CROSS-POLLINATING 15 



ating is impossible among plants which have imperfect flowers. 

 Other conditions or structures may make cross-pollinating neces- 

 sary (Fig. 9). Self-pollination is sometimes compulsory, as in 

 hidden flowers of the violets, blossoming under ground (Fig. 10). 

 Cross-Pollinating. Any plan or structure which requires that 

 the pollen be carried from one flower to another will enforce cross- 

 pollination. Certain perfect flowers may have their pistils and 

 stamens mature at different times. If the pollen is not "ripe" or 

 ready to be distributed at the time the pistil is ready to receive 

 the pollen, then pollen must come, if at all, from some other 

 flower where it is more mature. Again, if the pollen is ripe before 



FIG. 8. Cross-section of head of wild sunflower showing the individual flowers in different 

 stages of development. Note the curled stigmas at the left. (U. S. D. A.) 



or after the pistil in that flower, then the pollen can be useful only 

 in some other flower where the pistils are in the right condition. 



The structure may also be an important factor in causing 

 cross-pollination. In drooping flowers the pistils may be shorter 

 than the stamens so that the pollen will fall away from, instead of 

 toward, the pistil. In erect flowers the pistil may be longer than 

 the stamens so that the pollen cannot fall directly upon the stigma 

 or receiving surface of the pistil. 



In certain varieties of fruits, as in the apple, it has been found 

 that perfect flowers may be self -sterile. The pollen falling upon 

 the pistil of the same flower will not grow so well as pollen received 

 on the same stigma surface from other varieties of apples. It is 

 for this reason that two or more varieties of apples are frequently 



