434 VEGETABLE PHYSIOLOGY 
embryo is developed from it, which remains enclosed in the 
spore. In the Angiosperms fertilisation as followed not only 
by the formation of an embryo, but also bya large develop- 
ment of tissue arising in consequence of repeated divisions of 
the definitive nucleus, so that the spore contains a massive 
so-called endosperm in addition to the embryo, the latter 
being usually embedded in the former. This so-called en- 
dosperm has thus a different morphological value from the 
endosperm of the gymnospermous plant. 
One of the most remarkable features about the struc- 
ture and behaviour of the seed is the fact that soon after 
the embryo is formed it enters upon a period of rest, which 
in some cases is very prolonged. During this period the 
seed becomes detached from the parent plant. The 
resumption of its growth and development is known as the 
germination of the seed. This resting period does not occur 
during the development of the sporophyte in the Cryptogams. 
The embryo frequently attains a considerable size 
before its resting period commences. In this case it 
absorbs the contents of the cells of a considerable part, or 
sometimes the whole, of the endosperm, so that it fills 
more or less completely the cavity of the spore. 
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 
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. 
