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 
ig 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 ix 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 
