THE BEAN 



147 



its end brush out the pollen that has collected in the bottom 

 of the keel. The nine united stamens are pressed downward 

 with the keel, but the upper free 

 one is not. A space is thus left 

 between the free stamen and 

 the group of nine. To reach 

 the nectar, the bee must insert 

 its proboscis to one side of the 

 free stamen and into the trough 

 formed by the united stamens: 

 In doing this, the proboscis first 

 touches the stigma ; if there are 

 pollen grains on the proboscis 

 (which were collected on an 

 earlier visit of the bee to an- 

 other flower), they are brushed 

 off on to the stigma and are 

 held there by the sticky fluid 

 that begins to flow when the 

 stigma is touched. Next the 

 bee's proboscis touches the 

 brush of hairs on the style and 

 some of the pollen grains that 

 are held by the brush stick to 

 the proboscis. When the bee 

 leaves the flower, its proboscis 

 is prevented from again touch- 

 ing the stigma by the hairs on 

 the style, so that none of the 

 pollen from this flower is left on 

 its own stigma. The pollen 

 that is carried away on the 

 bee's proboscis may stick to the stigma of the next 

 flower that it visits. Thus the structure of the flower 

 favors cross-pollination that is, the landing of pollen 



FIG. 89. The female plant 

 of the lily (with neighboring cells) 

 at the time of fertilization : 

 a, pollen tube; b, two cells of 

 the female plant lying beside the 

 egg; c, a male nucleus uniting 

 with the egg nucleus, d; e, an- 

 other male nucleus which will 

 unite with two nuclei of the 

 female plant at /; g, three cells 

 of the female plant lying at the 

 opposite end from the egg. 



