444 VEGETABLE PHYSIOLOGY 



The seed may thus be a very complex structure ; it may 

 consist of the following parts : 



(1) The testa or skin, derived from the integuments of 



the ovule. 



(2) The perisperm, or remains of the "body of the megas- 



sporangium. 



(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 develop- 

 ment. (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 process 

 is a very widespread one, and in all plants which exhibit an 

 autothetic alternative of generations is the starting-point 

 of the development of the young sporophyte. 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 Bryophytes and Pteridophytes, 

 in which the antherozoid is ciliated (fig. 180), fertilisation 

 can only be brought about when the gametophytes are 

 moistened, as is the case from time to time. The anthero- 

 zoids sometimes arise in antheridia upon the same gameto- 

 phyte as the archegonia 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 the immediate neighbourhood of 

 one another, so that the transport of the antherozoids to 



