SHADE TREES. 225 



maple is quite different from that of the elm, horse-chestnut or red oak, 

 and their products of decomposition also differ. The accurate diagnosis 

 of trees killed by illuminating gas is highly specialized and technical. 

 Nevertheless the characteristic odors given to the tissue by illuminating 

 gas can be discerned quite accurately by one thoroughly familiar with 

 them. Sometimes these odors are found in all of the tissues of the trunk, 

 but more often they are confined to some special part of the tree or root. 

 They are far more pronounced at the base of the tree, and rarely found in 

 the top. Carolina poplars and willows often display peculiar reactions 

 to gas poisoning. The bark splits open and large masses of soft, paren- 

 chymous tissue are formed directly from the cambium layer. When the 

 tree dies this parenchymous ^ tissue decomposes into a mucilaginous 

 mass. (See Fig. 95.) Some species appear to be less often affected by 

 gas poisoning than others. It is a question whether there is much differ- 

 ence in susceptibility, however, as regards species. Trees like the elm 

 and maple, with a large spread of the roots, naturally become poisoned if 

 located near gas leaks, and some trees are adapted to more strenuous 

 conditions and possess a greater capacity for regeneration than others, 

 although they ma}^ be as susceptible to poisoning as trees of other species. 

 Coniferous trees possess the gi-eatest resistance to gas poisoning of any 

 species, and in many instances they have been observed surviving in an 

 apparently healthy condition when located dangerously near broken 

 mains, while deciduous trees located much farther away would always 

 succumb. In some cases where conifers have actually been poisoned to 

 quite an extent they have completely recovered. This response may in 

 part be explained by the protection furnished by the coating of micorhiza 

 on the roots of conifers. 



We know of no remedies wliicli can be applied to trees already poisoned 

 by gas, since the injury occurs below the surface of the ground, and the 

 effects are seldom noticeable until the poisoning is more or less pronounced. 

 If the leakage of gas could be discovered quickly and the leak repaired, 

 the effects on the roots might be prevented, but tliis is rarely the case. 

 Illuminating gas in small quantities acts as a stimulus to plants, and 

 there is a certain capacity for adaptation to poisons possessed by them, 

 although limited. By the time the effect appears in the foliage consider- 

 able gas has been absorbed by the roots, and since it is impossible to 

 eliminate the gas from the soil, absorption is bound to continue and the 

 tree is doomed. We have known of only a few instances (with the excep- 

 tion of the conifers above noted) in which trees have shoAvn even slight 

 sjanptoms of gas poisoning and survived for any length of time. In some 

 instances where only one root has been affected, and the poison has not 

 reached the trunk of the tree, amputation of the root may prevent further 

 injury, and is known to have been effective. There are many cases in 

 which trees have not suffered from gas poisoning although located near 

 large leaks, owing to the amputation, during the installation of curbings, 

 etc., of the larger roots which extended over the gas pipes. 



I Mass. Agr. Exp. Sta. Rept. 25, 1913, Pt. I., p. 51. 



