106 Plant Life and Evolution 



living trees. This secondary thickening occurs in 

 a slight degree in a few living ferns, and it is clear 

 that this peculiar development has arisen quite in- 

 dependently in all of the main groups of pterido- 

 phytes, but has been lost in most of the de- 

 scendants of the Paleozoic pteridophytes. In the 

 club-mosses there are developed minute sperms with 

 two cilia like those of the mosses, while in 

 the ferns and horsetails the sperms are very much 

 larger and have numerous cilia, and do not at all 

 resemble the sperms of any known bryophyte. 



While in the lower types of the pteridophytes, 

 like some species of Lycopodium and the lower 

 ferns, the gametophyte may reach a size comparable 

 with that of many liverworts, in the more special- 

 ized types there is a great reduction in the size 

 of the gametophyte which may live a very short 

 time. 



A peculiar modification of the gametophyte is 

 sometimes met with. This is the development of 

 a subterranean habit, with the entire loss of chloro- 

 phyll. The gametophyte thus becomes saprophytic, 

 living upon the leaf mold or the soil rich in organic 

 matter. To enable it to do this it has associated 

 itself with a fungus, much like that found in 

 the subterranean organs of certain saprophytic flow- 

 ering plants. In some way, not perfectly under- 

 stood, it is through the agency of this " endophyte " 

 supplied with carbon and perhaps also with nitrog- 

 enous matter. 



