SEXUALITY OF PLANTS 



45 



part of the course, but it is accomplished with a high 

 degree of certainty. 



The tube is a means of conveyance of 

 the male gametes to the ovule. The 

 mature pollen-grain contains two cells 

 (Fig. 26). One, the larger, is a vege- 

 tative cell, and takes no direct part 

 in propagation. Shortly after germi- 

 nation begins the other divides into 

 two, which are the male gametes. On 



FIG. 26. 



Pollen-grain, showing two 

 cells : the larger is vegeta- 

 tive; the smaller, to the 



germination these contents of the pollen- g - mete ~ s . (X540 .) A fter 



right, gives rise to the two 

 gametes. ( 



grain pass into the growingtube, and ad- 

 vance with it as it grows (Fig. 27, A ) . They are thus conveyed, 

 always protected within the tissue of the style, to the ovary, 



c 



B FIG. 27. A 



A, Pollen-tube of Orchis latifolia, teased out from the ovary. v = vegetative 

 nucleus; gg = gametes. ( x 500.) B, Pollen-tube of the same penetrating the 

 micropyle of the ovule ; just below its lip is the egg-apparatus, with the ovum 

 shaded. The male gametes are still in the tube. ( x 300.) After Strasburger. 



and finally to the ovule itself (Fig. 27, B). There they are 

 extruded through the soft tip of the tube into the embryo- 



