BARK-SCORCHING. 523 



gradually splits lengthwise and horizontally from the stem, and 

 falls off in pieces. Tlie injured sapwood turns brown, the 

 brown colour fading gradually towards the still sound wood. 



The exposed wood becomes rotten owing to the admission and 

 germination of spores of various species of Polyiwrus. By 

 strong and repeated insolation, the rot spreads in a wedge- 

 shaped manner down into the heart of the tree in the direction 

 of the medullary rays (Fig. 243). 



The destructiveness of the fungi is frequently hastened by 

 the fact that a strong growth of grass and herbage dries up 

 the surface moisture of the soil. 



2. Explanation. 



Bark-scorching is the result of powerful insolation. If the 

 sun beats directly on a stem, its west and south-west sides 

 become considerably heated. The southern side of a tree is 

 less heated owing to the frequent easterly winds, which blow 

 during hot anticyclones when the sky is clear, and skim past 

 the southern side of trees, and reduce the temperature of their 

 bark and sapwood on that side, whilst the W.S.W. side of the 

 tree is not affected by the east wind. This explanation is 

 confirmed by the fact, that when the southern side of a tree 

 is scorched, it has been found to be sheltered from easterly 

 winds by an adjoining dense wood. The greater effects of the 

 sun's rays on the W.S.W. side of a tree are also due to the fact 

 that the maximum daily temperature is in the afternoon, when 

 the sun has passed the meridian, and that the lower the sun 

 is, the more direct are its rays in the radial direction of the 

 stem, and the more intense are their effects. 



Vonhausen found that the maximum temperature on the 

 ^V.S.^^'. side of a tree, between its bark and sapwood, was 

 120" F. when the air-temperature was 91° F., while in 

 Bavaria, on the 18th August, 1892, with an air-temperature 

 of 96'8° F., Hartig observed a temperature of 131^ F. 

 between the bark and sapwood of some isolated 80-year-old 

 spruce trees. Cambium cells of European trees cannot with- 

 stand temperatures between 104" — 130° F. any better than 

 leaves and herbaceous shoots, which are speedily killed by 

 such temperatures. 



