132 Plant Life and Evolution 



The curious horsetails, or scouring rushes, be- 

 longing to the genus Equisetum, are the sole 

 survivors of a class, which in the early geo- 

 logical formations was represented by a great 

 variety of forms, some of which attained tree-like 

 dimensions. Some of the fossil species were hetero- 

 sporous, but there is no evidence that any of them 

 advanced far enough to develop seeds, and so far 

 as we know the seed habit was never attained by 

 members of this class of pteridophytes. 



Gymnosperms and Angiosperms. The seed- 

 plants, as they now exist, are commonly divided into 

 two very unequal classes, the Gymnosperms and the 

 Angiosperms. The gymnosperms, which include 

 the cone-bearing evergreens, are the older type, and 

 in these the gametophyte shows obvious resem- 

 blances to that of their pteridophytic forebears, and 

 the homologies are sufficiently evident. The angio- 

 sperms, on the other hand, the ordinary flowering 

 plants, which comprise an overwhelming majority 

 of existing seed-plants, show much less evidence of 

 their origin from lower forms, and at present it is 

 an open question whether or not they are at all re- 

 lated to any of the existing gymnosperms. It is, 

 moreover, very unlikely that all of the existing 

 gymnosperms have had a common origin. The two 

 lowest types, the cycads and Ginkgo, are, with very 

 little question, descended from fern-like ancestors, 

 presumably through some types of the seed-bearing 

 ferns of the Carboniferous. This is certainly true 



