74 Diagnosing of Diseases 
It stands to reason that the stronger and more shade- 
enduring species, having also a denser crown, will retard 
the development and kill out the light-needing varieties 
that may have been planted under its shade. The sensi- 
tiveness to withdrawal of light increases with deficiency of 
soil moisture. Pruning back of the stronger will give the 
weaker a chance for development. 
Care is necessary, however, with some species, especially 
conifers, in not opening up too rapidly, so that the foliage 
may gradually adapt itself to the new light conditions. 
Wherever there are trees planted in groups or as close 
neighbors this relative shade endurance becomes one of 
the most important factors in influencing the form develop- 
ment of the tree, and where light-needing species are in 
competition with shade-enduring, the former are likely to 
lose the battle. 
Mechanical Injuries. Finally, if none of the causes 
of disease so far discussed can be adduced as occa- 
sioning abnormal conditions, we must look for mechanical 
injuries. 
As we have seen, mechanical injuries to the bark and 
wood, by whatever agency inflicted, are first of all injurious 
by admitting destructive fungi. In addition, the bark, 
branch, or bole being broken or torn off in places, much of 
the underlying cambium and young wood is killed and 
rendered inactive in conducting water and food materials. 
As a consequence, some parts of the crown and of the root 
system are underfed and suffer in proportion to the extent 
of the injury, until the wound is healed and closed up. 
Usually, such injuries are one-sided and it is, therefore, a 
particular branch or one side of the tree which shows the 
influence of the under-feeding in the foliage or in dying 
portions. More regarding the nature and physiological 
