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.