CYTOLOGY OF REPRODUCTION IN ANGIOSPERMS 



141 



is the greatly extended intine, and into the tube move the cytoplasm and 

 nucleus of the tube cell together with the two male gametes. When 

 pollen is shed from the anther in the two-cell stage, the division of the 

 generative cell to form the male gametes takes place as the pollen tube 

 grows down through the style toward the ovary (Fig. 103). The course 

 of the tube is usually between the thin-walled cells, the intercellular sub- 



FiG. 103. — Development of male gametes and syngamy in lily {Lilium regale). 1. 

 germinating pollen grain. 2, generative cell and tube nucleus in pollen tube. '3-7, stages 

 in division of generative cell and nucleus in pollen tube. 8, tip of pollen tube containing 

 tube nucleus and two male gametes. 9, syngamy; one male gamete near egg nucleus, the 

 other leaving pollen tube (but not entering egg). {After D. C. Cooper.) 



stance being dissolved by enzymes from the tube. If the style has an 

 open central canal, the tube grows along against the cells lining it. When 

 the style is very long, the style and tube may wither away at the tip 

 before the growing end of the tube containing the c.ytoplasm and nuclei 

 reaches the ovule. Eventually the tube grows through the micropyle of 

 the ovule into the embryo sac where it ruptures, liberating the male 

 gametes and often the tube nucleus. 



Syngamy now occurs (Fig. 104). The male gametes just after enter- 

 ing the sac have been shown in numerous species to be complete cells, 



