434 VEGETABLE PHYSIOLOGY 



(2) The perisperm, or remains of the body of the 



megasporangium. 



(3) The embryo-sac or megaspore. 



(4) The endosperm derived from the definitive nucleus. 



(5) The embryo developed from the zygote. 



The antipodal cells generally disappear during the 

 development. (2) and (4) may be absent, having been 

 absorbed by the megaspore or by the embryo respectively 

 during their development. If either or both are present 

 the seed is said to be albuminous, the term albumen 

 embracing both perisperm and endosperm. 



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 



FIG. 182. AXTHEROZOIDS OF Moss (A) AND FERN (B). 



process is a very widespread one and is the starting point 

 of the development of the young sporophyte in all plants 

 above the Thallophytes. The mode of bringing the gametes 

 together varies with the habit of life of the plants. 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. 182), fertilisation can only be 

 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 



