ONTOGENESIS 175 



Although plants, in comparison with animals, are 

 handicapped in their ability to move about over the 

 surface of the earth and thus effect the maximum dis- 

 persal of the species, yet such a defect is compensated 

 by this ability to produce spores in enormous quanti- 

 ties. But in all except the very simplest plants 

 reproduction by germ-cells is also to be found. In 

 the plant world, accordingly, we find an alternation of 

 sexual and asexual generations. One plant (genera- 

 tion) produces gametes (whence it is called the 

 gametophyte) ; the zygote arising from the fusion of 

 two gametes develops into another plant (genera- 

 tion) which produces spores and is therefore called 

 the sporophyte. The gametophyte generation in 

 many groups is telescoped into the sporophyte, as it 

 were, and its true relations can be made clear only 

 in comparison with simpler types. 



Liverworts and Mosses. In the liverworts the 

 plant-body (thallus) has the form of a green, leaflike 

 structure, growing close to the ground, and sending 

 down minute, feeding root-hairs into the earth 

 from the lower surface. This is the gametophyte. 

 Along the edges are developed spermaries (anther- 

 idia) and ovaries (archegonia), which produce the 

 male and female gametes. The former are active, 

 and, swimming about in the dew or rain, meet and 

 fuse with the egg-cells. From the zygote thus 

 formed there arises, by repeated cell divisions, a 

 mass, of cells within the archegonium itself, which 

 becomes differentiated into a structure quite unlike 



