ONTOGENESIS 177 



the gametophyte and grows to a much greater size, 

 leaf after leaf being developed on the growing root- 

 stalk. Eventually, sporangia are developed in enor- 

 mous numbers on the under side of the leaves, usually 

 in clusters (sori). The spores, falling on damp soil, 

 sprout, and, cell division following cell division, 

 a mass of cells results which soon takes the form of a 

 tiny prothallium, the gametophyte of another 

 generation. In the more complex ferns two sorts of 

 spores are formed, large ones called megaspores and 

 smaller ones known as microspores. Both produce 

 gametophytes upon germination, but the prothallium 

 arising from a megaspore produces female gametes 

 only, whereas one arising from a microspore produces 

 male gametes only. We have here a sexual differ- 

 entiation in the sporophyte or non-sexual phase. 



Seed Plants. - - We have traced a progressive em- 

 phasis which the course of evolution has laid on 

 the sporophyte, compared with the gametophyte, 

 generation. In the liverwort the gametophyte is 

 the " plant " and the sporophyte a tiny parasite 

 upon it. In the higher mosses the gametophyte 

 is larger and the sporophyte, although still largely 

 dependent upon it, is able to contribute some of its 

 own food. In the ferns the sporophyte, although 

 dependent upon the gametophyte in the beginning, 

 soon shifts for itself and completely overshadows the 

 latter. Finally, in the seed plants, the representa- 

 tives of the plant world most familiar to us, the condi- 

 tions are reversed, and we find that the sporophyte is 



