62 MANUAL OF TREE DISEASES 



fibers are straight and may run round the tree when the grain 

 is spiral. At times, various other types of injury may occur, 

 such as : stripping of all the bark from the tree, shattering of 

 the top of the tree, splitting and otherwise shattering the 

 entire tree, and shattering the base and allowing the tree other- 

 wise uninjured to fall over. All of these types of violent in- 

 jury are caused by lightning in a way that is not understood. 

 It is known that lightning discharges are of high voltage. 

 When the damage is greatest at. the base of the tree, the posi- 

 tive charge was probably carried by the earth, and when the 

 top is shattered, the positive charge was in a stratum of air or 

 a cloud above the tree. Many divergent opinions concerning 

 the susceptibility or immunity of different species of trees to 

 lightning stroke have been stated. Recent investigations in 

 this country and in Europe have shown that trees of all kinds 

 are liable to lightning stroke and that those which are isolated, 

 on high ground, or are more deeply rooted than siu-rounding 

 trees are the most liable to be struck. The idea that trees of 

 a certain species are more often struck than others is usually 

 due to the fact that they are more abundant or more dominant 

 than other species. 



Another less common type of lightning-injury is when groups 

 of trees die within a few days after the stroke and others around 

 these die during the next few years. This type of injury is 

 explained by the fact that the positive charge was in the earth 

 and when the flash occurred the tissues of the roots and the 

 bark at the base of the tree were killed. A few of the trees 

 were injured to the extent that they died quickly, for want of 

 water and raw food materials from the soil; others around 

 which did not receive such a heavy shock were only injured 

 in such a way that they were prevented from sending prepared 

 food to the roots and died later. This type of injury then is 

 similar to the effects produced by girdling. 



The matter of protecting shade-trees from damage by elec- 



