444 THE VASCULAR PLANTS 



which leads through the coats of the ovule. (These coats 

 of the ovule are the integuments and the tissue they in- 

 close, the tissue in which the female gametophyte lies em- 

 bedded, is the nucellus. See page 276.) One of this lower 

 group of three cells, the largest one, is the egg. Only one 

 egg is produced by this gametophyte. Usually the pollen 

 tube enters the ovule by way of the micropyle and then 

 reaches the egg by burrowing its way through the nucel- 

 lus. 



Near the center of the female gametophyte you note a 

 pair of nuclei. The peculiar thing about this pair of 

 nuclei is that, about the time that a sperm fuses with the 

 egg, they fuse with each other. Thus they form what 

 is called the endosperm-nucleus. The endosperm-nucleus 

 produces the endosperm. 



The three nuclei at the other end appear to have no 

 special function; often after fertilization they disappear. 

 Sometimes they persist and contribute nourishment to the 

 embryo. 



This thing which we have been calling the female game- 

 tophyte, like other parts of the flower, received another 

 name long before its real nature was understood. Botanists 

 noted that in very young seeds the tiny embryo appeared 

 to be lying in a sort of sac. This sac was of course the 

 female gametophyte. They called it the embryo-sac, and 

 this name is still commonly used to describe the female 

 gametophyte of angiosperms. 



E. Double Fertilization. You recall that there are two 

 sperms in the pollen tube. Until quite recently it was 

 supposed that after one of these sperms fused with the 

 egg the other simply disappeared. It was thought that 



