I. SPHENOPHYLLUM.

The genus Sphenophyllum is placed in a special class, as representing a type which cannot be legitimately included in any of the existing groups of Vascular Cryptogams. Although this Palaeozoic genus possesses points of contact with various living plants, it is generally admitted by palaeobotanists that it constitutes a somewhat isolated type among the Pteridophytes of the Coal-Measures. Our knowledge of the anatomy of both vegetative shoots and strobili is now fairly complete, and the facts that we possess are in favour of excluding the genus from any of the three main divisions of the Pteridophyta.

In Scheuchzer’s Herbarium Diluvianum there is a careful drawing of some fragments of slender twigs, from an English locality, bearing verticils of cuneiform leaves, which the author compares with the common Galium[860]. As regards superficial external resemblance, the Galium of our hedgerows agrees very closely with what must have been the appearance of fresh green shoots of Sphenophyllum.

A twig of the same species of Sphenophyllum is figured by Schlotheim[861] in the first part of his work on fossil plants; he regards it as probably a fragment of some species of Palm. Sternberg[862] was the first to institute a generic name for this genus of plants, and specimens were described by him in 1825 as a species of the genus Rotularia. The name Sphenophyllites was proposed by Brongniart[863] in 1822 as a substitute for Schlotheim’s genus, and in a later work[864] the French author instituted the genus Sphenophyllum. Dawson[865] was the first to make any reference to the anatomy of this genus; but it is from the examination of the much more perfect material from St. Étienne, Autun, and other continental localities, the North of England and Pettycur in Scotland, by Renault, Williamson, Zeiller and Scott, that our more complete knowledge has been acquired.

The affinity of Sphenophyllum has always been a matter of speculation; it has been compared with Dicotyledons, Palms, Conifers (Ginkgo and Phyllocladus), and various Pteridophytes, such as Ophioglossum, Tmesipteris, Marsilia, Salvinia, Equisetum and the Lycopodiaceae[866].

DEFINITION.

We may define the genus Sphenophyllum as follows:—

Stem comparatively slender (1·5–15 mm.?), articulated, usually somewhat tumid at the nodes; the surface of the internodes is marked by more or less distinct ribs and grooves which do not alternate at the nodes, but follow a straight course from one internode to the next. A single branch is occasionally given off from a node. Adventitious roots are very rarely seen, their surface does not show the ridges and grooves of the foliage-shoots.

The leaves are borne in verticils at the nodes, those in the same whorl being usually of the same size, but in some forms two of the leaves are distinctly smaller than the others. Each verticil contains normally 6, 9, 12, 18 or more leaves, which are separate to the base and not fused into a sheath; the number of leaves in a verticil is not always a multiple of six. They vary in form from cuneiform with a narrow tapered base, and a lamina traversed by several forked veins, to narrow uninerved leaves and leaves with a lamina dissected into dichotomously branched linear segments. The leaves of successive whorls are superposed.

The strobili are long and narrow in form, having a length in some cases of 12 cm., and a diameter of 12 mm.; they occur as shortly stalked lateral branches, or terminate long leaf-bearing shoots. The axis of the cone bears whorls of numerous linear lanceolate bracts fused basally into a coherent funnel-shaped disc, bearing on its upper surface sporangiophores and sporangia.

The strobili are usually isosporous, but possibly heterosporous in some forms.

The stem is monostelic, with a triarch or hexarch triangular strand of centripetally developed primary xylem, consisting of reticulate, scalariform and spiral tracheae; the protoxylem elements being situated at the blunt corners of the xylem-strand. Foliar bundles are given off, either singly or in pairs, from each angle of the central primary strand. The secondary xylem consists of radially disposed reticulate or scalariform tracheae, developed from a cambium-layer. The phloem is made up of thin-walled elements, including sieve-tubes and parenchyma. Both xylem and phloem include secondary medullary rays of parenchymatous cells. The cortex consists in part of fairly thick-walled elements; in older stems the greater part of the cortical region is cut off by the development of deep-seated layers of periderm.

The roots are apparently diarch in structure, with a lacunar and smooth cortex.

•••••

The branch of Sphenophyllum emarginatum Brongn. given in fig. 109 shows the characteristic appearance of the genus as represented by this well-known species which Brongniart figured in 1822. The Indian species shown in fig. 111 illustrates the occurrence of unequal leaves in the same whorl, and in fig. 110, B, we have a form of verticil in which the leaves are deeply divided into filiform segments. A larger-leaved form is represented by S. Thoni, Mahr. (fig. 110, A), a species occasionally met with in Permian rocks.

No specimens of Sphenophyllum have so far been found attached to a thick stem; they always occur as slender shoots, which sometimes reach a considerable length. One of the longest examples known is in the collection of the Austrian Geological Survey; the axis is 4 mm. in breadth and 85 cm. long, bearing a slender branch 61 cm. in length. The manner of occurrence of the specimen as a curved slender stem on the surface of the rock suggests a weak plant, which must have depended for support on some external aid, either water or another plant. The anatomical structure and other features do not favour the suggestion of some writers that Sphenophyllum was a water-plant[867], but there would seem to be no serious obstacle in the way of regarding it as possibly a slender plant which flung itself on the branches and stems of stronger forest trees for support.

A. The anatomy of Sphenophyllum.

The following account of the structural features of the stem and root is based on the work of Renault[868], Williamson[869] and Williamson and Scott[870]. We may first consider such characters as have been recognised in different examples of the genus, and then notice briefly the distinguishing peculiarities of two well-marked specific types.

a. Stems.

i. Primary structure.

In a transverse section of a young Sphenophyllum stem such as that diagrammatically sketched in fig. 105, A, we find in the centre the xylem portion of a single stele with a characteristic triangular form. The primary xylem consists mainly of fairly large tracheae with numerous pits on their walls; towards the end of each arm the tracheids become scalariform, and at the apex there is a group of narrower spiral protoxylem elements. In the British species there is a single protoxylem group at the apex of each arm, but Renault has described some French stems in which the stele appears to be hexarch, having two protoxylem groups at the end of each of the three rays of the stele. The primary xylem strand of Sphenophyllum has therefore a root-like structure, the tracheids having been developed centripetally from the three initial protoxylem groups. This type of structure is typical of roots, but it also occurs in the stems of some recent Vascular Cryptogams.

Fig. 104. Diagrammatic longitudinal section of Sphenophyllum.
c, outer cortex; b, space next the stele, originally occupied by phloem etc.; a, xylem strand. (After Renault[871].) × 7.

As a rule the tissue next the xylem has not been petrified, but in exceptionally well-preserved examples it is seen to consist of a band of thin-walled elements, of which those in contact with the xylem may be spoken of as phloem, and those beyond as the pericycle. Succeeding this band of delicate tissue there is a broader band of thicker-walled and somewhat elongated elements, constituting the cortex. The specimen drawn in fig. 105, A, shows very prominent grooves in the cortex opposite the middle of each bay of the primary wood. It is these grooves that give to the ordinary casts of Sphenophyllum branches the appearance of longitudinal lines traversing each internode. In a longitudinal section of a stem, the cortical tissue (fig. 104, c) is found to be broader in the nodal regions, thus giving rise to the tumid nodes referred to in the diagnosis. The increased breadth at the nodes does not mean that the xylem is broader in these regions, as it is in Calamite stems. Small strands of vascular tissue are given off from the three edges of the triangular stele (fig. 105 A) at each node; these branch in passing through the cortex on their way to the verticils of leaves. The space b in the diagrammatic section of fig. 104 was originally occupied by the phloem and inner cortex. In some species of Sphenophyllum the apex of each arm of the xylem strand, as seen in transverse section, is occupied by a longitudinal canal surrounded by spiral tracheids, as in the primary xylem of the old stem shown in fig. 105, C.

Fig. 105. Sphenophyllum.

  1. Transverse section of young stem.
  2. Transverse section of the wood of a young stem; px, protoxylem; x, secondary xylem. (A and B. Sphenophyllum plurifoliatum.) × 20.
  3. Transverse section of an old stem; (S. insigne); a, phloem; b, periderm; c, fascicular secondary xylem; d, interfascicular secondary xylem. × 9. (No. 914 in the Williamson Collection.)
  4. Longitudinal section of the reticulate tracheae and medullary rays; r, r, r, of S. plurifoliatum. × 36.
  5. Similar section of S. insigne. × 75. (D and E after Williamson and Scott.)

ii. Secondary structure.

With the exception of very young twigs the petrified Sphenophyllum stems usually show a greater or less development of secondary wood. In the xylem-strand of fig. 105, B, the broad concave bays of the primary wood have been filled in by the development of two rows of large secondary tracheids, x, but opposite the protoxylem groups, px, there are no signs of cambial activity. In the unusually large stem represented by a rough sketch in fig. 105, C, the triangular primary xylem lies in the centre of a thick mass of secondary vascular tissue. The secondary and primary wood together have a diameter of about 5 mm.

After the bays between each protoxylem corner have been filled in, the formation of secondary wood proceeds uniformly along the stem radii, but the rows of tracheids and medullary rays which are developed opposite the corners of the primary strand, c, differ in certain characters from the broader masses of wood opposite the bays. For convenience, the secondary wood, c, opposite the protoxylem groups has been spoken of as fascicular wood, and the rest, d, as interfascicular wood.

The secondary xylem consists either of tracheae with numerous bordered pits on their radial walls (fig. 105, D), or of tracheae with broad and bordered scalariform pits (fig. 105, E). The suggestion of concentric rings of growth in the wood in fig. 105, C, is rather deceptive; there are no well-marked regular rings in Sphenophyllum stems, but irregular bands of smaller elements occasionally interrupt the uniformity of the secondary xylem. In some stems the medullary rays have the form of rows of parenchymatous cells, which in tangential longitudinal section are found to consist frequently of a single row of radially disposed elements; this type of medullary rays occurs in the species Sphenophyllum insigne, in which the tracheae are scalariform. Three medullary rays, r, are seen on the radial face of the scalariform tracheids in fig. 105, E, which represents a radial section of this species. In other species, e.g. S. plurifoliatum, the medullary rays have a peculiar and characteristic structure; in a transverse section of the stem they appear as groups of a few parenchymatous cells in the spaces between the truncated angles of the large tracheae (fig. 106). In longitudinal section these medullary-ray elements resemble thick bars stretching radially across the face of the tracheae (fig. 105, D, r); the apparent septa or bars are however thin-walled cells connecting the different groups of medullary-ray cells, as seen in a transverse section. These radial connecting cells are occasionally seen as short rays in transverse sections of stems.

The cambium and phloem elements are occasionally preserved in good specimens of older stems; the former consist of tabular flatted thin-walled cells, and the latter in some cases include large sieve-tubes and narrower parenchymatous elements.

The sections shown in fig. 107, E and F, illustrate the preservation of cambial and phloem tissue. In the transverse section of fig. 107, F, the secondary xylem with the medullary rays, r, is succeeded by a few tabular cambium cells, and external to these there are thin-walled elements of unequal size representing the phloem. In fig. 107, E, the scalariform tracheids are succeeded by narrow thin-walled cells, and the larger elements with transverse and oblique septa are no doubt sieve-tubes.

In the large stem of fig. 105, C, the xylem is succeeded by a band of tissue, a, which is no doubt phloem, and external to this there is a considerable development of periderm (b). The periderm in Sphenophyllum stems had a deep-seated origin, the phellogen or cork-cambium occasionally being formed in the secondary phloem-parenchyma, and in other cases in the pericycle, as in the stems of some living dicotyledons. Williamson and Scott[872] describe stems in which a succession of phellogens were formed at different levels, thus producing a scaly type of bark, such as we find in the Pine or the Plane tree.

SPHENOPHYLLUM PLURIFOLIATUM.

Before describing the structure of the strobili of Sphenophyllum, we may briefly point out the distinguishing features of two specific types of the genus recently described by Williamson and Scott. One of these species, S. insigne, was originally described by Williamson as an Asterophyllites; the numerous narrow linear leaves in each verticil led to the inclusion of the specimens in the latter genus. The material on which this species is founded is from the volcanic beds of Pettycur, Burntisland, on the coast of the Firth of Forth.

1. Sphenophyllum insigne (Will.). Figs. 105, C and E, and 107, E and F.

1891. Asterophyllites insignis. Williamson[873].

An intercellular space occurs at each angle of the three-rayed primary xylem strand, and spiral tracheae are abundant. The tracheae of the secondary wood have scalariform markings on the radial walls. Regular medullary rays extend through the secondary wood. The phloem contains large sieve-tubes.

This species occurs in the Calciferous sandstone rocks of Burntisland, and has lately been recorded from Germany. It characterises a lower horizon than S. plurifoliatum (Will. and Scott).

2. Sphenophyllum plurifoliatum (Williamson and Scott)[874]. Figs. 105, A, B, and D, and 106.

1891. Asterophyllites sphenophylloides. Will.[875]

The specific name plurifoliatum was proposed by Williamson and Scott for a type of stem originally described by Williamson[876] as an Asterophyllites, from the Coal-Measures of Oldham, Lancashire. This form of stem has not so far been connected with any of the older species founded on external characters, but it evidently bore foliage in which the leaves were deeply divided, as in Sphenophyllum trichomatosum (fig. 110, B).

Fig. 106. Sphenophyllum plurifoliatum, Will. and Scott.
From a photograph by Mr Highly from a section in the Williamson Collection (no. 899). × 27.

In this species there are no canals at the angles of the primary xylem, and there are fewer spiral tracheae than in S. insigne. The tracheae of the secondary wood have numerous small pits on the radial walls, and the medullary rays are chiefly composed of parenchymatous cells, which appear in transverse section as groups of cells between the truncated angles of the tracheae. The characters are fairly well seen in the xylem portion of a stele shown in fig. 106. The fascicular wood includes some rows of parenchymatous medullary-ray cells in addition to the characteristic groups, as seen in the figure. A slightly oblique transverse section of a stem is often convenient in the interpretation of histological features; one of the sections of S. plurifoliatum in the Williamson collection (no. 893), which has been cut somewhat obliquely, shows very clearly the differences in pitting exhibited by the different xylem elements.

b. Roots.

Our knowledge of the anatomy of Sphenophyllum roots is very limited. Renault has described a somewhat imperfect example of a silicified root from St. Étienne and Autun. The drawing in fig. 107, B, which is copied from one of Renault’s figures, shows a cylindrical mass of xylem with a small band of narrower elements occupying the centre, and surrounded by rows of larger secondary tracheae. The central bipolar band is described as the diarch primary xylem, around which the secondary pitted elements have been developed.

It is probable that the specimen described by Renault is a root of Sphenophyllum, but my impression gained from an examination of the section was that the diarch primary strand is not quite so clear as in the published figures. Until we possess better material we cannot attempt any very satisfactory description of the anatomical features of the roots of this genus.

A section of a Sphenophyllum stem has been figured by Felix[877], in which a lateral member is being given off; this may possibly represent the origin of an adventitious root, but the preservation is not sufficiently distinct to render this certain.

c. Leaves.

Renault[878] has described some silicified leaves of Sphenophyllum from Autun in which the laminae consist of thin-walled loose parenchyma, traversed by small groups of tracheids constituting the simple or forked veins. The epidermis is made up of a single layer of cells, with here and there indistinct indications of stomata. A more perfect stoma has, however, been described by Solms-Laubach from the epidermis of a bract in a strobilus (fig. 107, A).

Fig. 107. A. Stoma in a bract of Sphenophyllostachys. B. Root of Sphenophyllum. C. Sphenophyllostachys Römeri, Solms. s, sporangiophore, b, bract. D. Sporangium. E and F. Sections through the cambium, phloem and secondary xylem of Sphenophyllum insigne (Will.). s, sieve-tube. G. Sporangium and pedicel. A, C, D. After Solms-Laubach. B. After Renault. E–G. After Williamson and Scott. E. F. × 100. G. × 115.

d. Cones.

The history of the recognition of the cones of Sphenophyllum has already been briefly alluded to in chapter V., p. 100. The main points in the structure of the cones of this genus were known for several years, before the fact was established that they belonged to Sphenophyllum stems. In 1871 Williamson[879] published an account of an imperfect fossil strobilus from the Lower Coal-Measures of Oldham, Lancashire, under the name of Volkmannia Dawsoni. The generic term Volkmannia has been used by different writers for cones varying considerably in structural features; in the case of Williamson’s fossil, Weiss[880] substituted the name Bowmanites, a genus instituted by Binney[881] for a strobilus apparently of the same type as Volkmannia Dawsoni. In 1891 Williamson[882] described some additional specimens of Bowmanites Dawsoni, and, as in his earlier paper, he compared the strobilus with Asterophyllites and Sphenophyllum, but it was still a matter of speculation as to what was the form of the vegetative branches. Soon after the more complete account of the English cones was published, Zeiller[883] recognised a close agreement between some French and Belgian specimens of Sphenophyllum strobili and the strobilus described by Williamson. A closer comparison thoroughly established the connection between Bowmanites Dawsoni and Sphenophyllum; and there is little doubt that this strobilus belongs to the stem known as Sphenophyllum cuneifolium (Sternb.)—a well-known species of the genus.

STROBILUS.

The most important morphological features of the strobilus of Sphenophyllum may best be illustrated by a detailed account of one specific type, and by a brief reference to other forms which are characterised by certain differences in the number and attachment of the sporangia. When we know that a given strobilus must have grown on a Sphenophyllum stem, the obvious name to assign to it would seem to be that of the plant which bore it; but there are advantages in making use of special generic terms for detached cones, which cannot be referred with certainty to a particular species of stem. The genus Calamostachys affords an example of a name which is intended to denote that a cone so called belongs to a Calamarian plant; similarly such a name as Sphenophyllostachys may be used for Sphenophylloid cones which cannot be connected with certainty to particular species of Sphenophyllum. It has been suggested that the genus Bowmanites, first used for a cone which was afterwards recognised as belonging to a Sphenophyllum, should be employed instead of the sesquipedalian term Sphenophyllostachys. The latter is used here as being in accordance with a generally accepted and convenient system of nomenclature, and as a name which at once denotes the fact that the fossil is not only a cone but that it belongs to a Sphenophyllum.

Sphenophyllostachys Dawsoni (Will.). Figs. 107, A and G, 108.

Probably the strobilus of Sphenophyllum cuneifolium (Sternb.).

Fig. 108. Diagrammatic longitudinal section of a Sphenophyllum strobilus.
The upper figure represents a portion of a whorl of bracts. (The smaller figure, after Zeiller.)

The cone consists of a central axis bearing a number of verticils of bracts coherent in their lower portions in the form of a widely open funnel-shaped disc, which splits up peripherally into 14–20 linear-lanceolate segments. The free segments of each verticil have an obliquely ascending or almost vertical position, and extend upwards for a distance of about six internodes. The smaller drawing in fig. 108 shows the appearance in side view of the narrow bracts of a single whorl. A transverse section of a strobilus would include, therefore, sections of several concentric series of ascending bracts. The verticils of Sphenophyllostachys Dawsoni are probably superposed, but this point has not been definitely settled. From the upper surface of the coherent basal portion of each verticil, there are given off twice as many sporangiophores as there are free segments, and these are attached close to the line of junction of the axis of the cone and the funnel-shaped disc. Each sporangiophore has the form of a slender stalk which bends inwards at its distal end and bears a single sporangium (cf. fig. 107, D). The sporangiophores given off from the same verticil of bracts vary in length. All the sporangiophores are attached to the coherent bracts at the same distance from the axis of the cone; but as the sporangia between each verticil of bracts are arranged in two or three concentric series, it follows that the length of the sporangiophores varies considerably. The diagrammatic longitudinal section of a strobilus in fig. 108 shows three concentric series of sporangia between successive bract-verticils. A similar diagram was published by Williamson in 1892[884], and afterwards copied by Potonié[885], but in Williamson’s restoration the sporangiophores of the three series of sporangia are erroneously represented as arising from different points on the surface of the bracts. There is little doubt, as regards the strobilus of S. cuneifolium, that the sporangiophores were given off in a single series close to the axils of the bracts, as is partially shown in fig. 108.

The central part of the axis of the cone is occupied by a single triangular stele like that of the stem, except that each ray of the xylem strand has a comparatively broad blunt termination, and is not tapered to a narrow arm as in fig. 105, A and B. The wood consists of pitted tracheae, with two groups of protoxylem elements at each of the truncated angles of the solid strand of xylem. From the angles of the stele branches of vascular tissue pass out through the cortex to supply the sterile and fertile segments of each verticil. One of the transverse sections of the Sphenophyllum cone in the British Museum Collection (no. 1898 E) affords a good example of the misleading appearance occasionally presented by an intruded ‘rootlet’ of Stigmaria; the vascular tissue of the cone has disappeared, and a Stigmarian appendage with its vascular bundle occupies the position of the stelar tissues.

The bracts consist of parenchymatous tissue limited externally by an epidermis containing stomata. A single stoma with subsidiary cells is represented in fig. 107, A. The sporangiophores are composed internally of thin-walled cells with stronger cells towards the surface. The longer sporangiophores in a series may be more or less coherent for part of their length to the upper surface of the verticil of bracts. In fig. 108 the slender sporangiophores do not appear to come off always from the same portion of the bracts, but this is due to some of them lying on the surface of the latter during part of their course to support the external circle of sporangia. The hook-like distal end of a sporangiophore, towards the point of attachment of the sporangium, is characterised by the larger size and greater prominence of the surface cells; these larger cells, which pass over the upper surface of a sporangium base, probably constitute a kind of annulus which determines the dehiscence of the sporangial wall[886].

Fig. 107, G, represents a sporangiophore and its sporangium cut through transversely just below the point of attachment of the latter to the end of the hook-like termination of the former. The spores are characterised by an irregularly reticulate thickening of the outer coat or exospore, as seen in the figure.

One of the chief points of interest suggested by a Sphenophyllum cone is the exact morphological nature of the sporangiophores. Are they branches borne in the axils of bracts, or may we regard each sporangiophore as a modified leaf, which has become coherent with the whorls of sterile leaves? Or is a sporangiophore merely a stalk of a sporangium; or a ventral lobe of a leaf, of which the sterile bracts represent the dorsal lobes? Although it is impossible without the evidence of development to decide with certainty between these alternatives, it would seem most probable that a sporangiophore may be looked upon as a ventral lobe of a leaf, the sterile lobes forming the bracts or members of the sterile whorls of the cone. This question is discussed by Zeiller[887] and Williamson and Scott[888], also more recently by Scott[889] in his memoir on Cheirostrobus.

Sphenophyllostachys Römeri (Solms-Laubach)[890]. Fig. 107, C and D.

In another type of Sphenophyllum strobilus, recently described by Solms-Laubach, the incurved end of each sporangiophore bore two sporangia. In most respects this species, which has not been found in connection with a vegetative shoot, agrees with Sphenophyllostachys Dawsoni.

In fig. 107, C, which is copied from one of Solms-Laubach’s drawings[891], we have an oblique transverse section of part of a strobilus, including portions of two series of sporangia borne on one verticil of bracts, and at the right-hand edge the section has passed through the sporangia belonging to another whorl of bracts. There were probably three concentric series of sporangia attached to each verticil of bracts, as in the case of fig. 108. The unshaded area, b (fig. 107, C), represents the bracts of two successive sterile whorls in transverse section. The shaded areas are the sporangia, with their sporangiophores, s. The relative position of the sporangia and sporangiophores suggests that each pedicel bore two sporangia at its tip, instead of one, as in the strobilus of Sphenophyllum cuneifolium (Sternb.).

A further variation in the structure of the strobili is illustrated by some specimens of S. trichomatosum Stur, described by Kidston[892], from the Coal-Measures of Barnsley. Each whorl of bracts bears a single series of oval sporangia which appear to be sessile on the basal portion of the whorl. It is possible that delicate sporangiophores may have been present, but in the imperfect examples in Kidston’s collection[893] the sporangia present the appearance of being seated directly on the surface of the bracts. As the specimens do not show any internal structure, it would be unwise to lay too much stress on the apparent absence of the characteristic sporangiophores. In any case, Kidston’s cones afford an illustration of the occurrence of a single series of sporangia in each whorl, instead of the pluriseriate manner of occurrence in some other species.

The statement is occasionally met with that some Sphenophyllum cones possessed two kinds of spores, but we are still in want of satisfactory evidence that this was really the case. Renault has described an imperfect specimen, which he considers points to the heterosporous nature of a Sphenophyllum cone, but Zeiller and Williamson and Scott have expressed doubts as to the correctness of Renault’s conclusions. While admitting the possibility of undoubted heterosporous strobili being discovered, we are not in a position to refer to Sphenophyllum as having borne strobili containing two kinds of spores[894].

SPHENOPHYLLUM EMARGINATUM.

[The following are some of the specimens in the Williamson Cabinet which illustrate the structure of Sphenophyllum:—

S. plurifoliatum.874, 882, 884, 893, 894, 897, 899, 901, 903, 908, 1893.
S. insigne.910, 914, 919, 921, 922, 924, 926, 1420, 1898.
Sphenophyllostachys.1049A–1049C, 1898.]

B. Types of vegetative branches of Sphenophyllum.

1822.Sphenophyllites emarginatus, Brongniart[895].
1828.Sphenophyllum emarginatum, Brongniart[896].
1828.Sphenophyllum truncatum, Brongniart[896].
1828.Rotularia marsileaefolia, Bischoff[897].
1862.Sphenophyllum osnabrugense, Römer[898].

Fig. 109. Sphenophyllum emarginatum (Brongniart).
From a specimen in the Collection of Mr R. Kidston, Upper Coal-Measures, Radstock. ⅚ nat. size.

This species of Sphenophyllum bears verticils of six or eight wedge-shaped leaves varying in breadth and in the extent of dissection of the laminae; they are truncated distally, and terminate in a margin characterised by blunt or obtusely-rounded teeth, each of which receives a single vein. The larger leaves are usually more or less deeply divided by a median slit. The narrow base of each leaf receives a single vein which branches repeatedly in a dichotomous manner in the substance of the lamina. Several drawings have been given by Sterzel[899] in a memoir on Permian plants, showing the variation in leaf-form in Sphenophyllum emarginatum, but as Kidston[900] and Zeiller[901] have pointed out Sterzel’s specimens probably belong to S. cuneifolium (Sternb.).

Branches are given off singly from the nodes, and the cones are borne at the tips of branches or branchlets. The cone of S. emarginatum agrees very closely with that of S. cuneifolium, and is of the same type as that shown in fig. 108. The small branch of S. emarginatum represented in fig. 109 does not show clearly the detailed characters of the species, as the leaf-margins are not well preserved.

In one of the largest specimens of this species which I have seen, in the Leipzig Museum, the main stem has internodes of about 3·9 cm. in length, from which a lateral branch with much shorter internodes is given off from a node.

It is important to notice the close resemblance, as pointed out by Zeiller, between some of the narrower-leaved forms of S. emarginatum and S. cuneifolium (Sternb.)[902]; but in the latter species the margins of the leaves have sharp, and not blunt teeth.

The cone described and figured by Weiss[903] as Bowmanites germanicus, since investigated by Solms-Laubach[904], must be referred to this species. Geinitz[905] figured a cone in 1855 as that of S. emarginatum, but his determination of the species is a little doubtful. Good figures of the true cone of S. emarginatum have been given by Zeiller[906] in his Flore de Valenciennes, as well as in his important memoir on the fructification of Sphenophyllum.

LEAVES.

2. Sphenophyllum trichomatosum Stur. Fig. 110, B.

The finely-divided leaves of the single whorl shown in fig. 110, B (from the Middle Coal-Measures of Barnsley, Yorkshire), afford an example of a form of Sphenophyllum which is represented by such species as S. tenerrimum Ett.[907], S. trichomatosum Stur[908], and S. myriophyllum[909] Crép. Probably the specimen should be referred to S. trichomatosum, but it is almost impossible to speak with certainty as to the specific value of an isolated leaf-whorl of this form. It has long been known that the leaves of Sphenophyllum may vary considerably, as regards the size of the segments, on the same plant; and the occurrence of such finely-divided leaves has lent support to an opinion which was formerly held by some writers, that Asterophyllites and Sphenophyllum could not be regarded as well-defined separate genera. This heterophylly of Sphenophyllum has thus been responsible for certain mistaken opinions both as to the relation of the genus to Calamocladus[910] (Asterophyllites), and as regards the view that the finely-divided laminae belonged to submerged leaf-whorls, while the broader segments were those of floating or subaerial whorls.

There is a very close resemblance between some of the deeply-cut and linear segments of a Sphenophyllum and the leaves of Calamocladus, but in the former genus the linear segments are found to be connected basally into a narrow common sheath. The assertion[911] that the deeply-cut leaves occur on the lower portions of stems is not supported by the facts. Kidston[912] has pointed out that the cones are often borne on branches with such leaves, and the same author refers to a figure by Germar, in which entire and much-divided leaves occur mixed together in the same individual specimen. M. Zeiller recently pointed out to me a similar irregular association of broader and narrower leaf-segments on the same shoots in some large specimens in the École des Mines, Paris. Cones of Sphenophyllum tenerrimum have been figured by Stur[913] and others; they are characterised by their small size and by the dissection of the slender free portions of the narrow bracts[914].

3. Sphenophyllum Thoni Mahr. Fig. 110, A.

Another type of Sphenophyllum is illustrated by S. Thoni Mahr as shown in fig. 110, A. This species was first described by Mahr[915] from the Coal-Measures of Ilmenau, and has since been figured by Zeiller and other authors. Each whorl consists of six large obcuneiform leaves with the broad margin somewhat irregularly fringed. The unusually good specimen of which fig. 110, A, represents a single verticil was originally described and figured by Zeiller in 1880[916]; it is now in the École des Mines Museum, Paris.

Fig. 110.

  1. Sphenophyllum Thoni, Mahr. (After Zeiller.)
  2. Sphenophyllum trichomatosum, Stur. From a specimen in the Woodwardian Museum; from the Coal-Measures of Barnsley, Yorks. A and B ¾ nat. size.

The leaf-forms illustrated by figs. 109 and 110 are some of the more extreme types of Sphenophyllum leaves; but these are more or less connected by a series of intermediate forms. For a more complete systematic account of the different species the student should consult such works as those by Coemans and Kickx[917], Zeiller, Schimper, and others.

4. Sphenophyllum speciosum (Royle). Fig. 111.

1834. Trizygia speciosa, Royle[918].

The species shown in fig. 111 has been usually described as a separate genus Trizygia, a name instituted by Royle in 1834 for some Indian fossils from the Lower Gondwana rocks of India[919]. Zeiller[920] has lately pointed out the advisability of including this Asiatic type in the genus Sphenophyllum. The slender stem bears verticils of cuneate leaves in three pairs at each node, the anterior pair being smaller than the two lateral pairs. The characteristic Sphenophyllum venation is clearly seen in the enlarged leaf, fig. 111, B.

Fig. 111. Sphenophyllum speciosum (Royle).
A. Nat. size. B. enlarged leaf.
From the Raniganj Coal-field, India. (After Feistmantel.)

The inequality of the members of a single whorl, which characterises this Indian plant, is sometimes met with in European species. A specimen of Sphenophyllum oblongifolium, which Prof. Zeiller showed me in illustration of this point, was practically indistinguishable from Trizygia[921].

In some of the earlier descriptions of the Indian species the generic name Sphenophyllum[922] was used by McClelland and others, but the supposed difference in the leaf-whorls was made the ground of reverting to the distinct generic term Trizygia. Now that a similar type of leaf-whorl is known to occur in Sphenophyllum, it is better to adopt that genus rather than to allow the question of locality to unduly influence the choice of a separate generic name for an Indian plant.

GEOLOGICAL RANGE.

C. Affinities, range and habit of Sphenophyllum.

It has been pointed out in the description of Sphenophyllum, that the most widely separated families of recent plants have been selected by different authors as the nearest living allies of this Palaeozoic genus. It is now generally admitted that Sphenophyllum is a generic type apart; it cannot be classed in any family or sub-class of recent or fossil plants, without considerably extending or modifying the recognised characteristics of existing divisions of the plant-kingdom. The anatomical characters of the Sphenophyllum stem are such as one finds in some recent genera of the Lycopodineae, especially Psilotum. If the stele of Psilotum were composed internally of a solid strand of xylem, we should have a close correspondence between the centripetally-developed wood of this genus and that of Sphenophyllum. Similar comparisons might be drawn with other existing genera, but the more detailed consideration of the affinities of the Palaeozoic plant will be more easily dealt with after other members of the Pteridophytes have been described. The recent discovery of an entirely new type of Carboniferous strobilus in rocks of Calciferous sandstone age on the shores of the Firth of Forth has thrown new light on the position of Sphenophyllum. Cheirostrobus Pettycurensis, the new cone which Scott has described in an able memoir, affords certain points of contact with Sphenophyllum on the one hand and with Calamites on the other. This important question will be dealt with after we have given an account of Cheirostrobus[923]. To put the matter shortly, Sphenophyllum agrees with some Lycopodinous plants in its anatomical features; with the Equisetales it is connected by the verticillate disposition of the leaves, and some of the forms of Sphenophyllum strobili present features which also point to Equisetinous affinities.

In his Presidential address to the Botanical Section at the British Association Meeting of 1896 Scott[924] thus refers to the Sphenophyllums:—“We may hazard the guess that this interesting group may have been derived from some unknown form lying at the root of both Calamites and Lycopods. The existence of the Sphenophyllae certainly suggests the probability of a common origin for these two series.” The result of the subsequent investigation of the new cone Cheirostrobus amply justifies this opinion as to the position of Sphenophyllum.

It is probable that Sphenophyllum lived during the Devonian period, but the unsatisfactory specimens on which Dawson has founded a species of this age, S. antiquum[925], can hardly be said to afford positive evidence of the Pre-Carboniferous existence of the genus. From the Culm rocks and other strata older than the Coal-Measures, we have such species as S. insigne (Will.), Sphenophyllostachys Römeri (Solms-Laubach), and Sphenophyllum tenerrimum, Ett.[926] while S. emarginatum[5], Brongn. occurs in the Upper Coal-Measures and in the Transition rocks. S. cuneifolium[927] (Sternb.) has been recorded from the Transition, Middle and Lower Coal-Measures. Sphenophyllum oblongifolium, Germ.[928], is recorded from Lower Permian rocks, as is also S. Thoni[929], Mahr.

The comparison which has naturally been drawn between Sphenophyllum with its slender stems bearing occasionally dimorphic leaves, and water-plants is not, I believe, supported by the facts of anatomy or external characters. The entire and finely-dissected leaves do not exhibit that regularity of relative disposition which is characteristic of aquatic plants; the two forms of leaves may occur indiscriminately on the same branch. The well-developed and thick xylem is not in accordance with the anatomical features usually associated with water-plants. It is true that in some living dicotyledons of the family Leguminosae, which inhabit swampy places, the secondary xylem is represented by a thick mass of unlignified and thin-walled parenchyma, as in the genus Aeschynomene[930], from which the material of ‘pith’-helmets is obtained; but the wood of Sphenophyllum was obviously thick-walled and thoroughly lignified.

It is not improbable that the long and slender stems of this plant may have grown like small lianas in the Coal-Measure forests, supporting themselves to a large extent on the stouter branches of Calamites and other trees. The anatomical structure of a Sphenophyllum stem would seem to be in accord with the requirements of a climbing plant. It has been shewn[931] that in recent climbing plants the tracheae and sieve-tubes are characterised by their large diameter, a fact which may be correlated with the small diameter of climbing stems and the need for rapid transport of food material. In Sphenophyllum the tracheae of the xylem have a wide bore, and in S. insigne the phloem contains unusually wide sieve-tubes. The central position of the stele is another feature which is not inconsistent with a climbing habit. Schwendener and others[932] have demonstrated that in climbing organs, as in underground stems and roots, there is a tendency towards a centripetal concentration of mechanical or strengthening tissue. The axial xylem strand of Sphenophyllum would afford an efficient resistance to the tension or pulling force which climbing stems encounter.