Forest Devastation 51 



cankerworms have been a rather serious pest here and 

 there, particularly on the Manitoba maples. Numerous 

 gall-producing insects and leaf -curlers are also at work, but 

 usually the harm done is quite local and often confined to a 

 single tree or group of trees. These are the natural enemies 

 of the trees and under natural conditions are held in check. 

 The injury done is seldom widespread and affects trees 

 in streets, parks and gardens rather than those of the 

 forest. Local outbreaks can generally be controlled by 

 the mechanical means suggested in another chapter. 



Fungous diseases differ only in detail from insect attacks. 

 They are the attacks of one living organism upon another. 

 Any plant with green leaves is able to take food material 

 out of the air, and this, along with what it gets from the 

 soil, suffices for its support and growth. A plant which 

 has no green coloring matter in it cannot do this. It must 

 get its food ready made, and thus is compelled either to 

 live on the decaying bodies of dead plants or animals, or 

 else attack and live upon a growing plant. One of these 

 plants which lacks the green coloring matter is called a 

 fungus, and when it lives on another living plant it is a 

 parasite. The effect of the parasite upon the growing 

 host plant, if injurious, is spoken of as a disease, and trees 

 are subject to the attack of these fungous diseases. 



The fungus produces a threadlike body called a mycelium 

 which enters the host and grows through it, forming a 

 dense network. The food which should nourish the host 

 is absorbed by the parasite. When this food begins to fail, 

 either through the advance of the season or the death of 



