94 COLLEGE BOTANY 



held above the water and the pollen carried by the wind or 

 by insects. 



It is very evident that the anemophilous and entomophilous 

 methods will usually result in either close or cross-pollination. 

 In fact, nature appears to have provided against self-pollination 

 in the great majority of plants. In some plants the stamens and 

 pistils mature at different times and the pollen must be carried 

 to the stigma of another plant in order to be functional. Other 

 plants have various devices which result in their insect visitors 

 carrying the pollen to other flowers, but prevents its being de- 

 posited on the stigma of the same flower. 



It will be readily seen that much of the pollen must be lost 

 and wasted. Only a comparatively few pollen grains ever reach 

 the stigma of a flower of the same species and grow. However, 

 there must be one active pollen grain for every ovule that is 

 fertilized. Therefore, when we take into consideration the great 

 waste and the great number of seeds that are produced by some 

 plants, it will be readily seen that the number of pollen grains 

 produced must be enormous. An ordinary corn plant produces 

 about 50,000,000 pollen grains, or an average of 7000 for each 

 grain that is produced. Therefore, about one pollen grain in 

 every 7000 serves its function. 



Fertilization. The term fertilization is used in this connec- 

 tion to indicate the union of the male nucleus which is produced 

 by the pollen grain with the female nucleus which is produced 

 by the ovule. Unfortunately, the same word is used to indi- 

 cate the enrichment of the soil by the application of manures 

 and? chemicals. 



The pollen grain (or microspore] is originally a single cell 

 (Fig. 70). The nucleus divides into a tube nucleus and genera- 



