EEPEODUCTION 433 



is a certain amount of protoplasm existing in the spore, 

 lying around the wall and forming bridles across it, con- 

 necting the peripheral substance with that in the centre in 

 which the definitive nucleus is resting. 



There are no apparent archegonia ; the oosphere is one 

 of the three cells of the egg apparatus, the other two being 

 known as the synergidce. The oosphere is a product of 

 the last division of the original upper nucleus, the other 

 half being the polar nucleus which takes part in the fusion 

 described. 



After the fertilisation of the oosphere an embryo is 

 developed from it, which remains enclosed in the spore. 

 At the same time a large development of tissue takes place 

 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 

 embedded usually in the former. This so-called endosperm 

 has as we have seen 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. 



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