218 THE EVOLUTION OF PLANTS 



forms a part of a leaf is set apart to bear the 

 sporangia, as in the Royal Fern (Osmunda regalis), 

 while in others a whole leaf is modified for this 

 purpose (e. g. the Ostrich Fern, Struthiopteris 

 germanica). It may well be that the same differ- 

 ences existed among Equisetales and their allies, 

 the sporangium-bearing organ sometimes repre- 

 senting an entire leaf and sometimes a part of 

 one. 



The leaves in Sphenophylls are better de- 

 veloped than in most of the Horsetail class, and 

 suggest those of Ferns. This was the case also, 

 and in a more striking degree, with the leaves of 

 Pseudobornia, a genus which may be allied to 

 the Sphenophylls as well as to the Calamites. 

 It has been suggested that both groups may 

 have had a common origin from primitive Ferns, 

 a suggestion which is to a certain extent sup- 

 ported by the characters of the prothalli and 

 sexual organs in Equisetum. But any such con- 

 nection, if real, must lie very far back, and is at 

 present merely a subject for speculation. All 

 that the evidence really indicates is that the 

 Sphenophylls and Equisetales are branches of 

 the same stock, and that they started from 

 plants with large and well-developed leaves. 



The history of the common stock is still un- 

 known. Perhaps we may learn something about 

 it when the Devonian Flora has been more 



