The long and narrow cones referred to Bothrodendron minutifolium from English and French Coal-Measures are known only as impressions and it is not possible to say whether they were heterosporous or homosporous; the drawing given by Zeiller (Fig. 212, E) shows that the sporangia were of the same form as those in Lepidostrobus, but we have no more exact information as to their morphology. A recently published description of a petrified strobilus by Mr Watson affords a welcome addition to our knowledge. There is little doubt that this cone was borne by a species of Bothrodendron; the evidence for this conclusion is supplied by the agreement of the anatomical characters of the stele with that of the vegetative shoots originally described by Williamson as Lepidodendron mundum and by the constant association of the cones and vegetative shoots. In 1880 Williamson described a crushed cone containing both megaspores and microspores which he spoke of as “a diminutive organism, reminding us more of the dwarfed fruits of many living Selaginellas than of the large Lepidostrobi[647].” Watson’s specimens enable us to give a more complete account of this type. The axis of the strobilus bears short sporophylls bent upwards into a distal limb with a conspicuous ligule in a deep pit beyond the shortly stalked sporangium. The length of the strobilus is estimated at 10 mm.; the stele is of the same type as that of Bothrodendron mundum, but it differs from the specimens of the vegetative shoots so far found in having some secondary xylem. As shown in the sketch reproduced in [fig. 216] each sporophyll is characterised by two tangentially placed grooves, g, on the lower face, and by numerous transfusion tracheids, tr, above the vascular bundle, vb, immediately below the ligule, l. Megasporangia and microsporangia occur on the same cone, the megasporangia being on the lower sporophylls and containing a single tetrad of megaspores. Fig. 219, E, shows a radial longitudinal section of a microsporophyll bearing a sporangium on the adaxial side of the ligule, l, below which is the single vascular bundle and a group of short tracheids at t. The sporangia closely resemble those of species of Selaginella and Lycopodium and, as pointed out by Watson[648], they also recall the sporangia of the Palaeozoic genus Spencerites. Bothrostrobus is distinguished from Spencerites by the presence of a ligule, by the structure of the axis, and by the different form of the sporophylls. The occurrence of four spores only in the megasporangia is another character in which the extinct type resembles recent Lycopods. It is impossible to decide whether Watson’s cone represents a more or a less primitive type than Lepidostrobus: if we accept Professor Bower’s views in regard to the evolution of vegetative organs by the sterilisation of sporogenous tissue, we should probably place Lepidostrobus lower in the series than Bothrostrobus; but the greater resemblance between the fertile and vegetative shoots of Bothrodendron, as compared with the more pronounced difference in the case of Lepidodendron, may be regarded as an argument in favour of recognising Bothrodendron as the more primitive type.
Another possible example of a Bothrodendron cone has been described by Nathorst from Spitzbergen as Lepidostrobus Zeilleri[649]; this appears to consist of an axis bearing spirally disposed sporangia without any indication of sporophylls. This strobilus may belong to Bothrodendron tenerrimum.
Pinakodendron.
The name Pinakodendron[650] was instituted by the late Prof. Weiss for a type of stem closely resembling Bothrodendron but differing in the presence of a fine reticulation on the outer bark and in the form of the leaf-scars. Weiss’s genus has been recognised by Kidston in Dumfriesshire but our knowledge of the plant is as yet based solely on a few small specimens.
Omphalophloios (a genus of uncertain systematic position).
Figs. [193], C, [217].
This generic name was instituted by White[651] for certain specimens of large stems originally described by Lesquereux from the Coal-Measures of North America as Lepidodendron mammillatum and L. cyclostigma. The photograph reproduced in [fig. 193], C, for which I am indebted to Dr Kidston[652], represents a specimen described by him from the Upper Coal-Measures of Somerset as Omphalophloios anglicus, and identified with Lepidodendron anglicum of Sternberg.
The surface of the impression shown in [fig. 193], C, is characterised by clearly defined rhomboidal areas or cushions (fig. 217, E) like those of Lepidodendron, except in the absence of a median keel, and similar to those on some forms of Sigillaria Brardi. A short distance above the centre of each cushion is an oval or subcordate region bounded by a rim-like margin and containing a small oval scar, presumably that of a vascular strand. A triangular elevation which also shows a small pit (Fig. 217, E, a) occurs below the oval area. The appearance of the surface-features varies considerably on different parts of a single specimen. Fig. 217, D, represents one of the numerous figures published by White in his detailed account of the American material. Each cushion bears a widely open V-shaped ridge, which is described as a leaf-scar; above this is an oval area (2·5 mm. × 1·75 mm.), the surface of which is bounded by a narrow rim. Within the rim is a smaller concave oval region with a small pit near its upper end.
Fig. 217.
Omphalophloios. D. After White. E. After Kidston.
We cannot, in the absence of petrified material, arrive at any satisfactory conclusion as to the meaning of these surface-features. White considers that Omphalophloios is probably a rhizome of one of the arborescent Lycopods, but whether or not this is its true nature must be left for future discoveries. The fact that the rootlet bundles of some Stigmarian axes are accompanied by a parichnos strand, as Weiss has shown, may prepare us for the discovery of surface-features on Stigmariae not unlike those of Omphalophloios. (Fig. 193, C.)