BODY AND BRANCH DISEASES AND INJURIES 63 



trie currents carried by wires is of importance in every city. 

 Any device made of a substance which is a non-conductor 

 (porcelain, or rubber) and which keeps the wires from coming 

 into contact with the limbs, will prevent short circuiting. If 

 the additional injury caused by rubbing is to be avoided, some 

 arrangement must be made for locating the wires so that they 

 are taut and do not come within three or four inches of the limbs. 



References 



Stone, G. B. Electrioal injuries to trees. Massachusetts Agr. Exp. 

 Sta. Bui. 156 : 1-19, pis. 1-5. 1914. 



Plummer, Fred G. Lightning in relation to forest fires. U. S. Dept. 

 Agr. Forest Ser. Bui. Ill : 1-39, pis. 1-2, figs. 1-16. 1912. 



MacDougal, D. T. Effect of lightning on trees. Proe. N. Y. Bot. 

 Gard. 3 : 1902. 



Hartig, R. The effects of lightning. In Text- 

 book of the diseases of trees, pp. 302-304. 

 1894. 



Galls 



Caused by various insects, fungi and bacteria 



Trees of all kinds often develop large or 

 small galls of various types on the trunk 

 and limbs. The causal agents in many cases 

 have not been studied, but it is supposed 

 that certain insects, fungi and bacteria are 

 frequently responsible for the irritation 

 which results in the overgrowth in the 

 tissue. Mechanical injuries may in some 

 cases also cause galls. The causal agent 

 initiates abnormal division of the cells and 

 the tissues may continue the over-develop- 

 ment for years after the cause is gone. 

 The limb-galls of poplar are known to be 

 caused in some cases by the crown-gall 



■ Galls on 

 of oak. 



