. The inner edge of each vascular strand is now found to be occupied by a small irregularly circular canal (fig. 52, C, c, and D, c) in which may be seen some of the rings of protoxylem tracheids (D, a) which have been torn apart and almost completely destroyed. These canals, known as carinal canals, have arisen by the tearing and disruption of the thin-walled cells in the immediate neighbourhood of the protoxylem. Each carinal canal is bounded by a layer of elongated parenchymatous cells which form part of the xylem of the vascular bundle, and is succeeded internally by the general ground-tissue of the stem. The xylem parenchyma next a carinal canal is succeeded externally by phloem tissue, consisting of short protoplasmic cells and longer elements, without nuclei and poor in contents; the latter may be regarded as sieve-tubes. On either side of the phloem, the xylem occurs in two separate bands or groups of annular and reticulately thickened tracheids. In some species, e.g. Equisetum xylochaetum Metten.[488] and E. giganteum[489] L. a native of South America, the xylem has the form of two bands composed of fairly numerous tracheids, but in most species the xylem tracheids occur in small groups, as shown in the figure of E. maximum (fig. 52, D). In the shape of the vascular bundle, and in the formation of the carinal canal, there is a distinct resemblance between the vascular bundles of Equisetum and those of a monocotyledonous stem. These collateral stem-bundles of xylem and phloem traverse each internode as distinct strands, and at the nodes each strand forks into two branches (fig. 54, A), which anastomose with the alternating bundles passing into the stem from the leaf-sheath. Thus the vascular strands of each internode alternate in position with those of the next internode.

Fig. 54. A. Plan of the vascular bundles in the stem of an Equisetum; b, branches passing out to buds (after Strasburger); l, vascular strands passing to the leaf-segments. B. Longitudinal section through a node of E. arvense L. (after Duval-Jouve; × 20). Explanation in the text.

There are certain points connected with the vascular bundles in the nodal region of a shoot, which have an important bearing on the structure of fossil equisetaceous stems. Fig. 54 B represents a diagrammatic longitudinal section through the node of a rhizome of Equisetum arvense from which a root h is passing off in a downward direction, and a branch in an upward direction. The black band c in the parent stem shows the position of the vascular strands; in the region of the node the vascular tissue attains a considerable thickness, as seen at d in the figure. The bands passing out to the left from d go to supply the branch and root respectively. The increased breadth of the xylem strands at the node is due to the intercalation of a number of short tracheids. Fig. 55, 4 shows a transverse section through a mature node of Equisetum maximum; px marks the position of the protoxylem and e that of the endodermis. On comparing this section with that of the internodal vascular bundle in fig. 52, D, the much greater development of wood in the former is obvious; the carinal canal of the internodal bundle is absent in the section through a node. The disposition of the xylem tracheids in fig. 55, 4 shows a certain regularity which, though not very well marked, suggests the development of wood elements as the result of cambial activity. Longitudinal sections through the nodal region demonstrate the existence of “cells similar to those of an ordinary cambium, and a cell-formation resulting from their division which is similar to that in an ordinary secondary thickening.”[490] The short tracheids which make up this nodal mass of xylem differ from those in the internodal bundle in their smaller size, and in being reticulately thickened. There is, therefore, evidence that in the nodes of some Equisetum stems additional xylem elements are produced by a method of growth comparable with the cambial activity which brings about the growth in thickness of a forest-tree[491]. The significance of these statements will be realised when the structure of the extinct genus Calamites is described and compared with that of Equisetum.

The small drawing in fig. 55, 3 shows part of the ring of thick nodal wood; the section cuts through two bundles about their point of bifurcation, the strand x is passing out in a radial direction to a lateral branch, the strand to the right of x and the separate fragment of a strand to the left of x are portions of leaf-trace bundles on their way to the leaf-sheath. Reverting to fig. 54, B, the other structures seen in the section are the leaf-sheaths (l and m), the vallecular canal (f), the epidermis, cortex and pith (k, e and a) of the stem. The epidermis which has been ruptured by the root and branch is indicated at i, i; the dotted lines traversing the upper part of the pith of the lateral branch mark the position of a nodal diaphragm.

Fig. 55. 1. Transverse section of a root of Equisetum variegatum Schl., e endodermis, or outer layer of the phloeoterma (after Pfitzer; × 160). 2. Transverse section of rhizome of E. maximum, slightly enlarged. 3. Transverse section through a node of E. maximum, x, branch of vascular strand (slightly enlarged). 4. Transverse section through a node of E. maximum showing the mass of xylem, px protoxylem (× 175). (Figs. 3 and 4 after Cormack.)

Immediately external to each vascular strand, as seen in transverse section, there is a layer of cells containing starch, and this is followed by a distinct endodermis, of which the cells show the characteristic black dot in the cuticularised radial walls (fig. 52, D). Beyond the endodermis there is the large-celled parenchyma of the rest of the cortex. Tannin cells occur here and there scattered among the ground tissue. On the same radius on which each vascular strand occurs, the cortical parenchyma passes into a mass of sub-epidermal thick-walled mechanical tissue or stereome. Alternating with the ridges of stereome, the grooves are occupied by thin-walled chlorophyll-containing tissue which carries on most of the assimilating functions, and communicates with the external atmosphere by means of stomata arranged in vertical rows down each internode. The continuity of the cortical tissue is interrupted by the occurrence of large longitudinal vallecular canals alternating in position with the stem ridges and vascular strands (fig. 52, C, v). The epidermis consists of a single layer of cells, containing stomata, and with the outer cell-walls impregnated with silica.

In certain species of Equisetum, e.g. E. palustre L., the whole circle of vascular strands is enclosed by an endodermis, and has the structure typical of a monostelic stem. In others e.g. E. litorale Kühl. each vascular strand is surrounded by a separate endodermis, and in some forms e.g. E. sylvaticum L. there is an inner as well as an outer endodermal layer[492]. Without discussing the explanation given to this variation in the occurrence of the endodermis, it may be stated that in all species of Equisetum the stem may be regarded as monostelic[493].

In the rhizome the structure agrees in the main with that of the green shoots, but the vallecular canals attain a larger size, and the pith is solid. A slightly enlarged transverse section of a rhizome of Equisetum maximum is shown in fig. 55, 2, the small circles surrounding the pith mark the position of the vascular bundles and carinal canals; the much larger spaces between the central cylinder and the surface of the stem are the vallecular canals.

The central cylinder or stele of the root is of the diarch, triarch or tetrarch type; i.e. there may be 2, 3 or 4 groups of protoxylem in the xylem of the root stele. The axial portion is occupied by large tracheids, and the smaller tracheids of the xylem occur as radially disposed groups, alternating with groups of phloem. External to the xylem and phloem strands there occur two layers of cells, usually spoken of as a double endodermis, but it has been suggested that it is preferable to describe the double layer as the phloeoterma[494], of which the inner layer has the functions of a pericycle, and the outer that of an endodermis. A transverse section of a root is seen in fig. 55, 1, the dark cells on the left are part of a thick band of sclerenchyma in the cortex of the root, the layer e is the outer layer of the phloeoterma.

Without describing in detail the development[495] of the sporangia, it should be noted that the sporangial wall is at first 3 to 4 cells thick, but it eventually consists of a single layer. The cells have spiral thickening bands on the ventral surface, and rings on the cells where the longitudinal splitting takes place. Each sporangium is supplied by a vascular bundle which is given off from that of the sporangiophore axis. The strobili are isosporous.

II. FOSSIL EQUISETALES.

A. [EQUISETITES.] B. [PHYLLOTHECA.] C. [SCHIZONEURA.] D. [CALAMITES.] E. [ARCHAEOCALAMITES.]

In dealing with the fossil Equisetales, we will first consider the genera Equisetites, Phyllotheca and Schizoneura, and afterwards describe the older and better known genera Calamites and Archaeocalamites. A thoroughly satisfactory classification of the members of the Equisetales is practically impossible without more data than we at present possess. It has been the custom to include Equisetites, Phyllotheca and Schizoneura in the family Equisetaceae, and to refer Calamites and Archaeocalamites to the Calamarieae; such a division rests in part on assumption, and cannot be considered final. When we attempt to define the Equisetales and the two families Equisetaceae and Calamarieae, we find ourselves seriously hampered by lack of knowledge of certain important characters, which should be taken into account in framing diagnoses. There is little harm in retaining provisionally the two families already referred to, if we do not allow a purely arbitrary classification to prejudice our opinions as to the affinities of the several members of the Equisetales.

The Equisetaceae might be defined as a family including plants which were usually herbaceous but in some cases arborescent, bearing verticils of leaves in the form of sheaths more or less deeply divided into segments or teeth. The strobili were isosporous and consisted of a central axis bearing verticils of distally expanded sporophylls with sporangia, as in Equisetum. The genus Equisetites might be included in this family, but it must be admitted that we know next to nothing as to its anatomy, and we cannot be sure that the strobili were always isosporous.

The genus Schizoneura is too imperfectly known to be defined with any approach to completeness, or to be assigned to a family defined within certain prescribed limits. Phyllotheca is another genus about which we possess but little satisfactory knowledge; we are still without evidence as to its structure, and the descriptions of the few strobili that are known are not consistent. Recent work points to a probability of Phyllotheca being closely allied to Annularia, a genus included in the Calamarieae, and standing for a certain type of Calamitean foliage-shoots.

In comparing the Calamarieae with the Equisetaceae, the alternation of sterile and fertile whorls in the strobilus, and the free linear leaves at the nodes instead of leaf-sheaths are two characters made use of as distinguishing features of the genus Calamites as the type of the Calamarieae. On the other hand, the strobili of Phyllotheca appear to agree with those of Calamites rather than with those of Equisetum, and strobili of Archaeocalamites have been found exhibiting the typical Equisetum characters. The sheath-like form of the leaves is not necessarily peculiar to the Equisetaceae, and we have evidence that leaf-sheaths occurred on the nodes of Calamitean plants. In Archaeocalamites the leaves possess characteristic features, and can hardly be said to agree more closely with those of Calamites than with the leaves of Phyllotheca or Sphenophyllum, a genus belonging to another class of Pteridophytes.

On the whole, then, without discussing further the possibilities of a subdivision of the Equisetales, we may regard the genera Calamites, Archaeocalamites, Equisetites, Equisetum, Phyllotheca and Schizoneura as so many members of the Equisetales, without insisting on a classification which cannot be supported by satisfactory evidence.

Our knowledge of Calamites is fairly complete. Abundant and well-preserved material from the Coal-Measures of England, and from Permo-Carboniferous rocks of France, Germany and elsewhere, has enabled palaeobotanists to investigate the anatomical characters of both the vegetative and reproductive structures of this genus. We are in a position to give a detailed diagnosis of Calamitean stems, roots and strobili, and to determine the place of this type of plant in a system of classification. Calamites not only illustrates the possibilities of palaeobotanical research, but it demonstrates the importance of fossil forms as foundations on which to construct the most rational classification of existing plants. The close alliance between Calamites and the recent Equisetaceae has been clearly established, and certain characteristics of the former genus render necessary an extension and modification of the definition of the class to which both Calamites and Equisetites belong. The Calamites broaden our conception of the Equisetaceous alliance, and by their resemblance to other extinct Palaeozoic types they furnish us with important links towards a phylogenetic series, which the other members of the Equisetales do not supply.

From the Upper Devonian to the Permian epoch Calamites and other closely related types played a prominent part in the vegetation of the world. We have no good evidence for the existence of Calamites in Triassic times; in its place there were gigantic Equisetums which resembled modern Horse-tails in a remarkable degree. In the succeeding Jurassic period tree-like Equisetums were still in existence, and species of Equisetites are met with in rocks of this age in nearly all parts of the world. A few widely distributed species are known from Wealden rocks, but as we ascend the geologic series from the Jurassic strata, the Equisetums become less numerous and the individual plants gradually assume proportions practically identical with those of existing forms.

A. Equisetites.

The generic name Equisetites was proposed by Sternberg in 1838[496] as a convenient designation for fossil stems bearing a close resemblance to recent species of Equisetum. Some authors have preferred to apply the name Equisetum to fossil and recent species alike, but in spite of the apparent identity in the external characters of the fossil stems with those of existing Horse-tails, and a close similarity as regards the cones, there are certain reasons for retaining Sternberg’s generic name. It is important to avoid such nomenclature as might appear to express more than the facts admit. If the custom of adding the termination -ites to the root of a recent generic term is generally followed, it at once serves to show that the plants so named are fossil and not recent species. Moreover, in the case of fossil Equisetums we know nothing of their internal structure, and our comparisons are limited to external characters. Stems, cones, tubers, and leaves are often very well preserved as sandstone casts with distinct surface-markings, but we are still in want of petrified specimens. There is indeed evidence that some of the Triassic and Jurassic species of Equisetites, like the older Calamites, possessed the power of secondary growth in thickness, but our deductions are based solely on external characters.

In the following pages a few of the better known species of Equisetites are briefly described, the examples being chosen partly with a view to illustrate the geological history of the genus, and partly to contribute something towards a fuller knowledge of particular species. One of the most striking facts to be gleaned from a general survey of the past history of the Equisetaceae is the persistence since the latter part of the Palaeozoic period of that type of plant which is represented by existing Equisetums. There is perhaps no genus in existence which illustrates more vividly than Equisetum the survival of an extremely ancient group, which is represented to-day by numerous and widely spread species. The Equisetaceous characteristics mark an isolated division of existing Vascular Cryptogams, and without reference to extinct types it is practically impossible to do more than vaguely guess at the genealogical connections of the family. When we go back to Palaeozoic plants there are indications of guiding lines which point the way to connecting branches between the older Equisetales and other classes of Pteridophytes. The recently discovered genus Cheirostrobus[497] is especially important from this point of view.

LEAF-SHEATHS OF EQUISETITES.

The accurate description of species, and the determination of the value of such differences as are exhibited in the surface characters of structureless casts, are practically impossible in many of the fossil forms. In certain living Horse-tails we find striking differences between fertile and sterile shoots, and between branches of different orders. The isolated occurrence of fragments of fossil stems often leads to an artificial separation of ‘species’ largely founded on differences in diameter, or on slight variations in the form of the leaf-sheaths. It is wiser to admit that in many cases we are without the means of accurate diagnosis, and that the specific names applied to fossil Equisetums do not always possess much value as criteria of taxonomic differences.

The specimens of fossil Equisetums are usually readily recognised by the coherent leaf-segments in the form of nodal sheaths resembling those of recent species. The tissues of the cortex and central cylinder are occasionally represented by a thin layer of coal pressed on to the surface of a sandstone cast, or covering a flattened stem-impression on a piece of shale. It is sometimes possible under the microscope to recognise on the carbonised epidermal tissues the remains of a surface-ornamentation similar to that in recent species, which is due to the occurrence of siliceous patches on the superficial cells. Longitudinal rows of stomata may also be detected under favourable conditions of preservation. The nodal diaphragms of stems have occasionally been preserved apart, but such circular and radially-striated bodies may be misleading if found as isolated objects. Casts of the wide hollow pith of Equisetites, with longitudinal ridges and grooves, and fairly deep nodal constrictions, have often been mistaken for the medullary casts of Calamites.

Several species of Equisetites have been recorded from the Upper Coal-Measures and overlying Permian rocks, but these present special difficulties. In one instance described below, (Equisetites Hemingwayi Kidst.), the species was founded on a cast of what appeared to be a strobilus made up of sporophylls similar to those in an Equisetum cone. In other Permo-Carboniferous species the choice of the generic name Equisetites has been determined by the occurrence of leaf-sheaths either isolated or attached to the node of a stem. The question to consider is, how far may the Equisetum-like leaf-sheath be regarded as a characteristic feature of Equisetites as distinct from Calamites? In the genus Calamites the leaves are generally described as simple linear leaves arranged in a whorl at the nodes, but not coherent in the form of a sheath (fig. 85). The fusion of the segments into a continuous sheath or collar is regarded as a distinguishing characteristic of Equisetites and Equisetum. The typical leaf-sheath of a recent Horse-tail has already been described. In some species we have fairly large and persistent free teeth on the upper margin of the leaf-sheath, but in other Equisetums the rim of the sheath is practically straight and has a truncated appearance, the distal ends of the segments being separated from one another by very slight depressions, as in a portion of the sheath of Equisetum ramosissimum Desf. of fig. 58, C. In other leaf-sheaths of this species there are delicate and pointed teeth adherent to the margin of the coherent segments; the teeth are deciduous, and after they have fallen the sheath presents a truncated appearance. This difference between the sheaths to which the teeth are still attached and those from which they have fallen is illustrated by fig. 58, B and C; it is one which should be borne in mind in the description of fossil species, and has probably been responsible for erroneous specific diagnoses. In some recent Horse-tails the sheath is occasionally divided in one or two places by a slit reaching to the base of the coherent segments[498]; this shows a tendency of the segments towards the free manner of occurrence which is usually considered a Calamitean character. In certain fossils referred to the genus Annularia, the nodes bear whorls of long and narrow leaves which are fused basally into a collar (fig. 58, D). There are good grounds for believing that at least some Annularias were the foliage shoots of true Calamites. Again, in some species of Calamitina, a sub-genus of Calamites, the leaves appear to have been united basally into a narrow sheath. We see, then, that it is a mistake to attach great importance to the separate or coherent character of leaf-segments in attempting to draw a line between the true Calamites and Equisetites. Potonié[499] while pointing out that this distinction does not possess much value as a generic character, retains the genus Equisetites for certain Palaeozoic Equisetum-like leaf-sheaths.

Fig. 56. Calamitean leaf-sheath. From a specimen in the Woodwardian Museum. a, base of leaf-sheath; (very slightly reduced).

Fig. 56 represents a rather faint impression of a leaf-sheath and nodal diaphragm. The specimen is from the Coal-Measures of Ardwick, Manchester. The letter a probably points to the attachment of the sheath to the node of the stem. The flattened sheath is indistinctly divided into segments, and at the middle of the free margin there appears to be a single free tooth. The lower part of the specimen, as seen in the figure, shows the position of the nodal diaphragm. Between the diaphragm and the sheath there are several slight ridges converging towards the nodal line; these agree with the characteristic ridges and grooves of Calamite casts which are described in detail in Chapter X. There is another specimen in the British Museum which illustrates, rather more clearly than that shown in fig. 56, the association of a fused leaf-sheath with a type of cast usually regarded as belonging to a Calamitean stem. Some leaf-sheaths of Permian age described by Zeiller[500] as Equisetites Vaujolyi bear a close resemblance to the sheath in fig. 58 E. The nature of the true Calamite leaves is considered more fully on a later page.

PALAEOZOIC EQUISETITES.

The examples of supposed Equisetites sheaths referred to below may serve to illustrate the kind of evidence on which this genus has been recorded from Upper Palaeozoic rocks. I have retained the name Equisetites in the description of the species, but it would probably be better to speak of such specimens as ‘Calamitean leaf-sheaths’ rather than to describe them as definite species of Equisetites. We have not as yet any thoroughly satisfactory evidence that the Equisetites of Triassic and post-Triassic times existed in the vegetation of earlier periods.

In Grand’Eury’s Flore du Gard[501] a fossil strobilus is figured under the name Calamostachys tenuissima Grand’Eury, which consists of a slender axis bearing series of sporophylls and sporangia apparently resembling those of an Equisetum. There are no sterile appendages or bracts alternating with the sporophylls; and the absence of the former suggests a comparison with Equisetites rather than Calamites. Grand’Eury refers to the fossil as “parfois à peine perceptible,” and a recent examination of the specimen leads me to thoroughly endorse this description. It was impossible to recognise the features represented in Grand’Eury’s drawing. Setting aside this fossil, there are other strobili recorded by Renault[502] and referred by him to the genus Bornia (Archaeocalamites), which also exhibit the Equisetum-like character; the axis bears sporophylls only and no sterile bracts. It would appear then that in the Palaeozoic period the Equisetaceous strobilus, as we know it in Equisetum, was represented in some of the members of the Equisetales.

Fig. 57. A. Equisetites Hemingwayi Kidst. From a specimen in the British Museum. ⅔ nat. size. B. Diaphragm and sheath of an Equisetaceous plant, from the Coal-Measures. ⅔ nat. size. From a specimen in the British Museum.

1. Equisetites Hemingwayi Kidst. Fig. 57, A.

Mr Kidston[503] founded this species on a few specimens of cones found in the Middle Coal-Measures of Barnsley in Yorkshire. The best example of the cone described by Kidston has a length of 2·5 cm., and a breadth of 1·5 cm.; the surface is divided up into several hexagonal areas 4 mm. high and 5 mm. wide. Each of these plates shows a fairly prominent projecting point in its centre; this is regarded as the point of attachment of the sporangiophore axis which expanded distally into a hexagonal plate bearing sporangia. An examination of Mr Kidston’s specimens enabled me to recognise the close resemblance which he insists on between the fossils and such a recent Equisetaceous strobilus as that of Equisetum limosum Sm. Nothing is known of the structure of the fossils beyond the character of the superficial pattern of the impressions, and it is impossible to speak with absolute confidence as to their nature. The author of the species makes use of the generic name Equisetum; but in view of our ignorance of structural features it is better to adopt the more usual term Equisetites.

Since Kidston’s description was published I noticed a specimen in the British Museum collection which throws some further light on this doubtful fossil. Part of this specimen is shown in fig. 57, A. The stem is 21 cm. in length and about 5 mm. broad; it is divided into distinct nodes and internodes; the former being a little exaggerated in the drawing. The surface is marked by fine and irregular striations, and in one or two places there occur broken pieces of narrow linear leaves in the neighbourhood of a node. Portions of four cones occurring in contact with the stem, appear to be sessile on the nodes, but the preservation is not sufficiently good to enable one to speak with certainty as to the manner of attachment. Each cone consists of regular hexagonal depressions, which agree exactly with the surface characters of Kidston’s type-specimen. The manner of occurrence of the cones points to a lateral and not a terminal attachment. The stem does not show any traces of Equisetaceous leaf-sheaths at the nodes, and such fragments of leaves as occur appear to have the form of separate linear segments; they are not such as are met with on Equisetites. It agrees with some of the slender foliage-shoots of Calamitean plants often described under the generic name Asterophyllites. As regards the cones; they differ from the known Calamitean strobili in the absence of sterile bracts, and appear to consist entirely of distally expanded sporophylls as in Equisetum. The general impression afforded by the fossil is that we have not sufficient evidence for definitely associating this stem and cones with a true Equisetites. We may, however, adhere to this generic title until more satisfactory data are available.

2. Equisetites spatulatus Zeill. Fig. 58, A.

This species is chosen as an example of a French Equisetites of Permian age. It was recently founded by Zeiller[504] on some specimens of imperfect leaf-sheaths, and defined as follows:—

Sheaths spreading, erect, formed of numerous uninerved coherent leaves, convex on the dorsal surface, spatulate in form, 5–6 cm. in length and 2–3 mm. broad at the base, and 5–10 mm. broad at the apex, rounded at the distal end.

The specimen shown in fig. 58, A, represents part of a flattened sheath, the narrower crenulated end being the base of the sheath. The limits of the coherent segments and the position of the veins are clearly marked. Zeiller’s description accurately represents the character of the sheaths. They agree closely with an Equisetaceous leaf-sheath, but as I have already pointed out, we cannot feel certain that sheaths of this kind were not originally attached to a Calamite stem.

The portion of a leaf-sheath and a diaphragm represented in fig. 57, B, agrees closely with Zeiller’s examples. This specimen is from the English Coal-Measures, but it is not advisable to attempt any specific diagnosis on such fragmentary material. It is questionable, indeed, if these detached fossil leaf-sheaths should be designated by specific names. Another similar form of sheath, hardly distinguishable from Zeiller’s species, has recently been described by Potonié from the Permian (Rothliegende) of Thuringia.

Fig. 58.

1. Equisetites spatulatus, Zeill. Leaf-sheath. ⅘ nat. size. (After Zeiller.)
2. E. columnaris, Brongn. From a specimen in the British Museum. ¾ nat. size.
3. Equisetum ramosissimum, Desf. × 2.
4. Annularia stellata (Schloth.). Leaf-sheath. Slightly enlarged. (After Potonié.)
5. Equisetites zeaeformis (Schloth.). Leaf-sheath. ⅘ nat. size. (After Potonié.)
6. E. lateralis, Phill. From a specimen in the Scarborough Museum. Nat. size.

3. Equisetites zeaeformis (Schloth.)[505] Fig. 58, E.

The sheaths consist of linear segments fused laterally as in Equisetum. In some specimens the component parts of the sheath are more or less separate from one another, and in this form they are apparently identical with the leaves of Calamites (Calamitina) varians, Sternb. The example shown in fig. 58, E is probably a young leaf-sheath; the segments are fused, and each is traversed by a single vein represented by a dark line in the figure. The regular crenulated lower margin is the base of the sheath, and corresponds to the upper portion of fig. 58, A. This species affords, therefore, an interesting illustration of the difficulty of separating Equisetites leaves from those of true Calamites. Potonié has suggested that the leaf-sheath of a young Calamite might well be split up into distinct linear segments as the result of the increase in girth of the stem.

•••••

Other Palaeozoic species of Equisetites have been recorded, but with one exception these need not be dealt with, as they do not add anything to our knowledge of botanical importance. The specimen described in the Flore de Commentry as Equisetites Monyi, by Renault and Zeiller[506], differs from most of the other Palaeozoic species of Equisetites, in the fact that we have a stem with short internodes bearing a leaf-sheath at each node divided into comparatively long and distinct teeth. This species presents a close agreement with specimens of Calamitina, but Renault and Zeiller consider that it is generically distinct. They suggest that the English species, originally described and figured by Lindley and Hutton[507] as Hippurites gigantea, and now usually spoken of as Calamitina, should be named Equisetites. It would probably be better to adopt the name Calamitina for the French species. The type-specimen of this species is in the Natural History Museum, Paris.

EQUISETITES PLATYODON.

Fig. 59. Equisetites platyodon Brongn. (After Schoenlein, slightly reduced.)

When we pass from the Permian to the Triassic period, we find large casts of very modern-looking Equisetaceous stems which must clearly be referred to the genus Equisetites. The portion of a stem represented in fig. 59 known as Equisetites platyodon Brongn.[508] affords an example of a Triassic Equisetaceous stem with a clearly preserved leaf-sheath. The stem measures about 6 cm. in diameter. One of the oldest known Triassic species is Equisetites Mougeoti[509] (Brongn.) from the Bunter series of the Vosges.

The Keuper species E. arenaceus is, however, more completely known. The specimens referred to this species are very striking fossils; they agree in all external characters with recent Horse-tails but greatly exceed them in dimensions.

4. Equisetites arenaceus Bronn.

This plant has been found in the Triassic rocks of various parts of Germany and France; it occurs in the Lettenkohl group (Lower Keuper), as well as in the Middle Keuper of Stuttgart and elsewhere. The species may be defined as follows:—

Rhizome from 8–14 cm. in diameter, with short internodes, bearing lateral ovate tubers. Aerial shoots from 4–12 cm. in diameter, bearing whorls of branches, and leaf-sheaths made up of 110–120 coherent uni-nerved linear segments terminating in an apical lanceolate tooth. Strobili oval, consisting of crowded sporangiophores with pentagonal and hexagonal peltate terminations.

The casts of branches, rhizomes, tubers, buds and cones enable us to form a fairly exact estimate of the size and general appearance of this largest fossil Horse-tail. The Strassburg Museum contains many good examples of this species, and a few specimens may be seen in the British Museum. In the École des Mines, Paris, there are some exceptionally clear impressions of cones of this species from a lignite mine in the Vosges.

It is estimated that the plant reached a height of 8 to 10 meters, about equal to that of the tallest recent species of Equisetum, but in the diameter of the stems the Triassic plant far exceeded any existing species.

It is interesting to determine as far as possible, in the absence of petrified specimens, if this Keuper species increased in girth by means of a cambium. There are occasionally found sandstone casts of the pith-cavity which present an appearance very similar to that of Calamitean medullary casts[510]. The nodes are marked by comparatively deep constrictions, which probably represent the projecting nodal wood. The surface of the casts is traversed by regular ridges and grooves as in an ordinary Calamite, and it is probable that in Equisetites arenaceus, as in Calamites, these surface-features are the impression of the inner face of a cylinder of secondary wood (cf. p. 310). Excellent figures of this species of Equisetites are given by Schimper in his Atlas of fossil plants[511], also by Schimper and Koechlin-Schlumberger[512], and by Schoenlein and Schenk[513].

5. Equisetites columnaris Brongn. Figs. 11 and 58, B.

This species, which is by far the best known British Equisetites, was founded by Brongniart[514] on some specimens from the Lower Oolite beds of the Yorkshire coast. Casts of stems are familiar to those who have collected fossils on the coast between Whitby and Scarborough; they are often found in an erect position in the sandstone, and are usually described as occurring in the actual place of growth. As previously pointed out (p. 72), such stems have generally been deposited by water, and have assumed a vertical position (fig. 11). Young and Bird[515] figured a specimen of this species in 1822, and in view of its striking resemblance to the sugar-cane, they regarded the fossil as being of the same family as Saccharum officinarum, if not specifically identical.

A specimen was described by König[516] in 1829, from the Lower Oolite rocks of Brora in the north of Scotland under the name of Oncylogonatum carbonarium, but Brongniart[517] pointed out its identity with the English species Equisetites columnaris.

Our acquaintance with this species is practically limited to the casts of stems. A typical stem of E. columnaris measures 3 to 6 cm. in diameter and has fairly long internodes. The largest stem in the British Museum collection has internodes about 14 cm. long and a diameter of about 5 cm. In some cases the stem casts show irregular lateral projections in the neighbourhood of a node, but there is no evidence that the aerial shoots of this species gave off verticils of branches. In habit E. columnaris probably closely resembled such recent species as Equisetum hiemale L., E. trachyodon A. Br. and others.

The stems often show a distinct swelling at the nodes; this may be due, at least in part, to the existence of transverse nodal diaphragms which enabled the dead shoots to resist contraction in the region of the nodes. The leaf-sheaths consist of numerous long and narrow segments often truncated distally, as in fig. 58, B, and as in the sheath of such a recent Horse-tail as E. ramosissimum shown in fig. 58, C. In some specimens one occasionally finds indications of delicate acuminate teeth extending above the limits of a truncated sheath. Brongniart speaks of the existence of caducous acuminate teeth in his diagnosis of the species, and the example represented in fig. 58, B, demonstrates the existence of such deciduous appendages. There is a very close resemblance between the fossil sheath of fig. 58, B, with and without the teeth, and the leaf-sheath of the recent Equisetum in fig. 58, C. In some specimens of E. columnaris in which the cast is covered with a carbonaceous film, each segment in a leaf-sheath is seen to be slightly depressed in the median portion, which is often distinctly marked by numerous small dots, the edges of the segment being flat and smooth. The median region is that in which the stomata are found and on which deposits of silica occur.

6. Equisetites Beani (Bunb.). Figs. 60–62.

Bunbury[518] proposed the name Calamites Beani for some fossil stems from the Lower Oolite beds of the Yorkshire coast, which Bean had previously referred to in unpublished notes as C. giganteus. The latter name was not adopted by Bunbury on account of the possible confusion between this species and the Palaeozoic species Calamites gigas Brongn. The generic name Calamites must be replaced by Equisetites now that we are familiar with more perfect specimens which demonstrate the Equisetean characters of the plant.

Fig. 60. Equisetites Beani (Bunb.). ⅔ nat. size. [After Starkie Gardner (86) Pl. IX. fig. 2.]

Schimper[519] speaks of this species as possibly the pith-cast of Equisetites columnaris, but his opinion cannot be maintained; the species first described by Bunbury has considerably larger stems than those of E. columnaris. It is not impossible, however, that E. columnaris and E. Beani may be portions of the same species. The chief difference between these forms is that of size; but we have not sufficient data to justify the inclusion of both forms under one name. Zigno[520], in his work on the Oolitic Flora, figures an imperfect stem cast of E. Beani under the name of Calamites Beani, but the species has received little attention at the hands of recent writers. In 1886 Starkie Gardner[521] figured a specimen which was identified by Williamson as an example of Bunbury’s species; but the latter pointed out the greater resemblance, as regards the external appearance of the Jurassic stem, to some of the recent arborescent Gramineae[522] than to the Equisetaceae. Williamson, with his usual caution, adds that such appearances have very little taxonomic value. Fig. 60 is reproduced from the block used by Gardner in his memoir on Mesozoic Angiosperms; he quotes the specimen as possibly a Monocotyledonous stem. The fossil is an imperfect cast of a stem showing two clearly marked nodal regions, but no trace of leaf-sheaths. A recent examination of specimens in the museums of Whitby, Scarborough, York and London has convinced me that the plant named by Bunbury Calamites Beani is a large Equisetites. As a rule the specimens do not show any indications of the leaf-sheaths, but in a few cases the sheaths have left fairly distinct impressions.

Fig. 61. Equisetites Beani (Bunb.). From a specimen in the British Museum, ⅔ nat. size. (No. V. 2725.)

In the portion of stem shown in fig. 61 the impressions of the leaf segments are clearly marked. This specimen affords much better evidence of the Equisetaceous character of the plant than those which are simply internal casts. The narrow projecting lines extending upwards from the nodes in the figured specimen probably represent the divisions between the several segments of each leaf-sheath.

In the museums of Whitby and Scarborough there are some long specimens, in one case 44 cm. in length, and 33 cm. in circumference, which are probably casts of the broad pith-cavity. These casts are often transversely broken across at the nodes, so that they consist of three or four separate pieces which fit together by clean-cut faces. This manner of occurrence is most probably due to the existence of large and resistant nodal diaphragms which separated the sand-casts of adjacent internodes. In the York museum there are some large diaphragms, 10 cm. in diameter, preserved separately in a piece of rock containing a cast of Equisetites Beani. The nodal diaphragms of some of the Carboniferous Calamites were the seat of cork development[523], and it may be that the frequent preservation of Equisetaceous diaphragms in Triassic and Jurassic rocks is due to the protection afforded by a corky investment.

The stem shown in fig. 62 appears to be a portion of a shoot of E. Beani not far from its apical region. From the lower nodes there extend clearly marked and regular lines or slight grooves tapering gradually towards the next higher node; these are no doubt the impressions of segments of leaf-sheaths. The sheaths themselves have been detached and only their impressions remain. The flattened bands at the node of the stem in fig. 60, and shown also in fig. 61, mark the place of attachment of the leaf-sheaths. On some of these nodal bands one is able to recognise small scars which are most likely the casts of outgoing leaf-trace bundles.

Some of the internal casts of this species are marked by numerous closely arranged longitudinal lines, which are probably the impressions of the inner face of a central woody cylinder. In the smaller specimen shown in fig. 62 we have the apical portion of a shoot in which the uppermost internodes are in an unexpanded condition.

Fig. 62. Equisetites Beani (Bunb.). From a specimen in the Scarborough Museum. Very slightly reduced.

It is impossible to give a satisfactory diagnosis of this species without better material. The plant is characterised chiefly by the great breadth of the stem, and by the possession of leaf-sheaths consisting of numerous long and narrow segments. Equisetites Beani must have almost equalled in size the Triassic species, E. arenaceus, described above.

7. Equisetites lateralis Phill. Figs. 58, F, 63, and 64.

This species is described at some length as affording a useful illustration of the misleading character of certain features which are entirely due to methods of preservation. The specific name was proposed by Phillips in his first edition of the Geology of the Yorkshire Coast for some very imperfect stems from the Lower Oolite rocks near Whitby[524]. The choice of the term lateralis illustrates a misconception; it was given to the plant in the belief that certain characteristic wheel-like marks on the stems were the scars of branches. Lindley and Hutton[525] figured a specimen of this species in their Fossil Flora, and quoted a remark by “Mr Williamson junior” (afterwards Prof. Williamson) that the so-called scars often occur as isolated discs in the neighbourhood of the stems. Bunbury[526] described an example of the same species with narrow spreading leaves like those of a Palaeozoic Asterophyllites, and proposed this generic name as more appropriate than Equisetites. In all probability the example shown in fig. 63 is that which Bunbury described. It is certainly the same as one figured by Zigno[527] as Calamites lateralis in his Flora fossilis formationis Oolithicae.

Fig. 63. Equisetites lateralis Phill. From a specimen in the British Museum. Slightly reduced.

This specimen illustrates a further misconception in the diagnosis of the species. The long linear appendages spreading from the nodes are, I believe, slender branches and not leaves; they have not the form of delicate filmy markings on the rock face, but are comparatively thick and almost woody in appearance. The true leaves are distinctly indicated at the nodes, and exhibit the ordinary features of toothed sheaths.

Heer[528] proposed to transfer Phillips’ species to the genus Phyllotheca, and Schimper[529] preferred the generic term Schizoneura. The suggestion for the use of these two names would probably not have been made had the presence of the Equisetum sheaths been recognised.

The circular depressions a short distance above each node are the ‘branch scars’ of various writers. Schimper suggested that these radially marked circles might be displaced nodal diaphragms. Andrae[530] figured the same objects in 1853 but regarded them as branch scars, although in the specimen he describes, there are several of them lying apart from the stems, and to one of them is attached a portion of a leaf-sheath. Solms-Laubach[531] points out that the internodal position of these supposed scars is an obvious difficulty; we should not expect to find branches arising from an internode. After referring to some specimens in the Oxford museum, he adds—“In presence of these facts the usual explanation of these structures appears to me, as to Heer, very doubtful.... We are driven to the very arbitrary assumption that they represent the lowest nodes of the lateral branches which were inserted above the line of the nodes of the stem.” Circular discs similar to those of E. lateralis have been found in the Jurassic rocks of Siberia[532] and elsewhere. There are one or two examples of such discs from Siberia in the British Museum. If the nodal diaphragms were fairly hard and stout, it is easy to conceive that they might have been pressed out of their original position when the stems were flattened in the process of fossilisation. It is not quite clear what the radial spoke-like lines of the discs are due to; possibly they mark the position of bands of more resistant tissue or of outgoing strands of vascular bundles. A detached diaphragm is seen in fig. 64 C; in the centre it consists of a flat plate of tissue, and the peripheral region is traversed by the radiating lines. In the stem of fig. 64, A the deeply divided leaf-sheaths are clearly seen, and an imperfect impression of a diaphragm is preserved on the face of the middle internode. In fig. 64 B a flattened leaf-sheath is shown with the free acuminate teeth fused basally into a continuous collar[533]. The short piece of stem of Equisetites lateralis shown in fig. 58, F, shows how the free teeth may be outspread in a manner which bears some resemblance to the leaves of Phyllotheca, but a comparison with the specimens already described, and a careful examination of this specimen itself, demonstrate the generic identity of the species with Equisetites. The carbonaceous film on the surface of such stems as those of fig. 58, F, and 64, A, shows a characteristic shagreen texture which may possibly be due to the presence of silica in the epidermis as in recent Horse-tails.

There is another species of Equisetites, E. Münsteri, Schk., from a lower geological horizon which has been compared with E. lateralis, and lends support to the view that the so-called branch-scars are nodal diaphragms[534]. This species also affords additional evidence in favour of retaining the generic name Equisetites for Phillips’ species. Equisetites Münsteri is a typical Rhaetic plant; it has been found at Beyreuth and Kuhnbach, as well as in Switzerland, Hungary and elsewhere. A specimen of Equisetites originally described by Buckman as E. Brodii[535], from the Lower Lias of Worcestershire, may possibly be identical with E. Münsteri. The leaf-sheaths of this Rhaetic species consist of broad segments prolonged into acuminate teeth; some of the examples figured by Schenk[536] show clearly marked impressions of displaced nodal diaphragms exactly as in E. lateralis. Another form, Equisetum rotiferum described by Tenison-Woods[537] from Australia, is closely allied to, or possibly identical with E. lateralis.

Fig. 64. Equisetites lateralis Phill. A. Part of a stem showing leaf-sheaths and an imperfect diaphragm. B. A single flattened leaf-sheath. C. A detached nodal diaphragm. From a specimen in the York Museum. Slightly reduced.

8. Equisetites Burchardti Dunker[538]. Fig. 65.

This species of Equisetites is fairly common in the Wealden beds of the Sussex coast near Hastings, and also in Westphalia.

Fig. 65. Equisetites Burchardti Dunk. Showing a node with two tubers and a root. From a specimen in the British Museum. Nat. size.

It is characterised by having long and slender internodes, bearing at the nodes leaf-sheaths with five or six pointed segments, and by the frequent formation of branch-tubers. These tuberous branches closely resemble those which are formed on the underground shoots of Equisetum arvense L., E. sylvaticum L. and others; they occur either singly or in chains[539]. In the specimen shown in the figure the left-hand tuber is remarkably well preserved, its surface is somewhat sunk and shrivelled, and the apex is surrounded by a nodal leaf-sheath. A thin branched root is given off just below the point of insertion of the oval tuber.

No other species of Equisetites affords such numerous examples of tubers as this Wealden plant. By some of the earlier writers the detached tubers of E. Burchardti were described as fossil seeds under the name Carpolithus.

Fig. 66. Equisetites Yokoyamae Sew. From specimens in the British Museum. Nat. size.

The specimens shown in fig. 66 have been referred to another species, E. Yokoyamae Sew.[540]; they were obtained from the Wealden beds of Sussex, but according to Mr Rufford, who discovered them, the smaller tubers of this species are not found in association with those of E. Burchardti. The stems are very narrow and the tubers have a characteristic elliptical form; the species is of little value botanically, but it affords another instance of the common occurrence of these tuberous branches in the Wealden Equisetums.

Similar fossil tubers, on a much larger scale, have been found in association with the Triassic Equisetites arenaceus; with E. Parlatori Heer[541], a Tertiary species from Switzerland, and with other Mesozoic and Tertiary stems. E. Burejensis[542], described by Heer from the Jurassic rocks of Siberia, bears a close resemblance to the Wealden species.

•••••

The description of the above species by no means exhausts the material which is available towards a history of fossil Equisetums. The examples which have been selected may serve to illustrate the kind of specimens that are usually met with, as well as some of the possible sources of error which have to be borne in mind in the description of species.

Such Tertiary species as have been recorded need not be considered; they furnish us with no facts of particular interest from a morphological point of view. The wide distribution of Equisetites, especially during the Jurassic period, is one of the most interesting lessons to be learnt from a review of the fossil forms. No doubt a detailed comparison of the several species from different parts of the world would lead us to reduce the number of specific names; and at the same time it would emphasize the apparent identity of fossils which have been described from widely separated latitudes under different names.

Specimens of Equisetites are occasionally found in plant-bearing beds apart from the other members of a Flora; this isolated manner of occurrence suggests that the plant grew in a different station from that occupied by Cycads and other elements of the vegetation[543].

A selection of Triassic and Jurassic species arranged in a tabular form demonstrates the world-wide distribution of this persistent type of plant[544].

B. Phyllotheca.

The generic name Phyllotheca was proposed by Brongniart[545] in 1828 for some small fossil stems from the Hawkesbury river, near Port Jackson, Australia. The stems of this genus are divided into nodes and internodes and possess leaf-sheaths as in Equisetum, but Phyllotheca differs from other Equisetaceous plants in the form of the leaves and in the character of its sporophylls. We may define the genus as follows:—

Plants resembling in habit the recent Equisetums. Stems simple or branched, divided into distinct nodes and internodes, the latter marked by longitudinal ridges and grooves; from the nodes are given off leaf-sheaths consisting of linear-lanceolate uninerved segments coherent basally, but having the form of free narrow teeth for the greater part of their length. The long free teeth are usually spread out in the form of a cup and not adpressed to the stem, the tips of the teeth are often incurved.

The sporangia are borne on peltate sporangiophores attached to the stem between whorls of sterile leaves.

Our knowledge of Phyllotheca is unfortunately far from complete. The chief characteristic of the vegetative shoots consists in the cup-like leaf-sheaths; these are divided up into several linear segments, which differ from the teeth of an Equisetum leaf-sheath in their greater length and in their more open and spreading habit of growth. The large loose sheaths of the fertile shoots of some recent Horse-tails bear a certain resemblance to the sheaths of Phyllotheca. The diagnosis of the fertile shoots is founded principally on some Permian specimens of the genus described by Schmalhausen from Russia[546] and redescribed more recently by Solms-Laubach[547]. Prof. Zeiller[548] has, however, lately received some examples of Phyllotheca from the Coal-Measures of Asia Minor which bear strobili like those of the genus Annularia, a type which is dealt with in the succeeding chapter. A description of a few species will serve to illustrate the features usually associated with this generic type, as well as to emphasize the unsatisfactory state of our knowledge as to the real significance of such supposed generic characteristics.

There are a few fossil stems from Permian rocks of Siberia, from Jurassic strata in Italy, and from Lower Mesozoic and Permo-Carboniferous beds in South America, South Africa, India and Australia which do not conform in all points to the usually accepted definition of Equisetites, and so justify their inclusion in an allied genus. On the other hand there are numerous instances of stems or branches which have been referred to Phyllotheca on insufficient grounds. Our knowledge of this Equisetaceous plant has recently been extended by Zeiller[549], who has recorded its occurrence in the Coal-Measures of Asia Minor associated with typical Upper Carboniferous plants. The same author[550] has also brought forward good evidence for the Permian age of the beds in Siberia and Altai, where Phyllotheca has long been known. It is true that Zigno’s species of the genus occurs in Italian Jurassic rocks, but on the whole it would seem that this genus is rather a Permian than a Jurassic type. The species which Zeiller describes under the name Phyllotheca Rallii from the Coal-Measures of Herakleion (Asia Minor) shows some points of contact with Annularia. It is much to be desired, however, that we might learn more as to the reproductive organs of this member of the Equisetales; until we possess a closer acquaintance with the fructification we cannot hope to arrive at any satisfactory conclusion as to the exact position of the genus among the Calamarian and Equisetaceous forms. M. Zeiller[551] informs me that his specimens of P. Rallii, which are to be fully described in a forthcoming work, include fossil strobili resembling those of Annularia radiata. The verticils of linear leaves fused basally into a sheath agree in appearance with the star-like leaves of Annularia, but in Phyllotheca Rallii the segments appear to spread in all directions and are not extended in one plane as in the typical Annularia[552].

1. Phyllotheca deliquescens (Göpp.).

In an account of some fossil plants collected by Tchikatcheff in Altai, Göppert[553] describes and figures two imperfect stems of an Equisetum-like plant. Owing to the apparent absence of nodal lines on the surface of the stem the generic name Anarthrocanna is proposed for the fossils; and the manner in which the main axis appears to break up into slender branches suggested the specific name deliquescens. Schmalhausen[554] afterwards recognised the generic identity of Göppert’s fragments with the Indian and Australian stems referred to the genus Phyllotheca by McCoy[555] and Bunbury[556].

We may define the species as follows:—

Stem reaching a diameter of 2–3 cm. with internodes as much as 4 cm. long, the surface of which is traversed by longitudinal ridges and grooves which are continuous and not alternate at the nodes. Branches arise in verticils from the nodes. The leaves have the form of funnel-shaped sheaths split up into narrow and spreading linear segments, each of which is traversed by a median vein. The fertile shoot terminates in a loose strobilus bearing alternating whorls of sterile bracts and sporangiophores.

The specimens on which this diagnosis is founded are for the most part fragments of sterile branches. Some of these present the appearance of Calamitean stems in which the ridges and grooves continue in straight lines from one internode to the next. Similar stem-casts have been referred by some writers to the allied genus Schizoneura, and it would appear to be a hopeless task to decide with certainty under which generic designation such specimens should be described. The portion of stem shown in fig. 67 affords an example of an Equisetaceous plant, probably in the form of a cast of a hollow pith, which might be referred to either Phyllotheca or Schizoneura. The specimen was found in certain South African rocks which are probably of Permo-Carboniferous age[557]. It agrees closely with some stems from India described by Feistmantel[558] as Schizoneura gondwanensis, and it also resembles equally closely the Australian specimens referred by Feistmantel[559] to Phyllotheca australis and some stems of Phyllotheca indica figured by Bunbury[560].

The longitudinal ridges and grooves shown in fig. 67 probably represent the broad medullary rays and the projecting wedges of secondary wood surrounding a large hollow pith, as in Calamites. In the Calamitean casts the ridges and grooves of each internode usually alternate in position with those of the next, as in Equisetum (fig. 54, A), but in Phyllotheca, Schizoneura and Archaeocalamites there is no such regular alternation at the nodes of the internodal vascular strands.

Fig. 67. Phyllotheca? ¾ nat. size. From a South African specimen of Permo-Carboniferous age in the British Museum.

In Phyllotheca and Schizoneura there are no casts of ‘infranodal canals’ below each nodal line, but these are by no means always found in true Calamites. It is therefore practically impossible to determine the generic position of such fossils as that shown in fig. 67 without further evidence than is afforded by leafless casts.

A few examples of Phyllotheca deliquescens have been described by Schmalhausen in which a branch bears clusters of sporangiophores, alternating with verticils of sterile bracts. The sporangiophores appear to have the form of stalked peltate appendages bearing sporangia, very similar to the sporangiophores of Equisetum. Solms-Laubach[561] has examined the best of Schmalhausen’s specimens, and a carefully drawn figure of one of the fertile branches is given in his Fossil Botany.

The significance of this manner of occurrence of sporangiophores and whorls of sterile bracts on the fertile branch will be better understood after a description of the strobilus of Calamites. In Phyllotheca the sporangiophores appear to have been given off in whorls, which were separated from one another by whorls of sterile bracts, whereas in Equisetum there are no sterile appendages associated with the sporangiophores of the strobilus, with the exception of the annulus at the base of the cone. Heer[562] first drew attention to the fact that in Phyllotheca we have a form of strobilus or fertile shoot to a certain extent intermediate in character between Equisetum and Calamites.

In abnormal fertile shoots of Equisetum, sporophylls occasionally occur above and below a sterile leaf-sheath. Potonié[563] has figured such an example in which an apical strobilus is succeeded at a lower level by a sterile leaf-sheath, and this again by a second cluster of sporophylls. As Potonié points out, this alternation of fertile and sterile members affords an interesting resemblance between Phyllotheca and Equisetum. It suggests a partial reversion towards the Calamitean type of strobilus.

2. Phyllotheca Brongniarti Zigno. Fig. 68, A.

This species of Phyllotheca from the Lower Oolite rocks of Italy is known only in the form of sterile branches. The leaves are fused basally into an open cup-like sheath which is dissected into several spreading and incurved linear segments. The internodes are striated longitudinally; they are about 2 mm. in diameter and 10 mm. in length.

The specimen represented in fig. 68, A, was originally described by the Italian palaeobotanist Zigno[564]; it serves to illustrate the points of difference between this genus and the ordinary Equisetum. The open and spreading sheaths clasping the nodes and the erect solitary branches give the plant a distinctive appearance.

Fig. 68.

1. Phyllotheca Brongniarti, Zigno. Nat. size. (After Zigno.)
2. Calamocladus frondosus, Grand’Eury. (After Grand’Eury.) Slightly enlarged.
3. Phyllotheca indica, Bunb. Part of a leaf-sheath. From a specimen in the Museum of the Geological Society. Slightly enlarged.

3. Phyllotheca indica Bunb. and P. australis Brongn. Fig. 68, C.

Sir Charles Bunbury[565] described several imperfect specimens from the Nagpur district of India under this name, but he expressed the opinion that it was not clear to him if the plant was specifically distinct from the Phyllotheca australis Brongn. previously recorded from New South Wales. Feistmantel[566] subsequently described a few other Indian specimens, but did not materially add to our knowledge of the genus. Bunbury’s specimens were obtained from Bharatwádá in Nagpur, in beds belonging to the Damuda series of the Lower Gondwana rocks, usually regarded as of about the same age as the Permian rocks of Europe.

Phyllotheca indica is represented by broken and imperfect fragments of leaf-bearing stems. The species is thus diagnosed by Bunbury:—“Stem branched, furrowed; sheaths lax, somewhat bell-shaped, distinctly striated; leaves narrow linear, with a strong and distinct midrib, widely spreading and often recurved, nearly twice as long as the sheaths.” An examination of the specimens in the Museum of the Geological Society of London, on which this account was based, has led me to the opinion that it is practically impossible to distinguish the Indian examples from P. australis described by Brongniart[567] from New South Wales. The few specimens of the latter species which I have had an opportunity of examining bear out this view. In the smaller branches the axis of P. indica is divided into rather short internodes on which the ridges and grooves are faintly marked. In the larger stems the ridges and grooves are much more prominent, and continuous in direction from one internode to the next; a few branches are given off from the nodes of some of the specimens. The leaves are not very well preserved; they consist of a narrow collar-like basal sheath divided up into numerous, long and narrow segments, which are several times as long as the breadth of the sheath, and not merely twice as long as Bunbury described them. Each leaf-sheath has the form of a very shallow cup-like rim clasping the stem at a node, with long free spreading segments which are often bent back in their distal region. The general habit of the leafy branches appears to be identical with that of P. australis as figured by McCoy.

Prof. Zeiller informs me that in the type-specimen on which Brongniart founded the species, P. australis, the sheath appears to be closely applied to the stem with a verticil of narrow spreading segments radiating from its margin. It may be, therefore, that in the Australian form there was not such an open and cup-like sheath as in P. indica; but it would be difficult, without better material before us, to feel confidence in any well marked specific distinctions between the Indian and Australian Phyllothecas.

On the broader stems, such as that of fig. 67, we have clearly marked narrow grooves and broader and slightly convex ridges, which present an appearance identical with that of some Calamitean stems. In the specimen figured by Bunbury[568] in his Pl. X, fig. 6, there is a circular depression on the line of the node which represents the impression of the basal end of a branch; on the edges of the node there are indications of two other lateral branches. The nature of this stem-cast points umnistakeably to a woody stem like that of Calamites. The precise meaning of the ridges and grooves on the cast is described in the Chapter dealing with Calamitean plants.

CALAMOCLADUS.

Grand’Eury[569] in his monograph on the coal-basin of Gard, has recently described under the name of Calamocladus frondosus what he believes to be the leaf-bearing axes of a Calamitean plant. The thicker branches are almost exactly identical in appearance with the broader specimens of Phyllotheca. The finer branches of Calamocladus bear cup-like leaf-sheaths which are divided into long and narrow recurved segments (fig. 67, B), precisely as in Phyllotheca. These comparisons lead one to the opinion that the Phyllotheca of Australia and India may be a close ally of the Permo-Carboniferous Calamitean plants. The form of the leaf-whorls of Annularia (Calamarian leaf-bearing branches) and of Calamocladus is of the same type as in Phyllotheca; the character of the medullary casts is also the same. The nature of the fertile shoot of Phyllotheca described by Schmalhausen from Siberia, with its alternating whorls of sterile and fertile leaves, is another point of agreement between this genus and Calamitean plants. An Equisetaceous species has been described from the Newcastle Coal-Measures of Australia by Etheridge[570] in which there are two forms of leaves, some of which closely resemble those of Phyllotheca indica, while others are compared with the sterile bracts of Cingularia, a Calamitean genus instituted by Weiss[571].

When we turn to other recorded forms of Phyllotheca many of them appear on examination to have been placed in this genus on unsatisfactory grounds. Heer figures several stem fragments from the Jurassic rocks of Siberia as P. Sibirica Heer[572], and it was the resemblance between this form and the English Equisetites lateralis which led to the substitution of Phyllotheca for Equisetites in the latter species. Without examining Heer’s material it is impossible to criticise his conclusions with any completeness, but several of his specimens, appear to possess leaf-sheaths more like those of Equisetum than of Phyllotheca.

The frequent occurrence of isolated diaphragms and the comparatively long acuminate teeth of the leaf-sheath afford obvious points of resemblance to Equisetites lateralis. Some of the examples figured by Heer appear to be stem fragments, with numerous long and narrow filiform leaves different in appearance from those of other specimens which he figures. It may be that some of the less distinct pieces of stems are badly torn specimens in which the internodes have been divided into filiform threads. Heer also figures a fertile axis associated with the sterile stems, and this does not, as Heer admits, show the alternating sterile bracts such as Schmalhausen has described. So far as it is possible to judge from an examination of Heer’s figures and a few specimens from Siberia in the British Museum—and this is by no means a safe basis on which to found definite opinions—there appears to be little evidence in favour of separating the fossils described as Phyllotheca Sibirica from Equisetites. This Siberian form may indeed be specifically identical with Equisetites lateralis Phill.

Various species of Phyllotheca have been described from Jurassic and Upper Palaeozoic rocks in Australia. Some of these possess cup-like leaf-sheaths, and in the case of the thicker specimens they show continuous ridges and grooves on the internodes, as well as a habit of branching similar to that in some of the Italian Phyllothecas. In some of the stems it is however difficult to recognise any characters which justify the use of the term Phyllotheca. A fragment figured by Tenison-Woods[573] as a new species of Phyllotheca, P. carnosa, from Ipswich, Queensland, affords an example of the worthless material on which species have not infrequently been founded. The author of the species describes his single specimen as a “faint impression”; the figure accompanying his description suggests a fragment of some coniferous branch, as Feistmantel has pointed out in his monograph on Australian plants.

It is important that a thorough comparative examination should be made of the various fossil Phyllothecas with a view to determine their scientific value, and to discover how far the separation of Phyllotheca and Equisetites is legitimate in each case. There is too often a tendency to allow geographical distribution to decide the adoption of a particular generic name, and this seems to have been especially the case as regards several Mesozoic and Palaeozoic Southern Hemisphere plants.

The geological and geographical range of Phyllotheca is a question of considerable interest, but as already pointed out it is desirable to carefully examine the various records of the genus before attempting to generalise as to the range of the species. Phyllotheca is often spoken of as a characteristic member of the Glossopteris Flora of the Southern Hemisphere, and its geological age is usually considered to be Mesozoic rather than Palaeozoic.

C. Schizoneura.

The plants included under this genus were originally designated by Brongniart[574] Convallarites and classed as Monocotyledons. Some years later Schimper and Mougeot[575] had the opportunity of examining more perfect material from the Bunter beds of the Vosges, and proposed the new name Schizoneura in place of Brongniart’s term, on the grounds that the specimens were in all probability portions of Equisetaceous stems, and not Monocotyledons. Our knowledge of this genus is very limited, but the characteristics are on the whole better defined than in the case of Phyllotheca. The following diagnosis illustrates the chief features of Schizoneura.

Hollow stems with nodes and internodes as in Equisetum; the surface of the internodes is traversed by regular ridges and grooves, which are continuous and not alternate in their course from one internode to the next. The leaf-sheaths are large and consist of several coherent segments; the sheaths are usually split into two or more elongate ovate lobes, and each lobe contains more than one vein. Fertile shoots are unknown.

Two of the best known and most satisfactory species are Schizoneura gondwanensis Feist. and S. paradoxa Schimp. and Moug.

Schizoneura gondwanensis Feist. Fig. 69, A and B.

This species is represented by numerous specimens from the Lower Gondwana rocks of India[576]; it is characterised by narrow articulated stems which bear large leaf-sheaths at the nodes. The sheaths may have the form of two large and spreading elongate-oval lobes, each of which is traversed by several veins (fig. 69, B), or the lobes may be further dissected into long linear single-veined segments, as in fig. 69, A. It is supposed that in the young condition each node bears a leaf-sheath consisting of laterally coherent segments which, as development proceeds, split into two or more lobes. Feistmantel records this species from the Talchir, Damuda and Panchet divisions of the Lower Gondwana series of India; these divisions are regarded as equivalent to the Permo-Carboniferous and Triassic rocks of Europe. The two specimens shown in fig. 69 are from the Lower Gondwana rocks of the Raniganj Coal-field, India.

As already pointed out[577], some of the specimens of flat and broader stems referred by Feistmantel to Schizoneura are identical in appearance with stems which have been described from India and elsewhere as species of Phyllotheca.

Fig. 69. Schizoneura gondwanensis Feist. (After Feistmantel; slightly reduced.)

There are a few specimens of S. gondwanensis in the British Museum, but the genus is poorly represented in European collections.

A similar plant was described in 1844 by Schimper and Mougeot[578] from the Bunter rocks of the Vosges as Schizoneura paradoxa. This species bears a very close resemblance to the Indian forms, and indeed it is difficult to point to any distinction of taxonomic importance. Feistmantel considers that the European plant has rather fewer segments in the leaf-sheaths, and that the Indian plant had somewhat stronger stems. Both of these differences are such as might easily be found on branches of the same species. It is, however, interesting to notice the very close resemblance between the Lower Trias European plant and the somewhat older member of the Glossopteris flora recorded from India and other regions, which probably once formed part of that Southern Hemisphere Continent which is known as Gondwana Land[579].

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