74 THE EVOLUTION OF PLANTS 



are developed; these are the organs of motion 

 (fig. 7, B and c). The pollen-tube, distended with 

 water, now grows a little, and brings the end where 

 the sperms are, near the egg-cells; the sperms 

 begin to rotote while still in the tube and now show 

 themselves to be true, active spermatozoids. The 

 pollen-tube bursts; the spermatozoids are set 

 free, and by means of their actively vibrating cilia, 

 swim the short distance necessary, through the 

 water discharged from the pollen-tubes, reach the 

 egg-cells, unite with them, and so effect fertilisa- 

 tion. Each spermatozoid can fertilise one egg- 

 cell and one only. 



The Cycads and the Maidenhair-tree are the 

 highest plants which have retained the Crypto- 

 gamic method of fertilisation by actively moving 

 cells; it will be remembered that spermatozoids 

 are the agents of fertilisation in all Ferns, Mosses 

 and their allies, and in a large number of Seaweeds 

 and other Algae. In animals, too, the male cells 

 have the power of active movement. The higher 

 plants rise in the scale of evolution, the more do 

 they tend to lose their animal characters; some 

 of the lowest Algae have the power of movement 

 through most of their lives; in higher forms it is 

 only the spore-stage that keeps this power; hi 

 still higher plants, like Mosses and Ferns, the 

 spores too have lost their mobility, but the male 

 cells still keep it, as a means of reaching the egg- 



