DISEASE A DISTURBANCE. 11 



plies the water and mineral salts, is not independent of 

 the development of the foliage. Both are correlated, 

 and one must not overbalance the other. 



DISEASE A DISTURBANCE OF NORMAL FUNCTIONS. 



All diseases, properly speaking, can be traced to some 

 interference with the normal functions just described 

 and the economic equilibrium established in a sound 

 tree. 



Leaf diseases, by killing a greater part of the foli- 

 age, destroy the very organs in which food for the 

 growing tissues of the tree is prepared. The cambium 

 does not receive proper nourishment and can not grow, 

 while the hairs on the root tips, which must be renewed 

 continually, are no longer developed for lack of food, 

 with the result that when once the surplus of food in 

 storage in the wood is also exhausted no more water is 

 pumped up to the remaining leaves. 



Diseases of the bark of branches and trunk intercept 

 the flow of food coming down in the bark from the 

 leaves. The result is starvation of cambium and roots. 

 This explains how damage is done by mistletoes, by 

 Peridermium and other bark-destroying fungi, and by 

 bark beetles. The effect is the same as girdling the 

 tree with an ax. The bark also stores reserve mate- 

 rials, which are eliminated by the disease. 



Diseases of the sapwood cut off the water supply 

 which is pumped upward from the roots in the living 

 sapwood to the upper parts of the tree, and the leaves 

 suffer or die of drought. Any reserve materials stored 

 in the sapwood are lost to the tree. 



