130 



PLANT RELATIONS. 



of the common loosestrifes. Each flower has stamens of 

 two lengths, which, with the style, makes possible three 

 combinations. One flower has short stamens, middle-length 

 stamens, and long style ; another has short stamens, middle- 

 length style, and long stamens ; the third has short style, 

 middle-length stamens, and long stamens. In these cases 

 also the stigmas are intended to receive pollen from stamens 



Fig. 136. Yucca and Pronuba. In the lower figure to the right an opened flower 

 shows the pendent ovary with the stigma region at its apex. The upper figure to 

 the right shows the position of Pronuba when collecting pollen. The figure to the 

 left represents a cluster of capsules of Yucca, which shows the perforations made 

 by the larvae of Pronuba in escaping.— After Riley and Trelease. 



of their own length, and a transfer of pollen from flower to 

 flower is necessary. 



90. Self-pollination. — In considering these three general 

 methods of preventing self-pollination, it must not be sup- 

 posed that self-pollination is never provided for. It is pro- 

 vided for more extensively than was once supposed. It is 

 found that many plants, such as violets, in addition to the 

 usual showy, insect-pollinated flowers, produce flowers that 

 are not at all showy, in fact do not open, and are often not 

 prominently placed. The fact that these flowers are often 

 closed has suggested for them the name cleistogamous 



