Page:Proceedings of the Royal Society of London Vol 69.djvu/128

120 cylinder ; but that the latter becomes at first a concentric fibro- vas- cular tube (Biindelrohr of De Bary), with gaps for the branches alone, or with gaps for both leaves and branches.

The tubular nature of the central cylinder in the polystelic type may become subsequently disguised by the overlapping of the gaps and by the appearance of medullary strands, derived in all the cases investi- gated by the writer from the inner wall of the stelar tube. It seems better to describe these conditions as adelosiphonic instead of poly- stelic, since the latter term implies a misconception.

In the Osmundacese the writer believes he has found evidence of the derivation of the medullated monostelic and astelic types from the siphonostelic condition with internal phloem by the degeneration of the latter.

Osmunda cinnamomea shows all stages between the polystelic and astelic conditions ; 0. rff/alis still retains occasionally a brown scleren- chymatous pith, while in 0. claytoniana this phenomenon is quite absent. Similar examples of degeneracy are found among the Poly- podiacece. Potonie further believes that the so-called medullated monostelic central cylinder of the Gymnosperms is derived by de- generacy of the internal phloem from such types as Medullosa. The writer considers that there is good evidence for regarding the so-called medullated monostelic type of central cylinder as derived by specialisa- tion, accompanied by degeneracy, from the so-called polystelic type of Van Tieghem, and thus returns to the conception of the morpho- logy of nbro-vascular strands set forth in De Bary's ' Comparative Anatomy.'

The study of the development of the fibre-vascular skeleton of the higher plants seems to lead to the conclusion that is hardly less im- portant phylogenetically than the osseous skeleton has proved to be in the case of vertebrated animals. Where the tubular central cylinder exists there are two main types, the phyllosiphonic, where foliar gaps are constantly present, and the cladosiphonic, where foliar gaps are equally constantly absent. The central cylinder of the Filicales, Gymnosperms, and Angiosperms belongs to the former type, and that of the Lycopodiales and Equisetales to the latter. These distinctions appear to be of special importance, on account of the absence of constant and far-reaching criteria of taxonomy among the vascular plants. They moreover agree closely with evidence drawn from other available sources.

The writer is of opinion that there are two great primitive stocks of vascular plants, the Lycopsida and the Pteropsida. The Lycop- sida include the Lycopodiales and Equisetales, and are palingenetically microphyllous and cladosiphonic. The Pteropsida include the Filicales and Phaenogams, which are primitively megaphyllous and phyllosi- phonic.