SECTION IV 



INJURIES DUE TO ATMOSPHERIC 

 INFLUENCES AND FIRE 



THE ACTION OF FROST 



THE action of frost on plants, whether fatal or otherwise, can 

 be understood only when one has gained a clear idea of the 

 sources of heat of which plants can avail themselves. 



The metabolic processes which make the more highly 

 developed animals independent to a greater or less degree of the 

 influences of external heat constitute a factor in the vegetable 

 kingdom which may be neglected, in comparison with the effects 

 exerted on plants by the heat of the surrounding media. In 

 the case of the older classes of trees, especially those which are 

 covered by thick bark, the temperature of the lower and inner 

 portions of the tree is chiefly determined by that of the soil. 

 The temperature of the surrounding air has, however, most 

 influence on branches and twigs. 



At the time of active growth, and in fact whenever trans- 

 piration of water is proceeding energetically, the temperature 

 of the interior of a plant is brought into conformity with 

 that which prevails in the soil by means of the water that 

 is absorbed by the roots. This has been placed beyond 

 the shadow of a doubt by the following experiment. Two 

 trees alike in all respects and equally exposed to the sun 

 were selected, of which one was deprived of its branches. It 

 was then found that the temperature of the tree that had been 

 left intact was 1 8 F. lower than that of the tree which had been 

 pruned. When the former was also pruned, and the ascent of 



