244 MASS. EXPERIMENT STATION BULLETIN 170. 



with changes in the potential of the earth and atmosphere. The high 

 electrical resistance shown by plants in general, as already stated, serves 

 as a great protection against death from lightning and electric currents. 



Susceptibility of Different Trees to Lightning Stroke. — There has always 

 been much difference of opinion in regard to the susceptibility and non- 

 susceptibihty of various trees to lightning, and the data of the subject 

 gathered from this and that source are altogether too meager to admit of 

 reliable conclusions; but it is known that the location of the tree, nature 

 of the soil, elevation, etc., are- of great importance in determining sus- 

 ceptibility to lightning. 



It has already been pointed out that electrical resistance is influenced 

 by temperature, and the percentage of moisture in the tissues is also an 

 important factor. During thundershowers, trees become more or less 

 drenched with rain, and, according to Stahl,^ the more thoroughly wet 

 the tree is the less susceptible it becomes to lightning stroke. He bases 

 his observations on the fact that smooth-bark trees, like the beech and 

 others, which are considered more immune to lightning, become thoroughly 

 wet during storms, while the oak and other rough-bark trees do not. 

 Stahl's idea, therefore, is that smooth-bark trees possess a better water- 

 conducting surface and have a tendency to equalize the electrical tension 

 existing between the atmosphere and the ground, so that they are ren- 

 dered less susceptible to lightning. His deductions were based upon 

 experiments with electrical discharges made Avith the bark of different 

 species of trees containing various percentages of moisture. He further 

 observed that vertical limbs were more likely to become drenched than 

 horizontal, and that the lenticels and stomata play a role in the equaliza- 

 tion of the difference in electrical potential existing between the tissues 

 and the atmosphere. There appears to be no difference in the electri- 

 cal potential, at corresponding heights, under deciduous trees and in the 

 open air when there is no foliage, wliile the electrical potential will average 

 40 per cent, less under the trees than in the open air when the foliage is 

 developed. 



The potential of the air is usually negative, although occasionally 

 changing to positive. In the case of coniferous trees, however, like the 

 Norway spruce,^ we found that the potential under the foliage was in- 

 A^ariably positive or similar to that of the earth, which may be explained 

 on the theory that conifers are constantly discharging positive electricity 

 to such an extent that the air surrounding them becomes charged similar 

 to the earth. To what extent the film of water on the bark is ca})able of 

 equalizing the difference in electrical potential in the air surrounding the 

 trees, as well as in the ground and in the tissues themselves, has not been 

 wholly determined, but we had difficulty in obtaining potential readings 

 under the foliage of ehns in wet weather in our experiments covering 

 two summers. This may in part be explained by the improper installa- 



1 Stahl, E. Die Blitzgefahrdung der verschiedenen Baumarten, Jena, G. Fisher, 1912. 

 « Mass. (Hatch) Agr. Exp. Sta. Rept., 1905, p. 14. 



