In habit the fossil strobilus may be compared with the Triassic genus Pleuromeia, but the position of the sporangia on the sporophylls constitutes a well-marked difference. The most important result of Nathorst’s skillful treatment of this interesting fossil by chemical microscopic methods is the demonstration of the existence of a large heterosporous type of lycopodiaceous cone in a Rhaetic flora.
Poecilitostachys.
Under this generic name M. Fliche[235] has briefly described a fertile lycopodiaceous shoot from the Triassic rocks of Epinal in France: the type species Poecilitostachys Hangi consists of a cylindrical axis, 10 cm. × 5 mm., deprived of leaves and terminating in a rounded receptacle bearing a capitulum of bracts or fertile leaves. Detached megasporangia containing small globular bodies found in association with the capitulum are compared with the megasporangia of Isoetes.
CHAPTER XV.
Arborescent Lycopodiales.
Among the best known plants in the Palaeozoic floras are the genera Lepidodendron and Sigillaria, types which are often spoken of as Giant Club-Mosses or as ancestors of existing species of Lycopodium and Selaginella. Of these genera, but more particularly of Lepidodendron, we possess abundant records in a condition which have made it possible to obtain fairly complete information not only in regard to habit and external features but as to the anatomical characters of both vegetative and reproductive shoots. The structure of Lepidodendron differs too widely from that of recent Club-Mosses (species of Lycopodium) to justify the statement that this prominent member of the Palaeozoic vegetation may be regarded as a direct ancestor of any living plant. There is at least no doubt that Lepidodendron and Sigillaria must be included in the Pteridophyta. The description by Dr Scott[236] of the genus Lepidocarpon, founded on petrified specimens of strobili, demonstrated the existence of a type of lycopodiaceous plant in the Carboniferous period distinguished from all living representatives of the group by the possession of integumented megaspores, which may fairly be styled seeds. Lepidocarpon and another seed-bearing plant Miadesmia are described under a separate heading as lycopodiaceous types characterised by an important morphological feature, which among recent plants constitutes a differentiating character between the Pteridophytes and the Phanerogams.
Lepidodendron.
i. General.
The genus Lepidodendron included species comparable in size with existing forest trees. A tapered trunk rose vertically to a height of 100 feet or upwards from a dichotomously branched subterranean axis of which the spreading branches, clothed with numerous rootlets, grew in a horizontal direction probably in a swampy soil or possibly under water. A description by Mr Rodway[237] of Lycopods on the border of a savannah in Guiana forming a miniature forest of Pine-like Lycopodiums might, with the omission of the qualifying adjective, be applied with equal force to a grove of Lepidodendra. The equal dichotomy of many of the branches gave to the tree a habit in striking contrast to that of our modern forest trees, but, on the other hand, in close agreement with that of such recent species of Lycopodium as L. cernuum ([fig. 123]), L. obscurum ([fig. 124]) and other types. Linear or oval cones terminated some of the more slender branches ([fig. 188]) agreeing in size and form with the cones of the Spruce Fir and other conifers or with the male flowers of species of Araucaria, e.g. A. imbricata. Needle-like leaves, varying considerably in length in different species, covered the surface of young shoots in crowded spirals and their decurrent bases or leaf-cushions formed an encasing cylinder continuous with the outer cortex. The fact that leaves are usually found attached only to branches of comparatively small diameter would seem to show that Lepidodendron, though an evergreen, did not retain its foliage even for so long a period as do some recent conifers.