HOW POLLINATION AND FERTILIZATION TAKE PLACE 393 



the cherry and peach only one stigma is present in each blos- 

 som, whereas five are found in the apple and pear. 



By pollination is meant the transfer of pollen from the 

 anthers of the stamens to the stigmas of the pistils. In tree 

 fruits, when the stigmas are pollinated with pollen from the 

 same flower or from flowers of the same variety, the process is 

 called self-pollination; when the pollen is transferred from the 



(Md. Exp. Sta.) 



Fig. 157. Apple blossoms covered with paper 

 bags in pollination experiments. 



stamens of a different variety, cross-pollination is said to have 

 occurred. 



Pollination is generally followed rather quickly by ger- 

 mination of the pollen grain (Fig. 156) . Water and possibly 

 nutrients are absorbed from the surface of the stigma, the 

 grain swells, and a tube is forced out. The pollen tube grows 

 down through the style, enters the ovule, or unfertilized seed, 

 penetrates to the embryo sac, and discharges two male germs 

 or gametes. One of the male gametes fuses with the egg cell 



