PHYLOGENY 



Lycopsida (club mosses) are characterized bv similar scale-like leaves. 

 Another genus, Hijcnki, had whorled branches, and may have been an- 

 cestral to the Sphenopsida (horsetails). Finallv, some genera, such as 

 Pseudosporochnus, had the branch tips flattened out, and it is generally 

 believed that this led to the formation of broad leaves, as in many of the 

 Pteropsida. 



Thus the psilophytes appear to occupy a key position in the evolution 

 of the higher plants. They are the most primitive vascular plants known- 

 one might almost say they are the most primitive vascular plants imagin- 

 able—and it is highly probable that they arose from the green algae. The 

 psilophytes appear to have given rise to the three higher subphyla inde- 

 pendently, that is, each of the higher subphyla has arisen from a different 

 psilophytan ancestor, so that these represent parallel developments rather 

 than a sequence from primitive to advanced. It has also been suggested 

 that the bryophytes may have been derived from the psilophytes, but this 

 does not seem very probable in view of the fact that it would have re- 

 quired a reversal of the tendency toward dominance of the sporophyte, 

 as well as a suppression of the vascular bundles. Current opinion favors 

 the independent origin of both the bryophytes and the psilophytes from 

 the green algae. On the other hand, spores resembling those of land plants 



ANTHERIDIUM 



KCHEGONIUM 



FicuHK 38. r.silolii))! SroHoi'HYTK (i.kkt) AND Gamftophytk (hicmt) ( AfttT Lawsoii, 

 from Fuller and 'I'ippo, "Collfgc Botany," 2nd Ed., Henry Holt & Co., Inc., 1954.) 



JJ6' 



