Fossil plants, Vol. 2

Sphenophyllum.

The account of the Sphenophyllales given in the first volume[2] of this work must be extended and somewhat modified in the light of recent work on the fertile shoots of Sphenophyllum.

Sphenophyllostachys Dawsoni (Will.) was described as consisting of an axis bearing superposed whorls of bracts connate at the base in the form of a shallow funnel-shaped collar giving off from the upper surface and close to the axis of the cone two concentric series of sporangiophores. Occasionally there are three series, as represented in fig. 112. In another type of strobilus, Sphenophyllostachys Römeri[3] each sporangiophore terminates in two pendulous sporangia (fig. 113, A; see also fig. 107, C, vol. I.). It has already been pointed out that the common occurrence of detached strobili necessitates their description under distinct specific names; it is only by a rare accident that we can assign fossil cones to their vegetative shoots. There are, however, reasons for believing that Sphenophyllostachys Dawsoni is the strobilus of the plant originally described by Sternberg[4] from impressions of foliage-shoots as Rotularia cuneifolia. Another difficulty presented by petrified material is that of determining, with certainty, whether two imperfect specimens, differing from one another in features which do not appear to be of sufficient importance to warrant specific separation, are forms of one species or portions of specifically distinct cones. It has been pointed out by Scott[5] that the strobilus known as Sphenophyllostachys Dawsoni probably includes two distinct species, one being the cone of Sphenophyllum cuneifolium Sternb., and the other the cone of S. myriophyllum Crép[6]. The stem of S. myriophyllum agrees anatomically with the type known as Sphenophyllum plurifoliatum Will. and Scott[7].

In addition to the two types of cone already mentioned, Sphenophyllostachys Dawsoni and S. Römeri, others have been described by Kidston from carbonised impressions. One of these is the fertile branch of Sphenophyllum majus[8]. The basal portions of the bracts of each whorl form a narrow collar round the axis of the cone; the free portion of each bract consists of a lamina divided into two equal bifid lobes bearing on its upper surface one group, or possibly two groups, of four sessile sporangia between the narrow coherent bases of the laminae and the sinus between the terminal lobes (fig. 113, C). Another characteristic feature is the greater length of the internodes; this renders the cone less compact and less sharply differentiated from the vegetative shoots than those of other species. A specimen in Dr Kidston’s collection illustrates the peculiar character of the fertile portion of this species; it consists of an axis bearing a succession of lax sporophylls succeeded above and below by whorls of sterile leaves. In this species, therefore, we cannot speak of a compact strobilus at the end of a shoot of limited growth, but of axes in which sterile and fertile leaves are borne alternately[9], a condition recalling the alternation of foliage leaves and sporophylls in Tmesipteris and in Lycopodium Selago.

Another form of cone, also from the Middle Coal Measures, is referred by Kidston to Sphenophyllum trichomatosum Stur[10] (fig. 113, B): this is characterised by the more horizontal position of the bracts, which “do not appear to be so much or so suddenly bent upwards in their distal portion as in some other species of Sphenophyllum,” and by sessile sporangia borne singly on the upper face of each bract.

A more recent addition to our knowledge of the fertile shoots of Sphenophyllum is due to Scott who has described a new type of cone under the name Sphenophyllum fertile[11]. The petrified specimen on which the species was founded was discovered by Mr James Lomax in the Lower Coal Measures of Lancashire; it represents a portion of a cone 6 cm. long and approximately 12 mm. broad. The axis contains a single vascular cylinder agreeing in essentials with the type of stem structure known as Sphenophyllum plurifoliatum. The nodal regions, which exhibit the slight swelling characteristic of the genus, bear several (probably twelve) appendages connate at the base and forming a narrow flange encircling the axis. Each bract, the base of which forms part of the narrow collar surrounding the axis, consists of two lobes, ventral and dorsal, divided palmately into several (sometimes four) segments or sporangiophores (fig. 115). Each sporangiophore terminates distally in an oblong or oval lamina bearing two sporangia on its adaxial face (fig. 114). The space between the axis and the periphery of the cone is thus occupied by crowded peltate laminae, each with its pair of sporangia. A single vascular bundle supplies each sporangiophore and bifurcates in the distal lamina into two branches which extend to the bases of the sporangia. The sporangia agree in structure with those of other species of Sphenophyllum: the spores are of one size and elliptical, characterised by the presence of several sharp ridges or flanges encircling the spore-wall in the direction of the major-axis. Sphenophyllostachys fertilis differs from all previously recorded types in the absence of sterile bracts. The appendages of the cone-axis are all fertile, a striking contrast to the differentiation into protective and sporangia-bearing bracts which constitutes a constant feature in the cones of Sphenophyllum and Calamites. It is possible, as Scott suggests, that the absence of sterile segments is the result of modification of the more usual type of strobilus; instead of the dorsal and ventral lobes of the bracts sharing between them the duties of protection and spore-production, the whole of each bract is constructed on the plan of the maximum spore-output, the laminar terminations of the sporangiophores serving the purpose of protection. The cone may be described as more specialised than the normal type of strobilus for reproductive purposes[12].

It has been stated, on evidence which is unsatisfactory, that Sphenophyllum possesses two kinds of spores. While regarding the genus as homosporous on the evidence before us, it is interesting to find that cases occur in which the spores in the same sporangium exhibit a marked difference in size. Attention has been called by Williamson and Scott[13] to variation in the dimensions of spores: a more pronounced difference in size has been recorded by Mr Thoday[14] who gives 120μ as the maximum and 90μ as the minimum diameter of the spores in a cone of Sphenophyllostachys Dawsoni. The presence of several abortive spores in the sporangium (fig. 116) containing the larger spores favours the view that this difference in size may be the first step towards the development of heterospory.

It is clear that the types of strobilus designated Sphenophyllostachys (figs. 112–114) present a divergence of characters too great to be comprised under one genus; but in the absence of fuller information, we cannot do otherwise than follow the only logical custom of grouping them together as examples of strobili borne by plants which, in the present state of our knowledge, are most conveniently referred to the genus Sphenophyllum.

Cheirostrobus.

This generic name was applied by Dr Scott[15] to a calcified cone obtained by Mr James Bennie in 1883 from the Lower Carboniferous plant-beds of Pettycur near Burntisland on the Firth of Forth. Cheirostrobus is distinguished from Sphenophyllostachys by its greater breadth (3.5 cm.); externally it agrees more closely with the fertile shoots of Lepidodendron than with those of Sphenophyllum. A single vascular cylinder having the form of a fluted Doric column (fig. 117, B, x) occupies the axis of the cone: it consists for the most part of reticulate tracheae which tend to assume a short or isodiametric form in the central region; the smaller protoxylem tracheids with the spiral form of pitting constitute the sharp and prominent ridges at the periphery of the xylem-cylinder. In the outer part of the cylinder the metaxylem[16] consists exclusively of tracheae, but towards the centre of the axis these are associated with numerous parenchymatous cells.

The xylem is therefore centripetal in origin as in Sphenophyllum and in nearly all recent and fossil members of the Lycopodiales. In the type-specimen of Cheirostrobus the vascular cylinder of the cone consists entirely of primary xylem, but secondary xylem has been found in a more recently discovered specimen[17]. Secondary xylem occurs also in the peduncle of the cone. No appreciable remains of phloem have been found. The cortex consists of slightly elongated rather thick-walled tissue containing secretory sacs. Crowded superposed whorls of bracts (or sporophylls), usually twelve in each whorl, are borne on the axis and each sporophyll receives a single vascular bundle from one of the vertical ridges of the xylem column (fig. 117, A, lt). The members of each whorl are connate at the base: from this narrow collar each sporophyll branches into an upper or dorsal and a lower or ventral limb (fig. 117, A, f and s). Each limb divides palmately at a short distance from its origin into three slender segments, which extend in a horizontal direction and terminate in large laminar expansions (fig. 117, B, s) to afford a protective covering to the surface of the cone. The upper set of three segments, constituting sporangiophores (fig. 117, A, B, f) or fertile divisions of the sporophyll, expand distally into comparatively bulky laminae; each of these bears on its adaxial face four diagonally placed outgrowths which form the short pedicels of very long and narrow sporangia. The three lower segments—the sterile divisions of the sporophylls—(fig. 117, A, B, s) are similar to the upper set except in their greater length and in the kite-shaped form of their distal laminae which are provided with lateral lobes. The single vascular strand which supplies each sporophyll is represented at lt in fig. 117, B; at lt′ the strand has divided into four, the three upper bundles in the figure supply the sterile segments and the single lower bundle ultimately divides into three which supply the fertile segments. A pair of blunt processes (fig. A, s) extend downwards over the ends of the underlying fertile lamina and two slender prolongations extend upwards through several internodes.

An economical arrangement of the long and narrow sporangia and of the sporophyll-segments between the axis and the periphery of the cone is rendered possible by the interlocking of the sterile and fertile segments by means of a groove in the upper face of the latter for the accommodation of the former. The sporangia are characterised by their unusually long and narrow form: the length of a sporangium may reach 1 centimetre. In the structure of the wall the sporangia of Cheirostrobus agree closely with those of Calamostachys[18] and Sphenophyllostachys. The spores are of one size only. The vascular cylinder of the peduncle, originally described by Williamson[19] as the peduncle of a large Lepidostrobus (the cone of Lepidodendron), is characterised by the presence of a short radially disposed zone of secondary tracheids, a feature, as Scott points out, which may extend into the axis of the cone. It is noteworthy that the protoxylem elements are not always external, but occasionally occur internal to one or two of the outermost metaxylem tracheae: the usual exarch[20] structure of the central cylinder is not therefore absolutely constant, but may be replaced by a mesarch arrangement.

The presence of a few sterile leaves on the peduncle below the fertile portion of the cone, which agree in their lobed laminae with the sporophylls, is the only fact which we possess as to the form of the vegetative characters of the genus.

The above description is sufficient to indicate the extraordinary complexity and high degree of specialisation of Cheirostrobus. The sporophylls, with their trilobed segments, and the crowded sporangia of exceptional length attached only by a narrow base constitute striking peculiarities of the genus.

It is unfortunate that we are still without any satisfactory evidence as to the nature of the plant the cones of which have been made the type of a new genus and a new family. Cheirostrobus affords an interesting example of a type of reproductive shoot constructed on a plan sui generis, and may be classed with some other extinct genera as instances of the production in the course of evolution of architectural schemes which appear to have been ill adapted for competition with equally efficient though much simpler types. But the discovery of these isolated forms of restricted geological range among the relics of the Palaeozoic vegetation frequently supplies a key to phylogenetic problems. Cheirostrobus by its complex combination of features characteristic of the Equisetales, the Lycopodiales and the genus Sphenophyllum throws a welcome light on the inter-relationships of groups which represent divergent series. The combination of morphological features in this generalised type led the author of the genus to describe it as a descendant of an old stock which existed prior to the divergence of the Equisetales and Lycopodiales.

The discovery of this new type of strobilus naturally led to a search among Lower Carboniferous plants for vegetative shoots exhibiting characters conformable with the whorled and branched leaves of Cheirostrobus. In Sphenophyllum we have a genus obviously comparable with Cheirostrobus as regards the form and disposition of the leaves, but the differences between the cones and the striking similarity of the vascular cylinder of the latter to that of Lepidodendron demonstrate conclusively that we must look elsewhere for the vegetative members of the plant which produced cones of the Cheirostrobus type.

In 1902 Professor Nathorst[21] instituted the generic name Pseudobornia for plants of which imperfect examples had previously been referred by Heer[22] to Calamites under the name C. radiatus. Heer’s plants were obtained from Upper Devonian rocks of Bear Island in the Arctic seas and additional specimens were brought from the same locality by the Swedish Polar Expedition of 1898. Pseudobornia possesses jointed stems (fig. 117, D) bearing whorled and shortly stalked leaves, often four in number, at each node. The leaves are palmately branched with fine serrated edges (fig. 117, C). Certain specimens, which are no doubt correctly described by Nathorst as cones, are characterised by a thick axis bearing whorled leaves with sporangia on their lower surfaces, but the material is not sufficiently well preserved to render possible a recognition of structural details. It has been suggested by Scott that Pseudobornia may possibly be referable to the Sphenophyllales and that the stem of Cheirostrobus “may have had something in common with” Nathorst’s genus[23]. The beds in which the stems occur are of Upper Devonian age, while Cheirostrobus was found in Lower Carboniferous rocks: this difference in age is not, however, a serious objection to the validity of the comparison. We cannot do more than express the view that Pseudobornia, so far as can be ascertained without an examination of petrified material or of more perfect impressions of strobili, exhibits vegetative features not inconsistent with the morphological characters of the fertile shoots known as Cheirostrobus.

The institution of a special group-name for the reception of Sphenophyllum is justified by the sum of its morphological features, which do not sufficiently conform to those of any existing group of Pteridophytes to warrant its inclusion in a system of classification based on recent genera. In the case of Cheirostrobus we are limited to the characters of the cone and its peduncle. The suggestion that the Devonian fossils known as Pseudobornia may represent the foliage shoots of a plant closely related to Cheirostrobus has still to be proved correct. Although we may find justification in the highly complex and peculiar structure of Cheirostrobus for the recognition of the genus as a type of still another group of Pteridophytes, it would be unwise to take this step without additional knowledge.

The undoubted similarity between Cheirostrobus and Sphenophyllum coupled with striking points of difference favours the inclusion of the two genera in distinct families placed, for the present at least, in the group Sphenophyllales.

Group SPHENOPHYLLALES.

It has recently been proposed to include the family Psilotaceae, comprising the two recent genera Psilotum and Tmesipteris, as another subdivision of the Sphenophyllales. This proposal had been made by Professor Thomas[24] primarily on the ground that the sporophylls of Tmesipteris and Psilotum appear to afford the closest parallel among existing plants to the peculiar form of sporophyll characteristic of the Sphenophyllales. The morphological interpretation of the sporophylls of both Sphenophyllum and Cheirostrobus has been the source of considerable discussion[25]. If we regard each sporophyll as a leaf with two lobes, one fertile and one sterile, except in the case of Sphenophyllostachys fertilis in which both are fertile, an obvious comparison may be made with the fern Ophioglossum; but the difference between a single fern frond, consisting of a comparatively large sterile lamina bearing a fertile branch composed of a long axis with two rows of sporangia embedded in its tissues, and the whorled sporophylls of Sphenophyllum is considerable.

A brief reference may be made to the principal reasons which have led to the suggestion that the Psilotaceae should be included in the Sphenophyllales. The shoots of Tmesipteris bear simple foliage leaves spirally disposed on a slender axis, and in association with these occur sporophylls consisting of a short axis bearing a pair of small lobes and a bilocular synangium[26] (fig. 120, B). The synangium is seated on a very short stalk given off from its sporophyll at the base of the pair of laminae: the synangium with its short stalk may be spoken of as the sporangiophore. In most cases the synangium appears to be sessile on the sporophyll, but occasionally the much reduced stalk is prolonged and forms an obvious feature. Dr Scott[27] suggested that the Tmesipteris synangium with its axis may correspond to the ventral lobe (or sporangiophore) of Sphenophyllum. In the latter genus the whorled sporophylls consist in most species of a dorsal and a ventral lobe, the latter serving as a sporangiophore bearing one or more sporangia; in Tmesipteris the sporophylls are spirally disposed and each consists of a bilobed sterile portion bearing a septate sporangium or bilocular synangium on a very short ventral lobe. Professor Bower[28], in his account of the development and structure of the sporophylls of Tmesipteris, drew attention to the comparatively frequent occurrence of abnormal sporophylls and spoke of the plant as unstable. More recently Professor Thomas[29] of Auckland has carefully examined living plants, with the result that variations of different kinds are proved to be exceedingly common. He finds that sporophylls occur which exhibit repeated dichotomy of the axis (fig. 120, D, F) and thus each may bear four instead of two leaf-lobes and three synangia, one at the first fork and one at each of the forks of the second order[30].

Other abnormalities occur in which the synangium is raised on a distinct stalk instead of being more or less sessile at the point from which the leaf-lobes diverge. A third form of departure from the normal is that in which there is no synangium on the bilobed sporophyll, its place being taken by a leaf-lobe. The deduction from the occurrence of these abnormalities is that the synangium of Tmesipteris represents a ventral leaf-lobe, as Scott suggested. Professor Thomas draws attention to the resemblance between Tmesipteris sporophylls and the foliage-leaves of Sphenophyllum, which are either simple with dichotomously branched veins or the lamina is deeply divided into two or more segments. In some types of Sphenophyllostachys the bracts are simple (S. Dawsoni), but in others (Sphenophyllum majus, fig. 113, C) they are forked like the foliage-leaves and bear a close resemblance to the abnormal sporophylls of Tmesipteris. Moreover, in Sphenophyllostachys Römeri (fig. 113, A) each ventral lobe of a sporophyll bears two sporangia, a condition almost identical with that represented by the occasional occurrence of a synangium on a comparatively long stalk in Tmesipteris. Similarly the more elaborate sporophylls of Cheirostrobus may be compared with the branched sporophylls of Tmesipteris (fig. 120). This agreement between the sporophylls of the Palaeozoic and recent genera acquires additional importance from the very close resemblance between the exarch stele of Sphenophyllum and that of the genus Psilotum, which conforms to the Palaeozoic type not only in the centripetal character of the primary xylem and in its exarch structure, but also in the occasional occurrence of secondary xylem[31], and in the stellate form of its transverse section. The occasional mesarch structure of the stele of Cheirostrobus finds a parallel in the mesarch xylem groups in the stem of Tmesipteris. It is thus on the strength of these resemblances that Thomas and Bower would remove the Psilotaceae from the group Lycopodiales and unite them with Sphenophyllum and Cheirostrobus in the Sphenophyllales. While admitting the validity of the comparison briefly referred to above, I prefer to retain the Psilotaceae as a division of the Pteridophyta including only Psilotum and Tmesipteris.

In his recent book on The Origin of Land Flora, Prof. Bower raises objection to the use of the term ventral lobe in speaking of the sporangium-bearing stalk or sporangiophore borne on the sporophyll of Sphenophyllum. He points out that the use of this term implies the derivation of the sporangiophore by metamorphosis of part of a vegetative leaf, an opinion untenable in the absence of proof. The designation sporangiophore is no doubt preferable to that of ventral lobe as it carries with it no admission of particular morphological value; as a further concession to a non-committal attitude we may provisionally at least regard a sporangiophore as an organ sui generis “and not the result of modification of any other part[32].”

The view put forward by Prof. Lignier[33] that the Sphenophyllales are descendants of primitive ferns is not convincing, and his comparison of Sphenophyllum with Archaeopteris lacks force in view of our ignorance as to the nature of the reproductive organs of the latter genus. That the Sphenophyllales are connected with the Equisetales and with the Psilotales by important morphological features is clear; but the comparison between the sporophylls of the extinct genera with those of the existing genus Tmesipteris, though helpful and possibly based on true homology, cannot be considered as settling the morphological value of the sporangiophores of Sphenophyllum and Cheirostrobus.

I do not propose to discuss at length the different views in regard to the morphological nature of the sporangiophore of Sphenophyllum. The comparison, which we owe in the first instance to Scott, with the synangium of the Psilotales with its short stalk, though not accepted by Lignier as a comparison based on true homology, is one which appeals to many botanists and is probably the best so far suggested. The further question, whether these sporangiophores are to be called foliar or axial structures is one which has been answered by several authors, but it is improbable that we shall soon arrive at a decision likely to be accepted as final. Discussions of this kind tend to assume an exaggerated importance and frequently carry with them the implication that every appendage of the nature of a sporangiophore can be labelled either shoot or leaf. We treat the question from an academic standpoint and run a risk of ignoring the fact that the conception of stem and leaf is based on morphological characteristics, which have been evolved as the result of gradual differentiation of parts of one originally homogeneous whole. There is much that is attractive in the view recently propounded by Mr Tansley that a leaf is not an appendicular organ differing ab initio from the axis on which it is borne, but that it is in phylogenetic origin a “branch-system of a primitive undifferentiated sporangium-bearing thallus[34].” Admitting the probability that this view is correct, our faith in the importance of discussions on the morphological nature of sporangiophores is shaken, and we realise the possibility that our zeal for formality and classification may lead to results inconsistent with an evolutionary standpoint[35].