SHADE TREES. 



221 



cause leakage; and, finally, the wires, steel rails, etc., carrying electricity 

 are a constant source of danger to gas pipes, as is occasionally proved by 

 cases of electrolysis. 



A large amount of the gas manufactured is unaccounted for, often 

 averaging 10 per cent. This loss may be accounted for in part by dis- 

 crepancies in meter readings, etc., and should not be laid wholly to leak- 

 age, and a small percentage of unaccounted-for gas is of slight importance. 

 It should be stated in justice to many of the large gas producers that every 

 effort is usually made to prevent leakage and injury to trees. Some of 

 the most progressive manufacturers spare no 

 expense in constructing and maintaining their 

 lines, although it must be confessed that there is 

 great need for improvement in methods of convey- 

 ing this dangerous substance. The larger pipes, 

 which are more difficult to keep calked securely, 

 furnish better facilities to patrons; nevertheless 

 the danger from leakage is greater. There are 

 numerous connections in gas mains from which 

 the leakage is slight, perhaps only a few cubic feet 

 a day, while in others it is very great. Even small 

 leaks, if neglected, will injure trees in the course 

 of time, owing to the gradual saturation of the 

 soil with gas. 



There are several kinds of gas used for lighting 

 and heating, i.e., water gas, coal gas, gasoline gas, 

 acetylene gas and others, but their effects on the 

 plant are quite similar, and they are all very 

 poisonous to vegetation. The poisonous prop- 

 erties are largely confined to the numerous 

 products absorbed in small quantities by the soil 

 moisture, taken up by the roots and translocated 

 through the tissue. The reactions to the substances 

 are not quite the same in different locations nor 

 in different species of trees. Trees poisoned by 



illuminating gas usually show some characteristic post-mortem symptoms, 

 but the problem of diagnosis is greatly complicated by the fact that 

 many of these symptoms may be found in trees dying from other causes. 

 More or less rapid deterioration and increased brittleness of the wood 

 are quite characteristic symptoms, however. 



In summer, the first effects of gas poisoning may be seen in the foliage. 

 The leaves often turn yellow and drop off, while in other cases the leaves 

 will fall when still green, and, again, the leaves will turn a reddish brown 

 and die without falling. The upper part of the tree, being far from the 

 source of water supply, usually shows the effects of defoliation first. These 

 various symptoms occur before there is any evidence of abnormal tissue 

 above ground. But after the water in the soil containing the poisonous 



FIG. 93. Maple tree dy- 

 ing from the effects of 

 illuminating gas, with 

 characteristic fungous 

 (Polystictu*) growth. 



