INJURIES DUE TO ATMOSPHERIC INFLUENCES 283 



water consequently stopped, the temperature at once rose 1 8 F. 

 When the soil is frozen so that no water can enter by the 

 roots, the tree receives heat from the soil only by the process 

 of direct conduction. This, however, is always of sufficient im- 

 portance to explain why the temperature of the interior of 

 a tree, even during prolonged cold, rises as we descend ; and 

 also why a deep soil, in which the roots descend to long dis- 

 tances, has a more favourable thermic effect on trees than a 

 shallow soil. This also explains why a natural or artificial 

 covering on the soil is so useful in enabling fruit and ornamental 

 trees to resist the winter's cold. The reason also why certain 

 trees that are easily frosted when young are apparently less 

 sensitive to cold in later life or become "hardened," as it 

 is called is to be traced to the greater amount of heat which 

 the roots receive when they have penetrated to greater depths. 



The extraordinary rapidity with which shrubs and trees 

 become green in spring after a heavy shower of warm rain is 

 also due to the rise in temperature of the soil. Finally, the 

 early appearance of leaves on the smaller classes of trees in a 

 wood, as compared with the larger trees, is due to the fact that 

 the soil-strata in which the roots of the former are chiefly 

 distributed experience a rise in temperature at a time when the 

 cold of winter still prevails in the deeper strata, and it is from 

 the latter that the stronger and more vigorously developed roots 

 derive their heat. 



It is the temperature of the surrounding air that chiefly 

 determines the temperature of twigs and branches, as well 

 as of all the more delicate parts of plants generally. Heat 

 penetrates with extreme slowness into the interior of those 

 portions of a stem which are covered with a very thick periderm 

 or a layer of bark. It is only when insolation is uninterrupted 

 that the side of a tree which is exposed to the sun's rays may 

 become heated to such a pitch as to induce such pathological 

 phenomena as " Bark-scorching " and " Sun-cracks." As op- 

 posed to the heat which plants receive, we have the loss of heat 

 which they experience. Owing to the evaporation of water, heat 

 is directly abstracted from the tissues where this process is active- 

 The process of assimilation is also connected with loss of heat. 



The rate of cooling is, however, most influenced by radia- 



