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.
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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.
- Sphenophylleae: genus Sphenophyllum.
- Cheirostrobeae: genus Cheirostrobus.
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.
PSILOTACEAE
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].