160 ELEMENTARY STUDIES IN BOTANY 



are associated in the same flower, and the pollen is ready for 

 shedding and the stigma ready to receive at the same time. 

 In such a case nothing can prevent self-pollination except 

 some mechanical hindrance that makes it unlikely that the 

 pollen will reach the stigma. It is this situation that has 

 resulted in many of the irregularities and striking forms of 

 flowers, so that such flowers are among those most prized 

 in cultivation. Three notable examples will serve as illus- 

 trations. 



The sweet pea and its allies have what are called " butter- 

 fly-shaped " flowers. Two of the petals together form a 

 boat-shaped structure (keel) which encloses the several 

 stamens and the simple pistil (Fig. 126). The stigmatic 

 surface is on the top of the style and projects beyond the 

 anthers, whose shed pollen lodges on a hairy zone of the style 

 below the stigma. The projecting keel is the natural land- 

 ing place for a bee visiting the flower; and this keel is so 

 attached that the weight of the insect depresses it. This 

 depression of the keel causes the tip of the style to emerge 

 and to strike the body of the insect. It is a glancing blow, 

 so that after the tip has struck the insect, the surface of the 

 style is also rubbed against its body, brushing the lodged 

 pollen on to the insect. At the next flower visited, the stigma 

 strikes the pollen brushed off in the previous flower, and then 

 a new supply of pollen is brushed from the style. In this 

 way, each flower visited receives pollen from the preceding 

 one, and sends pollen to the next one. Of course, there are 

 large elements of chance in such a performance, but certainly 

 self-pollination is fairly well guarded against, and cross- 

 pollination is very likely to be secured. 



In the iris, often called " flag," each stamen is in a kind of 

 pocket between a petal and a petal-like style (Fig. 127). The 

 stigmatic surface is on the top of a flap or shelf which extends 

 from the style as a roof over the pocket. With the stamen 

 beneath this shelf and the stigmatic surface on top, it is clear 



