MORE ABOUT POLLINATION 



315 



pollen is concealed in the throat of the corolla and the 

 nectar is in the bottom of the spur, but both can be easily 

 reached by the tongue of bees. 



Yucca and Pronuba. — This is one of the most famous 

 cases of mutual dependence between a plant and its polli- 

 nating insect ; there is a sort of partnership in their repro- 

 ductive process, each being necessary to the other. 



The yuccas grow chiefly in deserts, especially in south- 

 western United States and Mexico; the Spanish bayonet 

 is a commonly cultivated species. (See Figure 134.) The 

 pronuba is a small moth most of whose 

 life is spent upon yucca plants. 



Pollination is performed by the female 

 moth. During the day she remains hid- 

 den within the flower. At twihght she 

 begins her work. First she visits the 

 stamens and collects a mass of pollen. 

 (See Figure 135.) Still holding the pol- 

 len, she moves about the flower and 

 finally comes to rest astride of one of the 

 stamens and with her head toward the 

 stigma. (See Figure 136.) While in this 

 position she thrusts her sharp-pointed 

 egg-depositing organ (the ovipositor) 

 down between the stamens, through the 

 wall of the ovary, and in among the 

 ovules. There she deposits an egg. As soon as this is 

 done, she rushes to the top of the hollow pistil and vig- 

 orously thrusts pollen down into it. She then repeats the 

 performance, going to the top of the pistil and thrusting 

 pollen into it after each egg is deposited. In this way the 

 same pistil is usually pollinated several times. It is as 



Fig. 135. — Pronuba 

 collecting pollen 

 from a stamen of 

 yucca. The sta- 

 mens open at the 

 top. 



