ANIMAL AND PLANT EEPEODUCTION 29 



differentiated, thus providing for protection of the 

 gametes. Such organs of various types and known 

 by different names have persisted throughout all the 

 higher plants. One may call them ovaries and spermaries 

 and thus keep in mind that in animals the same types of 

 change occurred. 



The final step in the general development of sexuality 

 is the restriction of the formation of sex organs to a par- 

 ticular phase in the plant's life, which on this account is 

 known as the gametophyte. The remaining stages are 

 known as non-sexual or sporophytic, because they are 

 characterized by the production of non-sexual reproduc- 

 tive cells, the spores. The liverworts, the mosses, the 

 ferns and the seed plants are thus set apart. 



Since these two phases alternate with each other, pairs 

 of reproductive cells of the gametophyte producing the 

 sporophyte, and the non-sexual spores of the latter giving 

 rise to the gametophyte, the sequence has retained the 

 name of alternation of generations. 



In the higher liverworts and mosses the gametophyte 

 carries on the greater part of the nutritive work, but in 

 the ferns the sporophyte becomes the dominant structure; 

 while in the seed plants the gametophyte has degenerated 

 until it consists of but two or three cell divisions. 



There is no question but that all of these numerous 

 changes are merely insurance against the future, some- 

 thing that may be said of seed production as a whole, since 

 the seed is but the younger generation nourished on the 

 parent stem. And it is interesting to note that just as 

 animals and plants paralleled each other in gamete pro- 

 tection and provisions for assuring fertilization, so also 

 the final step in each, the mammals and the seed plants, 



