92 THE MORPHOLOGY OF PTERIDOPHYTES 



component of the flora of those times, contemporaneous 

 with the tree-Hke Lepidodendrales. Whereas this species was 

 similar to most Selaginella species in having four megaspores 

 in each sporangium, others had sixteen or even thirty-two, 

 which suggests that they had not progressed so far in the 

 direction of heterospory. While there is no general agree- 

 ment among botanists as to how the various groups of the 

 Lycopsida are related to each other, it is generally supposed 

 that the heterosporous forms must have evolved from some 

 homosporous ancestor. 



In this connection, it is perhaps significant that, among 

 Selaginella species, the type regarded as the most primitive 

 (S. selaginoides) approaches most nearly to the Lycopodium 

 species which is regarded as the most primitive (L. selago). 

 Both are erect and isophyllous, with spirally arranged leaves 

 showing the least difference between fertile and sterile 

 regions and both having simple protostehc vascular systems. 

 The similarities extend even to the young embryo, as a com- 

 parison of Figs. loP and 14Q will show. The lack of a well 

 developed foot in each is interesting, and makes one wonder 

 whether it might have been absent from their ancestors also. 



The most important differences between these two plants, 

 therefore, seem to be the heterospory of Selaginella and its 

 possession of a hgule. If it be accepted that heterospory is 

 derived from homospory, there remains only the ligule to be 

 explained. This is, indeed, difficult. There is no obvious 

 reason why, in lycopods, this structure should invariably be 

 associated with heterospory. Selaginella is usually grouped 

 with Isoetes and the Lepidodendrales on the basis of the 

 possession of these two characters, yet on other grounds 

 Selaginella stands apart from Isoetes. The multiflagellate 

 antherozoids of the latter suggest very fundamental differ- 

 ences. On the basis of the number of flagella, Lycopodium 

 and Selaginella should be grouped together. Unfortunately, 

 of course, we have no knowledge of the antherozoids of the 

 Lepidodendrales, but one's guess would be that they were 



