140 SHADE-TREES IN TOWNS AND CITIES 



"The Commissioner of the Western Section believes that 

 this condition arises from the deposit, upon the leaves, of 

 the tar-dust stirred up by the intense circulation of vehicles. 



"The Public Roads Service will be informed of these 

 facts; and perhaps it will be necessary to decide to abandon 

 the tarring in the proximity of vegetation." 



While the above letter would seem to indicate that there 

 is a relation of cause and effect between the tarring of the 

 Bois de Boulogne and the damage to near-by trees the case 

 is not to be regarded as fully proved. The assigning of the 

 cause of the wilting of the foliage does not seem to be 

 conclusive. The statement about the dying of the stone- 

 crops the very next day after the tar was applied to the 

 road, would lead one to suspect that possibly fumes from 

 the tar were responsible for the injury rather than tar-dust, 

 for a fresh application of oil or tar effectively lays the dust. 



The subject needs further very careful study before any 

 definite conclusions can be drawn. The writer has begun 

 some experiments to determine the effect of dust collected 

 from oiled roads on the foliage of trees; but has not yet 

 reached any satisfactory results. 



ILLUMINATING GAS 



This is extremely poisonous, and is fatal to any tree the 

 roots of which are exposed to it for a sufficient length of 

 time. It is one of the hardships to which city-trees are 

 exposed that it is not always easy to prevent or foresee. 

 Frequently trees are killed before the leak is discovered. 



While poor construction of mains is frequently responsi- 

 ble for gas-leaks, breaks in mains occur from a great many 

 causes. Trolley-cars, steam-rollers, and other heavy traffic 

 on highways sometimes cause loosening of joints and even 



