ERRATA IN VOL. I

CHAPTER XII[1].

SPHENOPHYLLALES (concluded).

[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].

Fig. 112. Sketch of a radial longitudinal section of Sphenophyllostachys. There are usually two concentric series of sporangia on the sporophylls, not three as shown in the figure. The upper figure (after Zeiller) shows the linear bracts in surface-view.

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.

Fig. 113.

  1. Sphenophyllostachys Römeri. (Solms-Laubach.)
  2. Sphenophyllum trichomatosum Stur.
  3. Sphenophyllum majus. Bronn. (A–C. After Kidston.)

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.

Fig. 114. Sphenophyllostachys fertilis (Scott). (After Scott.) Diagram of a node in longitudinal section, showing one sporophyll and the base of the opposite one. v.l. ventral lobe of sporophyll; v.s. one of the segments into which it divides; v.s′. stump of another segment; d.l. dorsal lobe; d.s., d.s′. segments of dorsal lobe.

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].

Fig. 115. Sphenophyllostachys fertilis (Scott). (After Scott.) Diagram of a single sporophyll as it would appear in a transverse section of the cone; showing one lobe (dorsal or ventral). ax, part of axis to which the sporophylls are attached.

Fig. 116. Sphenophyllostachys Dawsoni. (After Thoday.) A. Larger spores; B, abortive spores; C, mature spores showing the characteristic spines.

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.

Fig. 117. A, B. Cheirostrobus pettycurensis Scott. (After Scott.)
C, D. Pseudobornia ursina Nath. (After Nathorst.)

  1. Diagrammatic radial longitudinal section of part of the cone-axis and two sporophylls. lt, bundle passing out to sporophyll; f, fertile segment of sporophyll showing two sporangia; s, sterile (lower) segment.
  2. Part of transverse section. x, stele; lt, lt′, bundles on their way to sporophylls; a, tips of sterile segments of lower sporophylls.
  3. Palmately branched leaf (½ natural size).
  4. Node of stem showing leaf-bases.

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.

PSEUDOBORNIA

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.