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 by a 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. 



