THE LIVING WORLD 



209 



The agent of impregnation is a pollen grain, every one of 

 which consists of a fluid particle of protoplasm, enclosed 

 in a membrane or wall, which is almost always double. 



As the yellow pollen dust flies about, one grain sooner 

 or later is wafted to the stigma of the pistil, or, as 

 in very many plants, is 

 carried there by insects 

 in search of food, who 

 have previously dusted 

 themselves with pollen in 

 visiting other plants. As 

 soon as the pollen-grain 

 finds itself there, the par- 

 ticle of protoplasm, enclosed 

 in its inner coat, passes 

 through its outer one and 

 descends through the inter- 

 cellular spaces of the style 

 till it reaches an ovule. DIAGRAM OF AN OVULE. 

 Then it passes through the Vertically bieected and showing th 

 micropyle, or minute aper- two coats which enclose it. 

 ture left in the ovule's two E Embryo sac. 



coats, and penetrates its M M ' cr <wlf /. ** opening 



through which the pollen- tube 

 substance till it reaches the 



embryo sac which it im- 



mu 4.v 

 pregnates. Then the upper 



thickening within it (before 



enters. 

 Upper thickening of cell-con- 



tents, in which the embryo 

 of ^ ^^ plant ^ ^ 



origin. 



spoken of) grows and de- T2 Lower thickening. 

 velops, from a mere formless mass of cells, into a miniature 

 plant with a minute rudimentary stem and root and two 

 pairs of leaves. Two of these leaves are termed the 

 cotyledons. They become of relatively enormous size, 

 enclosing the minute plant between them and constitut- 

 ing the great mass of the seed we familiarly know as " a 



o 



