134 Plant Life and Evolution 



The Gymnosperms Not a Homogeneous Class. 

 From the above statements it is evident that the 

 gymnosperms do not constitute a homogeneous as- 

 semblage of plants, but represent a more or less 

 heterogeneous collection of forms, which may very 

 well represent several quite unrelated lines of de- 

 scent. They all agree in having seeds of a primitive 

 type, usually exposed upon open leaves, or sporo- 

 phylls; whence their name of Gymnosperms, or 

 " naked-seeded " plants. They are evidently less 

 fitted to existing conditions than their rivals, the 

 angiosperms, which have largely superseded them 

 and have shown a far greater power of adaptation, 

 clearly indicated by their enormously greater vari- 

 ety of species and individuals. Probably there are 

 not more than five hundred living species of gymno- 

 sperms, \vhile of the angiosperms more than one 

 hundred thousand already have been described. 



THE CYCADS 



During the Paleozoic, especially in the Car- 

 boniferous, there arose a great assemblage of fern- 

 like plants, showing a wide range of structure, 

 many of them approaching both in the structure of 

 the tissues and in their reproductive parts the lower 

 types of seed-bearing plants. Some of these 

 pteridosperms, or seed-bearing ferns, were evidently 

 not very different in appearance from some of the 

 living ferns, especially those belonging to a small 



