22 STRUCTURAL BOTANY 



for the whole process of the germination of the micro- 

 spores can only go on in water. The microspores are, 

 however, so small that a very little water is sufficient, 

 such as we should find on the surface of the ground 

 after rain or heavy dew. 



The spermatozoids next become freed from their 

 mother-cells, the walls of which dissolve. As soon as the 

 spermatozoids are at liberty, or even sooner, their cilia 

 begin to lash about in the water, and when free the 

 spermatozoid sets off in active locomotion, exactly like 

 some water-animalcule. 



The movement is a double one : the spermatozoid 

 travels through the water with its narrow ciliated end 

 foremost, and at the same time it rotates about its own 

 axis. Its motion, in fact, is just like that of a rifle 

 bullet through the air, or that of the screw of a steamer 

 through the water. 



We must remember that the spermatozoids are of a 

 very minute size ; the body is about -^^^ of an inch 

 long, the cilia about twice that length. 



The spermatozoids are the bodies which perform the 

 act of fertilisation. Each spermatozoid corresponds to 

 one of the generative cells in the pollen-tube of Flower- 

 ing Plants (see Part I. pp. 178, 185, and 269). We 

 know that the generative cell chiefly consists of a very 

 large nucleus, with only a little protoplasm. This is also 

 true of the spermatozoid, which is all nucleus, except the 

 small part at the pointed end and the cilia, which are 

 protoplasmic. The spermatozoid is an actively moving 

 cell, which swims off on its own account, and may eventu- 

 ally find its way to an ovum. It thus differs from the 

 generative cell of the higher plants, which, except in 

 some of the Gymnosperms (see p. 303), is conveyed pas- 

 sively to its destination by the growth of the pollen-tube. 



