300 



TYPICAL FLOWERING PLANTS 



in it. When a pollen grain falls on a sticky stigma, the 

 inner coat pushes out through one of the thin places, 

 forming a tube into which all the contents of the pollen 

 grain flow. The contents, at this time, consist of two 

 nuclei and a small amount of protoplasm. The pollen 

 tube grows and pushes its way through the loose tissue 

 of the stigma till it reaches the ovary containing the 

 ovules (Figure 311, a, 6). 



The ovules are attached to the 

 sides of the ovary. Each has a 



Figure 311. — a, pollen grains growing 

 through pistil ; b, same magnified ; 

 c, nuclei of pollen and egg. 



thick coat called the integument which does not quite meet 

 at one spot, known as the micropyle. Inside the ovule is 

 the embryo sac containing the egg cell and a few other cells. 



When the pollen tube reaches the micropyle of an 

 ovule it enters, touches the egg cell, and bursts. The 

 male nucleus unites with the nucleus of the egg, and fer- 

 tilization is accomplished (Figure 311, c). The other nu- 

 cleus of the pollen tube usually unites with a nucleus 

 near the center of the embryo sac and helps to form tissue 

 which may be of use to the growing embryo or may form 

 a part of the mature seed. 



The fertilized egg cell soon begins to divide and grow, and 



