ELECTRICAL INJURIES TO TREES. 13 



tissues surrounding the channel are apparently not injured, but the 

 annular rings which are later formed outside the burned channel are 

 much broader, resulting in the formation of a ridge on the bark. (See 

 Fig. 11.) 



Earth Discharges. 



There are many cases of lightning that are apparently earth dis- 

 charges. Their effect on the tree is quite characteristic and not at all 

 similar to the ordinary fonns of lightning strokes. Our attention was 

 called several years ago to some shade trees to which lightning had 

 apparently caused some injury. These trees were maples 5 to 18 inches 

 in diameter, growing in soil composed mainly of gravel containing 

 oxide of iron, and underneath this a stratum of quicksand. A con- 

 siderable number of the trees showed the effects of repeated earth dis- 

 charges, in some cases becoming so disfig-ured that they had to be 

 replaced for the third time. These discharges occur during thunder 

 storms, and those who have observed them for many years relate that 

 they give rise to a dull, characteristic report resembling that caused 

 by throwing a wet cloth on a hard surface. The whole tree is not 

 affected as a rule, as the lightning stroke seldom follows up the main 

 trunk, but discharges at the points of several branches. As a rule, 

 however, one side of the trunk and one or more of the limbs on that 

 side are affected and the symmetry of the tree destroyed. The first 

 indication of the discharge is shown by the immediate wilting and sub- 

 sequent death of the leaves of the affected limbs, which also die later. 

 In the course of time cracks similar to those caused by frost, and later, 

 ridges due to healing, will be seen on the trunk, showing the path of 

 the discharge, and occasionally when the injury is considerable the 

 bark falls off near the affected part of the tree. The limbs, however, 

 are not always killed, frequently splitting (see Fig. 12, Plate V.), and 

 a cracking of the wood for some depth is now and then observed on 

 the trunk and limbs along the path of discharge. 



A very much larger number of trees show earth discharges than is 

 realized, MacDougaP has called attention to some trees which appear 

 to have been injured by earth discharges. 



Whether the chemical composition of the soil has any particular 

 bearing on earth discharges is not positively known. It is known, 

 however, that there frequently exist great differences in the electrical 

 potential between the earth and air during thunder storms, and that 

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

 stantly from negative to positive. Some observations made in our 

 laboratory with a Thomson self-recording quadrant electrometer and 



I Journal of the N. Y. Bet. Gardens, Vol. III., No. 31, July, 1902. 



