ECOLOGY OF FLOWERS; POLLIXATION 177 



210. Pollinatioa in Yucca. — The jTiccas and allied genera 

 are xerophytes (some of them low plants and others tree- 

 like) which abound especially in the desert and semi-desert 

 portions of the southwestern United States and in Mexico 

 (Plate ^"I). The flowers are white or whitish, borne in 

 great clusters, and are very conspicuous at night. The 

 stamens are shorter than the i)istil, and the pollen is sticky. 

 The pistil consists of three carpels M'hich form a tube 

 stigmatic on its mner surface. 



Pollination is unpossible ^rithout the aid of uisects. It is 



effected by a small moth {Pronuha) which throughout the 



day remains at rest within the flower (Fig. 132).i At 



dusk the female moth begms the work of pollinating the 



yucca flowers. She clings to the 



stamens and collects a mass of 



pollen (Fig. 133). C'ontmuing 



her work m the same flower, the 



moth stings one of the o^'aries 



and deposits an egg in an ovule, 



then mounts^ to the tip of the 



pistil and crowds some pollen 



into the mouth of the stigmatic ^''^- ^^- ^«^* °* '^''^oca 

 ,1 o !/• IT ■• -1 Moth. (Masnifled.) 



tube, belt-pollmation is thus 



p, mass of pollen held in position 

 secured. She then descends to hy spinous appendages of the 



the ovary and deposits another "^"^^'^ ^'""^■ 

 Ggg, ascends to the stigma to pollmate it, and so on. Each 

 grub A'S'hen hatched from the egg feeds on the ovule in which 

 the egg Avas laid ; but the ovules are very numerous, so 

 that many of them are left uneaten and ripen mto seeds. 



1 In the first flower (the lowest) the moth is gathering pollen ; in the second 

 she is pollinating the stigma ; in the third she is in the position of rest during 

 the day ; in the fourth in the position of rest when disturbed ; in the fifth ovi- 

 positing. 2 That is, travels away from the receptacle. 



