206 



PLANT STRUCTURES 



FIG. 177. Fertilization in the cotton plant, 

 a Dicotyledon, showing the pollen tube (P) 

 passing through the micropyle and con- 

 taining a single sperm (male cell), and hav- 

 ing entered the embryo-sac is in contact 

 with one of the synergids (S) on its way to 

 the egg (E). After DUGGAR. 



This completes the first 

 period of gametophyte de- 

 velopment, and it is ready 

 for fertilization. 



Fertilization. The 

 pollen-tube, carrying the 

 two male cells, has passed 

 down the style and en- 

 tered the micropyle (Fig. 

 173). It then reaches the 

 wall of the embryo -sac, 

 pierces it, and is in con- 

 tact with the egg -appa- 

 ratus. Usually it passes 

 along the side of one of 

 the synergids (Fig. 177), 

 feeding upon and disor- 

 ganizing it. When it 

 comes near the conspicu- 

 ous nucleus of the egg, 

 the tip of the tube breaks 

 and one male cell is dis- 

 charged into the cyto- 

 plasm of the egg (Fig. 

 178). The egg and the 

 male cell now fuse, and 

 an oospore, which invests 

 itself with a wall, is the 

 result. 



Second period. After 

 fertilization the gameto- 

 phyte begins its second 

 period of development. 

 The primary endosperm 

 nucleus begins a series of 

 divisions, and as a result 



