EEPEODUCTION 



435 



In the seeds of the Gymnosperms the endosperm repre- 

 sents the prothallium or gametophyte. 



The formation of the seed we have seen to depend 

 upon the fusion of the sexual cells or gametes. This 

 process is a very widespread one and is the starting point 

 of the development of the young sporophyte in all plants 



FIG. 180. ANTHEROZOIDS OF Moss (A) AND FERN (B). 



above the Thallophytes. The mode of bringing the gametes 

 together varies with the habit of life of the plant. Where 

 the male gamete is a motile antherozoid it makes its way to 

 the oosphere by means of its cilia, which enable it to swim 

 freely in water. In those forms with a terrestrial habit, 

 such as the Bryophyta and Pteridophyta, in which the 

 antherozoid is ciliated (fig. 180), fertilisation can only be 



FIG. 181. DEVELOPMENT OF THE ANTHERIDIUM IN THE FERN. (After Kny.) 



brought about when the gametophytes are moistened, as is 

 the case from time to time. The antherozoids sometimes 

 arise in antheridia upon the same gametophyte as the arche- 

 gonia with their oospheres, sometimes upon different ones. 

 In the heterosporous forms of course the latter is always 

 the case. A large number of such gametophytes, bearing 

 male and female cells respectively, are always produced in 



