34 INSECTS AND MAN 



leaves may turn red or yellow, and finally dry up, from the 

 same cause. Plants often hold out signals of distress, as it 

 were, when suffering root or stem injury. Cabbage leaves, 

 for instance, become more thickly coated with bloom, as 

 protection against excessive transpiration, and the same 

 result is achieved, in other plants, by a gradual shedding 

 of leaves. 



Not only is the passage of water and raw plant foods 

 interrupted by insect damage, the elaborated foods, also, are 

 often prevented from reaching the growing points, where 

 they are most needed. Various leaf -mining insects prevent 

 the passage of starch from those organs ; other insects so 

 damage certain steins that the organs below the wounds are 

 starved, while swellings arise above the wounds owing to 

 the accumulation of elaborated foodstuffs. 



Buds may be damaged, with the result that branches, 

 abnormal in number and position, are formed, or, in the 

 case of the terminal buds of some pines, severe distortion 

 follows insect injury. The excessive exudation of gum in 

 certain trees, such as peaches and cherries, is often the 

 result of damage by insects, and many trees are much 

 weakened, and even killed, after defoliation by insects, 

 owing to the strain imposed upon them by the production 

 of new crops of leaves to take the place of those lost. 



Of the injuries and diseases due to the presence of insects 

 and their products, front rank must be given to the work 

 of gall midges. Other allied injuries are those of the 

 Hessian fly, which causes a dark green colour and swellings 

 in wheat; the apple weevil, causing apples to swell and 

 turn brown ; leaf hoppers and thrips, causing leaf curl, etc. 

 Diseases, due to the indirect agency of insects, may enter 

 plants at the point of insect injury, or may even be carried 

 to the plants by the insects themselves. A few examples 

 of plant diseases, introduced by insects, may not be without 

 interest. Potato rot is caused by a bacillus known as 

 Bacillus solanacearum, and, experimentally at any rate, 



