INJURIES DUE TO ATMOSPHERIC INFLUENCES 297 



influence of the sun's rays that it expanded violently, and so 

 became detached from the wood. The question, however, has 

 not yet been settled by experiment, and unfortunately it is 

 scarcely possible in this way to determine the factors that 

 combine to produce sun-cracks. 



As a further result of dryness of the air and of excessively 

 strong sun, the premature withering and fall of leaves may here 

 be mentioned. In 1876 I -had the opportunity of observing this 

 in an intensified form in all the beech woods on south and west 



FIG. 159. Transverse section of an oak-stem showing numerous sun-cracks. One 



half natural size. 



slopes in the northern Harz. The beech pole-woods were 

 almost entirely defoliated in the end of August that is to 

 say, nearly two months before the normal time of the fall of 

 the leaf. As this state of things was manifest even on fairly 

 fresh ground, it must be attributed to an abnormal rate of 

 transpiration from the leaves during the hot dry summer, to 

 compensate for which water could not be conveyed quickly 

 enough from the ground. 



When plants have been kept in a humid atmosphere, as, for 

 instance, in a forcing-house, a conservatory, or under the shade 

 of a close wood, the shoots, but es pecially the leaves that are 



