INJURIES DUE TO ATMOSPHERIC INFLUENCES 295 



shade, and 1 04*9 F. on a felled area that was not exposed to 

 the wind, it was found that on the south-west side of eighty- 

 year-old spruces fully exposed to the sun the temperature was 

 131 F. between the wood and the bark. Four weeks later the 

 whole of the south-west side of most of the trees had died. 

 The high temperature may possibly be explained by the fact 

 that the trees had small crowns, and that consequently but little 

 water found its way up the younger wood-rings. By com- 

 paring the temperature of the cambium of beeches, spruces, and 

 pines of the same age and thickness, the influence of the cortex 

 and bark in modifying the temperature of trees fully exposed 

 to the sun was determined. On September 30, at 10 A.M., when 

 the temperature of the air was 69*8 F., the temperature on the 

 south-east side of the thin-barked beech was 98*6 F., of the thin r 

 barked spruce 82-4 F., and of the thick-barked pine 68 F. This 

 would appear to indicate that in trees with thin periderm or 

 bark the branches on isolated individuals come well down the 

 stem, so as to afford protection against the sun, a state of things 

 that one does not find to the same extent in trees with thick 

 bark. 



On standards in a young wood bark-scorching first appears, 

 and is most severe, near the surface of the ground. There are 

 two reasons for this. First, the rays reflected from the ground 

 increase the temperature ; and, secondly, the air-currents that 

 assist so materially in cooling those parts of a tree which are 

 exposed to the sun are interfered with by the young trees. 



Even the parenchymatous tissues of the injured parts of the 

 stem succumb to drought, and the alternate desiccation and 

 saturation with external moisture induces rapid decomposi- 

 tion, which of course speedily affects the internal portions of 

 the stem. Should parasitic tree-fungi effect an entrance, the 

 tree may be rapidly killed, but otherwise the decomposition 

 retains the simple character of wound-rot. 



I investigated and described a disease which I found in a 

 wood of Weymouth pines about forty years old. 1 This disease 

 both agrees with and differs from bark-scorching, and may be 

 designated "bark-drought." The extraordinary drought of 1876 

 had reduced the supplies of water in the trees of a wood growing 

 1 Untersuchungen, III. pp. 145 149. 



