XXXIV POLLINATION AND FERTILISATION 475 



Pollination may take place, as we have seen, by the agency 

 either of the \\ind or of insects. The microspores are 

 deposited on the stigma (a), where they germinate, each 

 sending off a pollen-tube (a and c^, /. /), which grows 

 downwards through the tissue' of the stigma and style to the 

 cavity of the venter, where it reaches a niegasporangium, 

 and entering at the micropyle (d, /. /), continues its course 

 through the nucellus, finally applying itself to the distal end 

 of the megaspore in the immediate neighbourhood of the 

 synergidae. 



In the meantime the nuclei of the microspore (c', nu, 

 ntt^) have passed into the end of the pollen-tube. The 

 vegetative nucleus undergoes degeneration, becoming 

 shrivelled and unaffected by dyes. The generative nuclei 

 wander to the apex of the pollen-tube and ultimately pass 

 through the softened cell-wall of its swollen end, one of them 

 entering the ovum and uniting with its nucleus in the usual 

 way, while the other fuses with the secondary nucleus of the 

 megasf)ore. 



The ovum is thus converted into an oosperm or unicellu- 

 lar embryo : it acquires a cell-wall and almost immediately 

 divides into two cells, of which that nearest the micropyle 

 becomes the suspensor (e, spsr), the other, or embryo 

 proper {emb), forming a solid aggregate of cells, the poly- 

 plast. By further differentiation rudiments of a stem (f, st), 

 a root (r) and either one or two cotyledons {ct) are formed, 

 and the embryo passes into the phyllula stage. 



While the early development of the embryo is going on, 

 the secondary nucleus of the megaspore divides repeatedly, 

 and the products of division (e, nu) becoming surrounded 

 by protoplasm, a number of cells are produced, which, by 

 further multiplication, fill up all that part of the megaspore 

 which is not occupied by the embryo. The tissue thus 



