316 



FLOWERS 



Fig. 136. - 



-Pronuba depositing eggs in the 

 ovary of yucca. 



though the moth were following an instinct which impels 

 it in this way to provide food for its young. 



The ovary of yucca 

 contains many ovules. 

 While they are develop- 

 ing into seeds, the egg of 

 the moth is developing 

 into a grub, the larva. 

 The grub eats a number 

 of the young seeds, and 

 then bores its way out of 

 the ovary and drops to 

 the ground. Ripe ovaries of yucca show a number of holes 

 from which pronuba grubs have escaped. (See Figure 

 137.) In this process a good many seeds are sacrificed, 

 but still more come to maturity, which, 

 without the aid of pronuba, they are 

 quite unable to attain. On both sides 

 there seems to be fair payment for the 

 service rendered. 



The pronuba usually thrusts the pollen 

 into the pistil of the same flower in which 

 she collects it. Note that this is there- 

 fore a case of insect-pollination which is 

 not cross-pollination ; also that in this 

 case close-pollination is not synonymous 

 with self-pollination: Sometimes, how- 

 ever, after collecting pollen, the moth 

 flies to another flower before beginning to lay eggs and 

 to pollinate the pistils. In this way the pronuba may 

 effect geitonogamy or even xenogamy. 



Fig and Wasp. — Here is an even more complicated 



Fig. 137. — Fruit of 

 yucca showing holes 

 made by escaping 

 grubs of pronuba. 



