Additional Papers. 



321 



lined above, will show how severely it may be injured by having nails and 

 staples driven into it. Not only are many of the delicate cells destroyed, 

 but the holes formed admit sunlight, frost, and rain, each of which do 

 their destructive work by killing the cambium cells in the vicinity of the 

 wound. Figure 6 shows a tree which has been injured by a barbed wire. 



Fig. 6. Young Elm injured by barbed wire. 



The increased number of telephone and electric wires in proximity 

 to the branches of street trees is a serious menace to their welfare. Many 

 of the wires conducting currents for trolley lines and electric lights carry 

 such heavy charges of electricity that they burn notches wherever they 

 touch the trees. The damage is done in wet weather when the trees are 

 covered with a film of water.^ 



In almost every town there are numbers of trees which have been in- 



3 For an admirable study of this subject, see Bull. 91, Mass. Exp. Sta. 

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