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 (&) on its way to 

 the egg (E). After DUGGAB. 



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 (Fig. 177). When 

 it comes near the egg, the 

 tip of the tube breaks and 

 the two male cells are dis- 

 charged into the embryo- 

 sac. One male cell passes 

 to the egg and the two 

 nuclei fuse, the resulting 

 cell being the oospore, 

 which develops the em- 

 bryo. The other male 

 cell passes to the endo- 

 sperm nucleus and fuses 

 with it, the cell resulting 

 from this triple fusion of 

 a male cell and two polar 

 nuclei developing the 

 endosperm (Fig. 178). 

 These two simultaneous 

 acts of fertilization are 

 spoken of as " double fer- 

 tilization." 



Endosperm. After 

 fertilization, the primary 

 endosperm nucleus begins 

 a series of divisions, and 

 as a result the sac becomes 



