2G8 STRUCTURAL BOTANY 



the outside by a layer of cuticle. At two places on 

 opposite sides of the grain the cuticle becomes 

 separated from the cellulose wall within ; the spaces 

 thus formed are at first filled with water, so that a 

 kind of blister arises on each side of the pollen-grain. 

 The water is absorbed and the spaces become filled 

 with air when the pollen is ripe. This curious 

 arrangement has the effect of reducing the weight 

 of the pollen-grain in proportion to its surface, these 

 lateral expansions acting as veritable wings. Thus 

 the grain is more easily carried to a distance by the 

 wind. 



Before the pollen-grain is ripe, important changes 

 go on in its interior. Each pollen-grain, when first 

 formed by division of its mother-cell, is merely a 

 single cell with one nucleus. But before the de- 

 hiscence of the pollen-sac takes place, a series of 

 divisions goes on in each pollen-grain. First, a small 

 cell is cut off on the side of the grain farthest from 

 the insertion of the wings. This little cell is separated 

 from the large one by a thin wall shaped like a watch- 

 glass. Two more divisions take place in the large 

 cell, cutting off two more small cells, so that there is 

 now a row of three small cells placed one upon 

 another, and projecting into the cavity of the large 

 cell The first two of the small cells collapse, the 

 third grows bigger and divides into two, forming a 

 little stalk-cell and a larger terminal cell, which is 

 long enough to reach quite half way across the cavity 

 of the grain (Fig. Ill, A and B). 



The pollen-grain is now ripe, and we see that 



