242 MASS. EXPERIMENT STATION BULLETIN 170. 



like an avalanche, causing a severe shattering of the tissue, while less 

 powerful discharges may remove a strip of wood only a few inches wide 

 and 1 or 2 inches thick. Lightning often takes a spiral course, following 

 the grain of the wood, which is sometimes very irregular. Even when 

 strips of wood 4 or 5 inches wide and 2 or 3 inches 

 thick are removed, in which case the electrical energy 

 is enormous, the path of the discharge is shown only by 

 a dark-colored streak 2 or 3 millimeters wide. 



Sometimes trees are killed outright by lightning with- 

 out being shattered or displaying any other of the com- 

 mon effects. In such cases the discharge is apparently 

 dispersed so as to cause no visible mechanical injury to 

 the tree, but the girdling of a large or small area of the 

 living zone or cambium layer of the trunk would be 

 sufficient to cause its death. This might follow as a 

 result of an earth discharge either destroying the vital 

 tissue directly or by a dissipation of heat over a surface 

 film of moisture. In some instances the leaves wilt im- 

 mediately and die, indicating injury from heat. How- 

 ever, in a very large number of instances neither death 

 nor mechanical injury of any importance takes place. 

 Hundreds of trees are annually struck by lightning 

 that never show any effects except to those capable 

 of interpreting the small narrow ridges which later 

 make their appearance on the trunk. In such cases 

 the lightning discharge follows the line of least resist- 

 ance, the cambium zone, burning a small chan- 

 nel usually about 1 millimeter in diameter. The 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. 



Earth Discharges. There are many cases of light- 

 ning that are apparently earth discharges. Their effect 

 on the tree is quite characteristic and not at all 

 similar to the ordinary forms 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 

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

 charges, in some cases becoming so disfigured that they had to be replaced 

 for the third time. These discharges occur during thunderstorms, 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 



FIG, 101. Showing 

 ridge on elm tree 

 caused by feeble 

 lightning dis- 

 charge. 



