130 PLANT STUDIES 
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 RILET 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 
