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TEXTBOOK OF BOTANY 



170. Pollination. The pollen of the bean is usually car- 

 ried from the stamen to the pistil by insects. Insect-pol- 

 lination is more economical and effective than the wind- 

 pollination which is depended upon by the pine, because in- 

 sects, flying directly from flower to flower, are likely to carry 

 more of the pollen to the exact place where it is needed. The 

 insects that pollinate the bean are, most commonly at least, 

 large bumble-bees. The pollen drops out of the pollen sacs 

 when it is ripe and is caught in the bottom of the keel. None 

 of it falls upon the stigma, because the style projects beyond 



FIG. 88. A , the three macrospores into which the macrospore mother 

 cell of the bean (d, Fig. 87) has divided. B and C, stages in the develop- 

 ment of the largest of the three macrospores into a female plant. After 

 Miss Mabel Brown. 



the stamens. In the keel, the pollen is covered by the wings 

 and other parts of the flower. It cannot be blown or washed 

 away, and it is protected from moisture, which is harmful 

 to it as it is to the pollen of most plants. 



When a bee alights upon one of the wings of the flower, 

 both the wings and the keel are pressed downward. This 

 allows the style to push out of the keel, and the hairs near 



