CONCLUSION 227 



great advantage over the sporogonium-theory; 

 we know plenty of intermediate stages between a 

 thallus and a leafy stem, but no one ever saw an 

 intermediate stage between a sporogonium and 

 a leafy stem. 



We may now ask, how does the thallus-theory 

 fit in with the fossil record? The record is con- 

 sistent with the theory but affords it no decisive 

 support. In some of the early Ferns there is a 

 certain resemblance in structure between leaf and 

 stem; it may even be difficult to tell from the 

 anatomy which organ we have before us. It is 

 doubtful, however, whether this occasional resem- 

 blance is really a sign that the plants were primi- 

 tive and their organs not yet fully differentiated. 

 The resemblance between leaf and stem may only 

 have been due to reduction in both, or to some 

 special alteration in the fertile fronds. 



Some of the forked forms of leaf met with 

 in early Palaeozoic Ferns and in Pseudobornia 

 rather suggest a thallus; in a general way the 

 prevalence of large compound fronds in early 

 times is consistent with the theory, for one can 

 easily see how certain branches of a much- 

 divided thallus might have become changed into 

 compound leaves; precisely the same thing is 

 seen in certain Liverworts at the present day, 

 though of course this in the sexual and not the 

 asexual generation. Though the fossil records do 



