REPRODUCTION IN PLANTS 85 



pollen are on their inner surfaces, next to the style and ovary. 

 When the pollen is shaken out of the anthers, it first collects 

 in the space between the anthers and the pistil. In the natu- 

 ral position of the pansy blossom it should be remembered 

 that the pistil is directed downward, and the end of the stigma 

 rests on the lower petal, with the stigmatic cup opening out- 

 ward and away from the anthers. Between the two anthers, 

 on the under side of the pistil, and at the end nearest the 

 stigma, there is a V-shaped notch from which the pollen may 

 readily escape when the flower is shaken by the wind or 

 insects. Since the notch is immediately over the groove in the 

 lower petal, the pollen falls into this groove and cannot un- 

 aided get into the stigmatic cup, since, as before stated, this 

 cup opens away from the direction in which the pollen must 

 fall. That the pansy pretty effectually prevents self-polli- 

 nation the following results of some of Darwin's experiments 

 along this line show. Two vigorous pansy plants were 

 selected for the experiment. One was covered with a net so 

 that the bumblebees could not get at the flowers, and the 

 other was left uncovered. In the uncovered one 105 fine 

 capsules were formed, while on the covered one only 18 were 

 formed, and in these only a few good seeds developed; 

 and Darwin states that even the few seeds formed were prob- 

 ably due to the agency of tiny insects that the net could not 

 exclude. 



In many other flowers in which both pistil and stamens are 

 present we find other devices for preventing self-pollination. 

 Some of these are as follows: In apple and pear blossoms 

 the stamens usually ripen at different times from the stigmas 

 in the same blossom, so that self-pollination in such cases is 

 impossible. Likewise, when stamens and pistils are in differ- 

 ent flowers, as in the pumpkin, corn, and willow, cross-polli- 

 nation is obviously necessary, if seeds are to be formed. 



