xxx POLLINATION AND FERTILIZATION 383 



nectar, a saccharine fluid* secreted by certain parts. The 

 microspores are deposited on the stigma (A), where they 

 germinate, each sending off a pollen-tube (A and c 2 , /. /), 

 which grows downwards through the tissue of the stigma and 

 style to the cavity of the venter, where it reaches a megaspo- 

 rangium, and entering at the micropyle (D, p. /), 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 2 , nu, 

 iiu*) have passed into the end of the pollen-tube. The 

 nucleus of the larger cell undergoes degeneration, becoming 

 shrivelled and unaffected by dyes ; that of the smaller cell 

 divides by karyokinesis. One of the two daughter-nuclei 

 thus formed also degenerates, the other, accompanied by 

 its directive spheres, passes through the softened cell-wall of 

 the swollen end of the pollen-tube and enters the ovum, 

 uniting with its nucleus in the usual way. 



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 

 polyplast. By further differentiation rudiments of a stem 

 (F, st), a root (r) and either one or two cotyledons (<:/) 

 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 

 formed is called the endosperm (F. end), and occupies pre- 



