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thought is required in order to assimilate it. The morpliological redescription 

 of the fertile parts of microphyllous genera such as, for example, the cone of 

 Equisetum or the synangia of Psilotum, in terms of sporangiophores, is fairly 

 easy to envisage even if one does not wish to go to the length of using the ter- 

 minology of the "telome" as postulated by Zimmermann (1930, 1938). The case 

 of the megaphyllous ferns and seed plants is, however, more complicated, and 

 again it may be helpful to quote Halle. In discussing naked forked axes bear- 

 ing terminal sporangia known as Dawsonites and thought to be the fertile parts 

 of PsilopJiyton he remarks : 



". . . the sporangia of Dawsonites recall those of certain Upper Devonian and Carbonif- 

 erous ferns generally considered as primitive, as for instance Dimeripteris, or perhaps 

 Stauro2)teris, according to Lignier (1908-11). . . . The chief points of resemblance between 

 the fertile fronds of certain Primifilices and Dawsonites arcuatus are the large size of 

 the sporangia and their apical position on branches of special fronds or pinnae without 

 developed laminae. Among the fronds of the Lower Carboniferous and Upper Devonian, 

 the common occurrence of "modified" fronds bearing sporangia but no flattened pinnules 

 is very striking. ... In the Lower Devonian, finally, we find frondlike structures bearing 

 sporangia but no fronds with developed laminae. One can hardly escape the conclusion 

 that the "modified" fertile fronds may represent the primitive state in this case and that 

 the flattened pinnules are a later development as suggested by Professor Lignier. The 

 sporangia would then be pre-existent in respect to the laminae of the pinnules. 



This last sentence has a bearing, not only on our view of the nature of 

 primitive ferns, but, by an extension which Halle himself visualized (1937), 

 can also be applied to the seed plants if, as may have been the case, the seed 

 also is older than the lamina of a leaf. The fossil evidence is inconclusive here 

 and it would be out of place to discuss it further. It is, however, necessary to 

 refer to it in passing because it raises the point of view that, although the mega- 

 phyllous types at present known contain both ferns and seed plants, the mega- 

 phyllous habit which they both share may be homoplastic and the only common 

 ancestor uniting the Filicales and Gymnosperms may in fact be the Psilophytales. 



Lycopsida 



Articulata 



Pftrqpslda 



Figure 1. Phylogeny of the living and fossil 

 Pteridopliyta, redrawn from Zimmermann, 1938. 



