Seed-Plants 133 



of the cycads, which both in form and internal 

 structure reveal unmistakably their fern ancestry. 

 Moreover, the discovery of fern-like sperms in these, 

 and the actual development of green tissue in 

 the gametophyte under certain conditions, show 

 that they are very much less reduced in these respects 

 than some of the living heterosporous pteridophytes, 

 and indicate still further the extremely primitive 

 nature of these low seed-bearing plants. 



In the conifers, the adaptation to terrestrial con- 

 ditions is complete, and all trace of their aquatic 

 ancestry is finally lost. These are the predominant 

 type of gymnosperms at the present day, and are 

 not certainly met with until the last of the Paleozoic 

 formations, where in the form of some Permian 

 species there is evidence of the beginning of the 

 coniferous series. As we have already indicated, 

 there is some reason to suppose that these conifers 

 may be descendants of some of the gigantic club- 

 mosses of the Carboniferous, but some eminent au- 

 thorities believe that the conifers also are descended 

 from fern-like ancestors. 



Gnetales. The last order of the gymnosperms, 

 the Gnetales, is a small one containing three very 

 peculiar genera, with a small number of species, 

 evidently not at all closely related either to each 

 other or to the other gymnosperms. Unfortunately 

 these plants are practically unknown in a fossil 

 condition, and at present it is impossible to deter- 

 mine their exact position in the system. 



