DAMAGE TO TREES BY GASES 
167 
The presence of a sudden leak of large volume, result¬ 
ing from a break in a main, will usually show itself through 
changes in the appearance of all vegetation within a nearby 
circle. When the tree is in leaf, a flowing leak of this nature 
may generally be located by noting where the damage is 
greatest, for it will be found that the leak is near the center 
of the area affected, and nearest the particular tree which 
appears to suffer most seriously. The exact location may 
then be determined by the sense of smell. 
When the trees are in foliage, gas poisoning which 
results from a sudden flow of gas makes itself immediately 
visible by the action of the gas on the leaves. As a result 
of the poison the leaves turn yellow, wilt, and finally fall. 
Later, the bark is apt to drop off in small patches, while 
dark blue stains may be found in the inner bark and in the 
roots. In the case of a smaller and more gradual leak the 
leaves turn yellow and droop, foliage is gradually thinned 
by the falling of some of the leaves, and a general 
appearance of unhealthiness and loss of vigor shows itself 
in the entire tree. 
While the signs mentioned may result from other 
causes, and do not always indicate gas poisoning, their 
appearance makes it important that an immediate investi¬ 
gation should be made for the location of a possible leak. 
The surest proof of the presence of gas is its odor, which is 
so pervasive that it cannot be overlooked and is not likely 
to be confused with any other scent. When there is the 
slightest suggestion of this odor, steps should be taken at 
once to locate its origin. Even if the sense of smell does 
not detect gas when the symptoms of trouble appear, a 
careful search should quickly be made for the purpose of 
determining whether or not gas is to blame. In this search, 
one or more holes should be made in the ground with a 
crow-bar, to a depth of three feet or more. When the 
