THE PLANT AS A HOST 39 



of a ball of pollen. As soon as this is ready, it 

 is carried off to another flower, where the insect 

 lays an egg at the base of the pistil, by thrusting 

 its ovipositor down through the soft tissue. This 

 done, the moth mounts to the summit of the 

 style and places the ball of pollen upon the stig- 

 matic surface, thereby ensuring the fertilisation of 

 the ovules. When the caterpillar hatches, it finds 

 itself supplied with food material in the surround- 

 ing tissues ; there the larva will remain until the 

 ripening of the seed, when it eats its way out, 

 crawls to the ground, and then turns to a chrysalis 

 under the surface. It is believed that the Yucca 

 never sets seed unless it has received the attention 

 of the little moth. In return the grub of the 

 insect is provided with food and protection. The 

 ravages of the little guest do not materially affect 

 the seed production, seeing that the number of 

 ovules is considerable. 



As a general rule, the numerous galls which 

 are found so abundantly on the Oak, and also on 

 certain other trees and plants, cannot be said to 

 be harmful to the vegetable. Now and again, 

 when the insects producing them become very 

 abundant, growth may be stunted by the presence 

 of the galls ; or, as in the case of the dreaded 

 grape louse of Southern Europe (Phylloxera), the 



