ANGIOSPEEMiE 



183 



Fig. 935. 



' whicli descended to help form the definitive nucleus, is the female 

 gamete, or oosphere, while the other two are known as the 

 aynergidce. In some few cases the definitive nucleus is consti- 

 tuted by only the one from the chalazal end ; there are then 

 two oospheres, of which only one becomes fertilised. In 

 rare cases the synergidse are fertile gametes, though this very 

 seldom happens. 



At this stage the development of the prothallium remains 

 suspended, and nothing further takes place unless the oosphere 

 becomes fertilised. The 

 egg apparatus lies almost 

 always immediately below 

 the micropyle, and is thus 

 not covered by a mass of 

 tissue as in the Q-ymno- 

 sperms. The pollen tube 

 reaches the micropyle as 

 already described ; its apex 

 becomes more or less muci- 

 laginous, as does the wall 

 of the embryo sac, and one 

 of the male gametes makes 

 its way throiigh to the 

 oosphere, with which it 

 fuses, nucleus with nucleus 

 and protoplasm with pro- 

 toplasm. The fertilised 

 oosphere, now become a 

 zygote, surrounds itself 

 with a cell-wall, and the 

 synergidae become disor- 

 ganised and disappear. 



The further fate of the zygote differs in the two classes into 

 which the Angiosperms are divided and will be discussed later. 

 The embryo sac becomes filled with a tissue known as the endo- 

 sperm, which has, however, a different morphological yalae from 

 the tissue in the maorospore of the Gymnosperms, though it bears 

 the same name. 



This so-caUed endosperm is derived from the definitive 

 nucleus of the embryo sac. This body divides repeatedly, form- 

 ing a number of nuclei which become disposed in a layer all over 

 the surface of the wall of the embryo sac. Round each of 

 these an aggregation of protoplasm takes place, so that the sac 



Fig. 935. Macrosporajigium (ovule) of an 

 Angiosperm. mac. Maorospore. oos. 

 Oosphere. p.t. Pollen-tube. 



