148 THE CULTIVATED EVERGREENS 



bark may be torn from the trunk, either in continuous Hues 

 or at various isolated points. The whole or a portion of a tree 

 may be killed, and this often takes place without any external 

 signs of injury. It is probably due to a killing of the root 

 system or to a girdling of a large or small zone of the living 

 tissues. Many trees are struck, however, which show no ill 

 effects to the untrained observer. There is apparently no 

 difference in the susceptibility of the various kinds of trees 

 to injury by lightning, but it is more likely to occur on those 

 which are more or less isolated on high ground and which 

 have deep root systems. 



High-tension line wires may cause a local burning or a 

 partial destruction of trees, due to leakage from the wires 

 when they are in contact with the trees or when they are 

 poorly insulated. This leakage occurs particularly in wet 

 weather when there is a film of moisture on the surface of the 

 tree. In rare cases, trees may be killed outright. 



Apart from their unsightly character, such wounds, unless 

 properly cared for, allow many destructive fungi to gain access 

 to the wood of the tree. They may easily be prevented by 

 insisting on the proper insulation of the wires. 



Injury due to illuminating gases. 



There are several kinds of gases used for illuminating and 

 heating purposes, such as water gas, coal gas, gasoline gas, 

 acetylene gas, and others, all of which are more or less poison- 

 ous to vegetation. The injury caused to trees is due to leaky 

 mains, from which the gases penetrate into the soil about the 

 root systems. Certain of the substances are absorbed with the 

 soil-moisture, producing various reactions in the tissues of 

 the trees. Conifers are very resistant to gas poisoning and may 

 completely recover from an exposure. 



