178 NATURAL mSTORV. [cH. X. 



and exposed to the action of the fire, is bent to the 

 side acted upon by that element. Another effect of 

 the curling of the shoot is, that the leaves, which, 

 if extended, would stand far apart, are so drawn to- 

 gether as to form a complete covering, and thus the 

 insects are at once defended from wind and rain, 

 and concealed from the view of their natural ene- 

 mies. 



If shoots can be bent by insects which attach 

 themselves to this portion of a plant, the leaves 

 must of course be more easily susceptible of this 

 operation. 



Generally, they seek the under surface of the 

 leaves, probably as that part which affords them 

 most shelter. If the upper surface of a gooseberry, 

 currant, or apple leaf be examined, it will be found 

 studded with pale, reddish, or citron-coloured emi- 

 nences ; and on the under surface, cavities will be 

 discovered answering to these eminences, and peo- 

 pled with aphides. These portions of the leaf are 

 thicker than the rest. 



A still more remarkable change of form is caused 

 in the plant by the developement of galls formed bv 

 the plant-louse. When opened, they will be found 

 hollow, and filled with a colony of these creatures. 

 In size they vary from that of a nut to that of the 

 human fist. 



