294 



DISEASES OF TREES 



progress only in a frosty year distinguishes it from fungoid 

 canker, which spreads every year. It is further to be noted that 

 the late frost also kills the wood at the exposed region as far in 

 as the medulla. The products that result from the decomposi- 

 tion of the contents of the dead cells distribute themselves more 

 or less both up and down the stem, whereas in the case of 

 fungoid canker the exposed wood usually becomes brown only 

 on the surface. 



In the case of many trees, especially exotic dicotyledons, the 

 small fissures in the cortex which are induced by cold prove the 

 primary cause of canker. 



BARK-SCORCHING, SUN-CRACKS, AND DEFICIENCY OF 



LIGHT 



In science and in practice two entirely different phenomena 

 are referred to under the first of these terms. The more 

 frequent phenomenon, which I shall specially designate bark- 

 scorching, is caused during the months of July or August by the 

 action of unusually strong sunshine on the bark of smooth- 

 stemmed trees which have been suddenly exposed after growing 

 up in a close wood. 



The trees that suffer most from bark-scorching are the beech, 

 hornbeam, spruce, Weymouth pine, and silver fir.* The 

 commonest causes of exposure are the formation of roads, 

 railways, or rides, or the retention of certain trees for the 

 production of seeds, or as standards. 



The injury to the bark by drying up and exfoliation occurs 

 almost always on the south-west side, the reason being that this 

 is the side on which the sun's rays impinge at the time of the 

 maximum daily temperature. 



The extensive clear-felling of spruce woods that had been 

 entirely or largely defoliated in Upper Bavaria by the nun 

 moth afforded an opportunity for some careful observations on 

 the temperature of isolated trees. On August 18, the warmest 

 day of 1892, when the thermometer registered 96-8 F. in the 



* [These injuries occur not unfrequently even in our climate. It should be 

 noted that the word " bark" is here used in a somewhat loose N sense : true 

 bark is dead, and it is the living tissues below which suffer. ED.] 



