270 THE YUCCA MOTH 



penetrate certain mysteries without presupposing a stage 

 manager. 



The American yucca is well known the plant that 

 sends up a cascade of ivory white bells from the centre 

 of a sheaf of leaves like painted tin. It condescends to 

 flower under our cloudy skies, but never bears seed in 

 this country, not, apparently, because of our cloudiness, 

 but for want of the offices of a little moth (Pronuba). The 

 anthers in the yucca blossom are only half as long as the 

 pistil, which has its orifice at the extreme tip, so that the 

 pollen from the anthers can never reach it unless helped 

 by external agency. Pronuba lays her egg3 by means of 

 a sharp ovipositor near the base of the pistil among the 

 embryo ovules. But these ovules, upon which the moth's 

 grubs depend for food, would never develop unless the 

 pistil were properly fertilised. Pronuba is careful to 

 attend to this. She enters a flower, collects a pellet of 

 pollen from the anthers, flies with it to another flower, in 

 the ovary of which she deposits her eggs, and then swiftly 

 plugs the orifice of the pistil with the pellet of pollen 

 brought from the other flower. A double purpose is thus 

 effected; the flowers are cross-fertilised as is essential to 

 the vigour of future seedlings, and the ovules, swelling 

 into succulent seeds, afford provender to the young brood 

 of Pronuba. The yucca depends for propagation on what 

 the grubs leave untouched. In this complex process 

 there is a distinct act of volition on the part of Pronuba, 

 quite different from the casual transference of pollen 

 from flower to flower on the hairy bodies of bees and flies. 

 But science is dumb if you ask her how Pronuba learned 

 her part ; she can only report the performance. 



