120 Anatomy and Development of the Stem in Pteridophyta, &c. 



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. 



