48 



LIGHTNING. 



Lightning affects trees in different ways, the most common effect 

 upon vegetation being a shattering of the tissues. It is surmised 

 that some trees are more susceptible to lightning strokes than others, 

 but little is known about the subject. 



A stroke of lightning is frequently dispersed in such a way as to 

 travel over more or less of the cambium zone of the tree, thus gird- 

 ling it and causing its death, and trees have 

 been observed which were killed instantly by 

 a lightning stroke but which showed no frac- 

 turing whatsoever, in which case the light- 

 ning, although not powerful enough to dislo- 

 cate any of the bark, had destroyed a consider- 

 able portion of the vital layer of the tree. 



More frequently, however, trees receive only 

 a slight discharge, which burns out a very 

 Fig. 27. Cross section Small hole near the cambium, and the result 

 •of a piece of wood taken of such a discharge will not be noticeable 



from a tree struck by light- ., , 1x1 



ning, showing the forma- ""^il two Or three years afterward. In such 

 tion of ridge on the out- cases a ridge forms on the bark, revealing the 



side of the bark, x, small 1 r i- 1 • • 



dead area corresponding P^th of discharge. An examination of the 

 to path of lightning dis- tissue will disclose a small hole, usually not 



charge. , 1 1 1 i r • • 1 



larger than the head or a pin, running down 

 near the cambium layer. A wound of even this size acts as a stimu- 

 lus and induces a marked growth of the cambium. These cases are 

 very common but often overlooked. 



Less common, are earth discharges, which originate in the 

 soil and discharge from the limbs, but there are enough authen- 

 tic cases to prove they occur, and they occasionally cause injury. It 

 is known that a great difference frequently exists between the elec- 

 trical potential of the earth and air during thunder storms, and that 

 the electrical conditions of the clouds and earth may change in- 

 stantly from negative to positive. It is then that earth discharges 

 >Tnay take place. 



