FKUITS AND SEEDS 231 



membrane, as a perforation or disruption of the cell-wall has 

 not been observed. One can only see that the contents of 

 the pollen tube become squeezed towards the embryo sac by a 

 constricting of the micropyle, and that the tube becomes firmly 

 applied to one of the synergidse. Then the nucleus and vacuole 

 of this cell disappear ; it and its sister-cell (Fig. 33, s, s) be- 

 come dim and granular, lose their shape, and are ultimately seen 

 attached to the egg-cell as colourless and irregular viscid masses. 



This completes the act of fertilisation, the immediate effect 

 of which is that the egg-cell (e), having temporarily two 

 nuclei (which afterwards fuse), surrounds itself with a delicate 

 membrane. 



This stage of the development is shown in Fig. 32. Umhr 

 represents the fertilised egg-cell, and we 

 notice that during or shortly after fertili- 

 sation several changes have taken place 

 in the embryo sac. Before fertilisation, 

 as is mentioned above, a fusion of one of 

 the nuclei of the micropylar end with 

 a nucleus from the antipodal end has 

 given rise to a single new nucleus near the 

 centre of this embryo sac. Now by repeated 

 divisions of this nucleus, a large number ^'s- 33- 



of nuclei (Endsp) have been formed, which 

 are chiefly found in the layer of protoplasm lining 

 the inner wall of the embryo sac. Here each nucleus 

 becomes the centre of a new cell, and by further division 

 the embryo sac becomes entirely filled by a new cellular 

 tissue. This tissue, which fills the entire embryo sac when 

 the latter has completed its growth, is called the albumen 

 or endosperm. It is the tissue in which are stored up the 

 reserve substances which have to nourish the young embryo 

 during its further growth, and it forms the greater part of 

 some seeds. In the case of the wheat-grain and other grass- 

 seeds, we see the small embryo attached to one end of the 

 seed, and the rest of the tissue filling up the former embryo 

 sac is the starchy endosperm, very much more considerable in 

 bulk than the embryo. If we imagine the centre of this 

 mealy endosperm to be hollow, and not filled up with cells, we 



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