54 WIND AND TREE-FORM [CH. IV 



temperature for long periods, provided they are dry and 

 thoroughly ripe. 



Wind, snow and beating hail undoubtedly do much 

 damage to the crown of trees and shrubs by mechanical 

 breakages, and by weighing them down till fractures 

 result, or by bruising the buds and twigs, and it has been 

 pointed out that the whipping action of Birches in high 

 winds may so injure the twigs and branches of surround- 

 ing trees, that an important gain of area may result, 

 and it is quite true that these mechanical injuries have 

 their effects in modifying the shapes of trees, &c. ; but 

 the alternate actions of wet and frost, and the desiccating 

 action of cold dry winds, are more important in the con- 

 nection here discussed. 



