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 



