17 



would not cause any perceptible burning but which would produce a 

 retarding effect in the plants' activities thus resulting in a weakened 

 condition of the tree. If, for example, a tree was subjected to a cur- 

 rent of a strength equivalent to that represented between r and mx 

 in Figure 3, page 6, (retardation current), it would be subject to 

 over stimulation which might result in its becoming sickly and ulti- 

 mately dying. Indeed, such a case might not only be possible, but 

 is extremely probable, inasmuch as a few instances have come under 

 our observation where trees were apparently dying from this cause, 

 without any indications of burning. Such cases, however, are 

 extremely rare. Then again lightning arresters, when placed near 

 trees, may succeed in causing injury by discharges. This opinion, 

 however, is merely conjectural, although it has the support of some 

 observations. It is known that occasional leakage takes place 

 through the soil, as is manifested by the behavior of sensitive instru- 

 ments commonly used in laboratories. Probably the amount of 

 ground leakage which may occur through imperfect rail connections, 

 thus charging water and gas pipes and occasionally charging the 

 soil, would not cause any perceptible injury to trees. 



On the whole, the cases of killing trees by escaping electricity are 

 very rare, and by no means so numerous as is generally believed. 

 Because a large number of trees, adjacent to electric railroads, 

 happen to become sickly, it must not be concluded that electricity is 

 the cause of these abnormal conditions in all cases. In cities and 

 towns, where most of the sickly specimens exist, there are various 

 adverse conditions with which trees have to contend. It is, there- 

 fore, quite essential, in diagnosing diseased conditions that these 

 various factors be taken into consideration before forming definite 

 opinions as to the real cause responsible for their abnormal 

 condition. 



LIGHTNING. 



The common effects of lightning strokes upon trees are so well 

 known that it is not necessary to dwell upon them here. Lightning, 

 however, does not always strike a tree in the same manner, or pro- 

 duce the same results, and the peculiar effects which it displays are 

 often interesting. Sometimes trees are killed outright by lightning 

 without displaying any of the common effects of shattering. Appar- 



