PLANT GENETICS 



19 



elongates, forming a pollen tube which passes down the style. 

 This tube grows through the tissue of the pistil and reaches the 

 embryo sac. The generative nucleus passes into the pollen tube 

 and divides, forming two nuclei which are the male gametes. 

 The pollen tube grows through the tissues of the pistil until it 

 reaches the embryo sac, and the tip of the tube breaks after it 

 penetrates the wall of the embryo sac. In fertilization one of 



FIG. 5. Longitudinal section of the lower portion of the embryo sac of 

 maize at the time of fertilization; pn, polar nuclei fusing; sn, sperm nucleus 

 fusing with a polar nucleus, pn; e, egg; sn, sperm nucleus in the egg; pt, pollen 

 tube; syn, synergid; v, vacuole. (After Miller.) 



these gametes of the pollen tube unites with the egg cell to form 

 the embryo of the seed and the other unites with two polar nuclei 

 to form the endosperm (see Fig. 5). 



If we represent the chromosome number of each body cell by 

 2x, each gamete would be represented by x, the embryo formed 

 by the union of the generative cell with the egg cell would be 

 2x and the endosperm tissue 3x. 



