THE FLOWER 



243 



FIG. 352. Pod 

 of yucca pierced by 

 the Pronuba yuc- 

 casella. 



the wind, it is clearly to the advantage of a plant to attract 

 such visitors, even at the expense of a little honey, or of a 

 liberal toll out of the pollen they distribute. 



278. Special partnerships. Some plants have adapted 

 themselves to the visits of one particular kind of insect so 

 completely that they would die out if that 

 species were to become extinct. The well- 

 known alliance between red clover and the 

 bumblebee was brought to light when the 

 plant was first introduced into Australia. 

 It grew luxuriantly and blossomed pro- 

 fusely, but would never set seed till the 

 bumblebee was introduced to 

 keep it company. 



A remarkable partnership of 

 this kind exists between the 

 pronuba, or yucca moth, and 

 the flowering yuccas, of which the bear's grass 

 and Spanish bayonet are familiar examples. 

 The pods of these plants are never perfect, but 

 all show a constriction at or near the middle, 



such as is some- 

 times seen in 

 the sides of 

 wormy plums 

 and pears. 

 This is caused by the larvae 

 of the moth, which feed upon 

 the unripe seeds. A glance 

 under the nodding perianth 

 of a yucca blossom (Fig. 354) 

 will show that the short stamens are curved back from the 

 pistil in such a manner that, under ordinary circumstances, 

 the pollen cannot reach the stigma except by the rarest 

 accident. But the yucca moth, as soon as she has deposited 

 her eggs in the seed vessel, takes care to provide a crop of 



FIG. 353. 

 Pronuba polli- 

 nating pistil of 

 yucca. 



FIG. 354. Moth resting on yucca 

 blossom. 



