REPRODUCTION 197 



mass of cytoplasm of its own, sometimes with a separate wall, 

 and is known as the generative cell. Shortly after the pollen has 

 reached the stigma the thick wall of the pollen grain bursts at 

 one point and out of the grain proceeds a thin-walled pollen-tube. 

 Near the tip of this moves the tube-nucleus, followed by the 

 generative cell (Fig. 111). This tube bores through the tissues 



Fiu. 111. — Germinating pollen of squash. The pollen grain has burst and a 

 pollen tube is starting down through the style. Near the end of the tube is the 

 tube nucleus. Some distance behind is the generative cell, from which are later 

 developed the two male cells. {From A. I. Weinstein). 



of the style and carries the contents of the pollen grain down into 

 the ovary and to the mouth of an ovule (Fig. 1 10, C). Meanwhile 

 the generative cell divides into two male cells, which are the true 

 male gametes. 



By this time the ovule has become prepared for fertilization. 

 It possesses one or two coats or integuments, later developing 

 into the seed-coats, which cover it except at one point, the mouth 

 or micropyle. Here the pollen-tube usually enters. Inside the 

 integuments is a thin nutritive layer, the nucellus. Within this, 

 in turn, and ordinarily occupying the whole central portion of the 

 ovule, is the embryo-sac. This is a small, sap-filled cavity with 

 three cells at each end and a naked nucleus, the endosperm 

 7uicleus, near its center. The three cells at the end of the sac 

 farthest from the micropyle play no part in fertilization or seed 

 development. Of the three at the micropj-lar end, however, 

 One is usually distinguishable by its greater size and is the female 



