310 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



princeps, and this form, as set forth above, would seem to furnish us with an intermediate 

 stage required by Lignier's liypothesis. From tliis point of view the whole pteriodophytic 

 stock would be monophyletic, the Lycopsida and Pteropsida being derived from a common 

 form already vascular. It would thus not be necessary to assume parallel evolution of a 

 similar vascular system along two different lines. The leaves, on the other hand, would 

 be morphologically diphyletic. If the microphyllous habit is regarded as primitive, it 

 would not be necessary to derive the large fronds of the Filicales from the leaves of the 

 Lycopsida through a process of progressive development. In the Filicales, only the paleae 

 or other emergences on the rachis would be homologous with the leaves of the Lycopsida. 

 Such emergences, in the shape of hairs or spines, are strikingly common in Palaeozoic 

 fronds: . . . Finally attention may be called to the coincidence that the circinnate verna- 

 tion of the fern fronds is paralleled in the branches of Psilophyton princeps. 



These quotations have been given at length because they convey very clearly 

 the gist of one of the main conclusions which these plants have brought perma- 

 nently into botany, namely, the conclusion concerning the fundamental differ- 

 ence and relative order of origin of microphyllous and megaphyllous forms. It 

 is possible that the speculations quoted might have been slightly different in 

 detail had they been based on Rhynia rather than on Psilophyton. Thus one 

 may suspect that less stress might have been laid on the idea of "enation" as 

 opposed to the suggestion, actually made by Tansley (1908) and considered by 

 Lignier (1911), that microphylls in origin are small lateral branches (i.e., small 

 "cauloids" and not enations) as opposed to the large branch systems of mega- 

 phylls. Even if this possibility is left open, however, the essential contribution 

 to thought regarding the nature of megaphylls is unaffected and it ties up so 

 closely with the anatomical considerations already mentioned (cf. Jeffrey, 1897) 

 with regard not only to tlie ferns but to gymnosperms and angiosperms as well 

 (all of which are megaphyllous though not necessarily monophyletic) that it 

 seems unlikely now to be seriously challenged. 



Since Gaspe, Koragen and Rhynie, the knowledge of Middle and Lower De- 

 vonian floras, and with them of the Psilophytales, has been greatly extended, at 

 first by the work of Lang on Scottish rocks (1927-1937), and later by an exten- 

 sion to other countries, notably by Kraiisel and Weyland (1923-1935) in Ger- 

 many, Dorf (1933-1934) in the United States (Wyoming), Stockmanns (1940) 

 in Belgium, Hoeg (1942) in Norway (on rocks from Spitzbergen). An excel- 

 lent early summary will be found in Hoeg (1937), and a later one in Croft and 

 Lang (1942). Finally an extension to still earlier geological formations, all 

 nevertheless containing vascular plants of the Psilophytalean affinity, has been 

 made by Lang (1937) for the Downtonian of Britain and by Lang and Cookson 

 (1930, 1935) and by Cookson (1935) on the Silurian of Australia, which is spe- 

 cially important for showing that somewhat more complex and larger types 

 than Rhynia or Psilophyton existed at an even earlier date. We are therefore 

 only in possession of knowledge of the merest fragments of what must have been 

 not only the dominant, but a very varied group, at tlie dawn of the fossil record 

 of land plants. 



The impact of this new knowledge on the taxonomic system is still undoubt- 

 edly incomplete. The primitive nature of rootless types has emphasized the 

 importance of the living Psilotaceae. On the other hand, the undoubtedly axial 

 nature of the spore-bearing members has affected so fundamentally the previously 

 prevalent concepts of the nature of "sporophylls" that a complete revolution in 



