300 DISEASES OF TREES 



covered with snow, and should the apex of such a branch become 

 frozen into the upper layers, it may readily happen that during 

 the gradual melting and shrinking of the snow the branch is 

 forcibly detached from the stem altogether. Such wounds 

 frequently form the means of entrance for the above-named 

 parasitic fungi. 



Gales may fracture stems or tear trees up by the roots, but 

 such injuries fall rather within the limits of a treatise on 

 sylviculture or forest management than of pathology. 



INJURIES DUE TO FIRE, COAL SMOKE, AND LIGHTNING 



Attention may here be directed to the fact that the destructive 

 effects of the passage of fire over the ground of a wood depend 

 not only upon the intensity and duration of the conflagration, 

 but also upon the species and age of the trees, or, in other words, 

 upon the amount of protection afforded by the cortex and bark. 

 As is known, the lower portions of the bark of old pines may be 

 perfectly black and charred without the cambium being killed. 

 This is due to the low conductivity of the bark for heat.* If ro 

 brownness is to be observed in the younger layers of the bast, 

 it is evident that the fire can have done no damage. On the 

 other hand, trees with thin bark are very sensitive to fire, and by 

 making a few incisions in the cortex one may determine whether 

 it has been killed or not. Although trees whose lower cortex is 

 damaged may produce fresh leaves, one must not be deceived 

 by such a state of things. Trees that are no thicker than one's 

 arm become green in spring when the lower cortex is charred 

 or withered right round. A similar state of things occurs with 

 beech-saplings that have been barked by mice, but in both cases 

 the trees ultimately wither up entirely. During the growing 

 season the starch that is stored up in the stem at a lower level 

 than the dead cortex is utilized by the cambium which is no 

 longer nourished from above in the formation of the wood-ring^ 

 so that when the trees die in the course of the summer the 

 stools, being destitute of reserve supplies, are unable to produce 



* [In cases where the cambium is scorched for some distance, but not 

 entirely, round the stem, the remnant of living cambium may slowly creep 

 round and form callus over the injured side : years afterwards, on felling 

 such parts of the stem present " ring-shakes. 1 ' ED.] 



