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 Alge, the Volvocacez, the Siphonea, 
and Confervacez, and less perfectly in several other 
groups. 
