340 NATURE STUDY AND AGRICULTURE 



ditions the vegetative cells of the body produced small cells 

 within themselves, and these escaped into the water. Such 

 special reproductive cells are called spores, and as they 

 swim actively by means of cilia they are called swimming 

 spores. These spores swim about for a time, but gradu- 

 ally settle down, and each one produces a new plant. 



Still later in the history of plants, another kind of 

 reproduction was developed. Swimming spores much 

 smaller than the ordinary ones were produced, and these 

 were unable to give rise to new plants. However, they 

 came together and fused in pairs, and the resulting cell, 

 produced by the fusion of two cells, did have the power of 

 producing a new plant. These pairing cells are gametes 

 and this process of cell fusion is fertilization. This 

 method of reproduction is the sexual method, and this 

 transformation of swimming spores into fusing spores or 

 gametes is the origin of sex. 



There have thus been developed three methods of re- 

 production, in the following order: vegetative multiplica- 

 tion, reproduction by spores, and sexual reproduction. All 

 three methods are retained by all the higher plants. 



A further advance in sex reproduction was secured by 

 what is called the differentiation of sex. When sex first 

 appeared, the pairing gametes were alike, but later they 

 became different. One became much larger than the 

 other and lost its cilia and hence the power of locomotion. 

 The other retained its small size and activity. These two 

 kinds of gametes have received different names, the large 

 passive one being the egg, and the small active one the 

 sperm. In this way not only sex, but two sexes were de- 

 veloped. The cell produced by the fusion of sperm and 



