41 



tree generally assumes a dark color, indicating an absence of life, 

 but this feature is not always noticeable. The sap wood is often 

 found to be discolored, and it has peculiar, characteristic odors 

 which assist in diagnosis. Sometimes, however, especially when the 

 tree is injured by gas in late sum- 

 mer, at which time the flow of sap 

 is not so active as it is in the spring, 

 the odors of the wood are not so 

 marked. Where slow poisoning 

 occurs this same condition of the 

 tissue is noticeable. If only one 

 root becomes affected with gas, that 

 portion of the tree nearest it will 

 show the effect first. Generally, 

 however, the top of the tree first 

 shows the effects by defoliation and 

 loss of bark. The presence of fungi 

 on trees (Schizophyllum, Polystictus, 

 etc.) affected by gas is of common 

 occurrence and often significant, 

 and they frequently make their ap- 

 pearance shortly after a tree has 

 been injured. Trees affected by 

 gas disintegrate very rapidly, and 

 should never be allowed to stand 

 long after dying, as they become 

 brittle and are a source of danger. 

 Gas escaping into the soil from a 

 leak follows the line of least resist- 

 ance. P'or this reason, if leakage 

 occurs in the street in front of a 

 house one can usually detect the ,. c, ■ .1 j .. ^ « <. 



-' I'lg. 21. Showing the destructive enact 



odor of gas in the cellar, as the gas of wires on the growth of trees. 



will follow the exterior of the pipe 



leading into the cellar, and it often escapes into sewers, underground 

 conduits, hydrants, etc. There is considerable difference in the re- 

 sistance of soils to gas. In gravelly soils we have known gas to 

 travel 2,000 feet when the ground was frozen and escape into the 



