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 hght 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. INIore regarding the nature and physiological 



