POLLINATION AND FERTILIZATION 127 



unable to set any seods if that species of insect is not at hand 

 to carry their pollen. One famous example of this depend- 

 ence of the flower on a particular insect is that of the common 

 fig, which may bear large and juicy fruits without insect visits, 

 but cannot produce seed that will grow without being polli- 

 nated by a small species of wasp. Another instance is that of 

 the yuccas (Sect. 121). 



121. Pollination in yucca. The yuccas are mainly plants of 

 desert or semi-desert regions, especially characteristic of the 

 southwestern United States and Mexico. One species, the 

 jVdam's-needle, or Spanish dagger, is a native of the Atlan- 

 tic and GuK coast, and commonly cultivated. The flowers 

 of yuccas are white or nearly so, mostly with large spreading 

 corollas, and borne in great clusters, of one of which Fig. 116 

 represents only a small portion. The stamens are somewhat 

 shorter than the carpels, with abundant sticky pollen, and the 

 pistil consists of three carpels which are joined to form a tube, 

 which is stigmatic on its inner surface. Pollination is impos- 

 sible without insect aid, and this is furnished by a small moth 

 (^Pronuba). Unlike most cases of insect-pollination, that per- 

 formed hy the yucca moth is self-poUinatinn. 



The flowers of yucca are fully open and in condition for 

 pollination during only a short period. Throughout the day 

 the female moth remains at rest within the flower, almost hid- 

 den by the stamens (Fig. 116). At dusk she begins active 

 work, first crawling to the anthers, on the surfaces of which 

 the pollen generally remains in a lump after its expulsion 

 from the pollen sacs. She collects pollen into a mass, held as 

 shown in Fig. 117, which is sometimes three times the size of 

 her head. She then crawls over or within the flower, with 

 occasional sudden starts, until finally she takes a position 

 astride of one stamen and with her head toward the stigma, 

 as shown in the top flower of Fig. 116. Lowermg the abdo- 

 men between the stamens, she now thi'usts the sharp tip of 

 the egg-depositing apparatus (^ovipomtor) into the soft ovary 

 wall and inserts an egg into an ovule. After depositing an egg. 



