STREET TREES. 29 



be killed and the quicker the poison will act. Conversely a 

 small leak, or one at some distance from a tree, may induce 

 so slow an action that the real cause of the tree's failure 

 cannot be determined. The activity of gas poison depends 

 also upon the character of the soil, the material of which the 

 road and sidewalk are made, the species and the vigor of 

 the tree. For instance, clay soil will allow gas to escape 

 from it less quickly than sand, but if the road surface above 

 it is gas tight tree roots in either will have little chance. 



Xot infrequently an actual leak can be lo- Locating 

 ?ated by observing that trees and other vegeta- a Ieak< 

 tkai are affected with variable severity within a circle having 

 a radius up to one hundred feet. The source of the trouble 

 will be found near the center of the circle and near the tree 

 that suffers most. A poison case may develop so rapidly 

 from a large, sudden leak that several trees will be killed 

 within a day or two. Ordinarily, however, leaks are small 

 and the poison spreads slowly. Sometimes only a part of a 

 single tree is affected. 



But it is often difficult to establish a case of signs of 

 poison because most of the evidences may be SHS poison - 

 produced by other causes. If, however, the leaves develop 

 less size than usual, or wilt after they unfold usually be- 

 ginning at the ends of the branches or if they droop sud- 

 denly in mid-season, a gas case may be suspected. Later 

 and surer signs are dark blue stains in the inner bark. These 

 stains will often be found in the roots when they do not ap- 

 pear in the body. Loosening of the bark usually follows 

 death, whether gas or something else be the cause. One who 

 is not expert should hesitate to declare a case of this kind. 

 The odor of gas is the surest guide to its presence. If that, 

 or a failing tree, indicates a leaky main drive a crowbar 

 three feet into the ground as nearly as possible over the joint 

 that may be faulty. Then withdraw it and put your nostrils 

 to the hole. Don't try with a match for that may cause an 

 explosion. If gas is present lose no time in having the 

 leaky joint, or joints, exposed and made tight. Where trees 

 are endangered it is important to free the soil of poison at 



