THE CONDITIONS OF PLANT LIFE 27 



tions germinate without fusion, showing that they may 

 properly be considered simply as modifications of cells 

 once purely non-sexual in character. 



As the sexual cells become more differentiated, the 

 difference in size becomes very marked, the female cell 

 being many times larger than the male (Fig. 6, D, E). 

 The former also shows a tendency to become passive 

 before fertilization, even in such forms as still retain 

 the primitive ciliated condition, and finally all power of 

 motion is lost, and usually the female cell, or egg, is re- 

 tained within the cell where it is formed, and is there 

 fertilized by the small, active male cell or spermatozoid. 



As the plant body becomes multicellular, the repro- 

 ductive function, except in the lowest types, becomes 

 restricted to special cells which differ in appearance 

 and size from the vegetative cells. This is accompa- 

 nied by the more perfect differentiation of the sexual 

 cells, resulting as already stated in the formation of a 

 large, passive female cell or egg, and a small, actively 

 motile male cell or spermatozoid. In extreme cases, 

 such as the ferns and mosses, the spermatozoid is mainly 

 reduced to the nuclear substance of the mother-cell, a 

 small portion only, including the locomotive organs 

 (cilia), being composed of the cytoplasm or cell-plasm. 



It is an interesting fact that a very similar evolution 

 of the sexual cells has taken place in the animal king- 

 dom, and has also developed independently in several 

 widely separated groups of plants. Thus we have still 

 existing, every phase of development of these sexual 

 cells in the Brown Algse, the Volvocacese, the Siphonese, 

 and Confervacese, and less perfectly in several other 

 groups. 



