Street Trees. 307 



to control such dangers means the enforcing of old laws and perhaps 

 the adoption of new ones. 



(a) Gas. The number of trees suffering from the effects of gas 

 appears, from reports, to be increasing. The introduction of larger gas 

 mains to meet the increasing demands of consumers has magnified the 

 opportunity for leakage into the surrounding soil; and this, together 

 with the fact that street surfaces are paved or macadamized, which 

 tends to confine the escaping gas, explains the sickly and unhealthy 

 appearance of thousands of shade trees. Professor G. E. Stone, of 

 the Massachusetts Agricultural College, for a number of years has 

 conducted experiments on the injury to trees by gas escaping into the 

 soil, and he has demonstrated the fact that hundreds of trees in cities 

 and towns are killed each year by this means. The degree to which 

 trees may be injured depends on the quantity of gas coming in contact 

 with the roots. In many localities the volume of escaping gas may 

 be but a few cubic feet each day. The trees affected by small amounts 

 very often show no marked effects the first few months or even years; 

 however, with this continual flow into a soil from which it cannot 

 escape, the time will finally come when the trees will be killed. 

 When large quantities of gas escape each day, the trees whose roots 

 penetrate the soil that is vitiated, will show signs of injury in a few 

 months, and sometimes in a few weeks. An instance of this is cited, 

 in which, in one small city with four miles of laid pipe, there were 

 along the line of the pipe, two years after laying, about one hundred 

 trees that had been injured beyond recovery. Here Professor Stone 

 estimated that three or four hundred additional trees had been so 

 strongly affected by the gas that they were certain to die pre- 

 maturely. 



One of the injuries resulting from escaping gas is the presence of a 

 greater or less amount of dead wood throughout the specimen each 

 year. This condition is frequently seen when the leakage is small, 

 and the ground, being aerated because of its open character or because 

 of the absence of a water-tight layer of road material, does not 

 become sufficiently charged to cause immediate death to the tree. 

 In the case of severe injury to large trees, causing their complete or 

 partial defoliation, there remains little hope of recover}', and their 

 immediate removal is advised. 



The detection of trees that are suffering from gas in the soil 

 requires the services of one who has had experience with trees thus 

 affected. The most common indications of gas-poisoning are the yellow- 

 ing and drying of the leaves during early summer and their subsequent 



