[Chap. XXXIU SEXUAL REPRODUCTION IN FLOWERING PLANTS 383 



until careful observations showed certain peculiarities in the time of 

 flower opening. Each flower opens twice. In some varieties this may be 

 the morning of one day and the afternoon of the next day; in other 

 varieties, the afternoon of one day and the morning of the third day. 

 At the first opening the pistils are mature but the pollen has not been 

 shed. At the second opening the anthers dehisce, but the stigmas are 

 too mature for the pollen to germinate. Evidently self-pollination in 

 avocado does not result in fertilization and the production of fruit. If 

 these two varieties of avocado grow sufficiently close together, cross- 

 pollination and fertilization may take place, and good yields of fruit 

 result. The whole process is, however, complicated by the erratic opening 

 of the flowers, and by the insects that transfer the pollen. 



Dissemination of pollen. In the flowers that remain closed during 

 ripening, as in oats and wheat, gravity plays an important part in pollina- 

 tion. A few plants, such as eel grass, certain pond weeds, and the water 

 buttercup, are pollinated by water-borne pollen. Often the pistillate 

 flowers develop just at the surface of the water, while the staminate 

 flowers may be formed below the surface. The staminate flowers rise to 

 the surface in large numbers, and the pollen eventually comes in contact 

 with the stigmas. 



In the great majority of land plants, particularly where cross-pollina- 

 tion occurs, wind and insects are the most important agents of pollina- 



,*!^ 





Fig. 170. Various forms of pollen grains. From W. Hamilton Gibson. 



tion. Pollen grains vary widely in form (Fig. 170). Pollen may be car- 

 ried many miles by air currents, but the pollen of most plants under 



