12 DISKASKS ()! HKCIIU'OCS FOItKST TREES. 



Very easily <ii-c.>l<ired: Black oak ((Jurrcux sp.), hickory (1 linn in sp.), scarlet oak 

 ((jucrcus coci-ima Mueiich.), chestnut i < 'n*tanea dentata (Marsh.) Borkh.), and Spanish 

 cik i (Jin crux dujitiitu i Marsh, i Stuhvorth i. 



Partially resistant to discoloration: Tulip poplar (Liriodendron tulipifera L.), white 

 oak ((jiii'i-rii* nlba L.), chestnut oak ((J. prinus L.), and post oak (Q. minor (Marsh, i 

 Sargent I. 



nuite ivsi.-iani io discoloration: IJlack irum (Xyssa sylvatica Marsh.), white pine 

 (Pinus strobus L.), maple (An,- sp. i. {)irch pine (Pui//.v rii/idn Mill.), and hemlock 

 ( Tsui/a sp.). 



A great variation in susceptibility to gas injury has been noted by 

 the senior writer in different regions, largely because the general 

 growth conditions have a direct relation to the question of smoke and 

 gas resistance. Trees growing in good soil, not too close together, 

 so that they grow vigorously, are more resistant than those grown 

 on poorer soils or crowded together. Whether smoke or gas has any 

 direct influence on the soil, thereby causing disease in the roots, has 

 not yet been definitely determined. Haselhoff and Lindau (36) con- 

 clude that the smoke or gas has no influence whatever on the soil, 

 while Wieler (112) cites a striking instance showing that soil from a 

 smoky district is very injurious to healthy trees planted in it. After 

 three years' growth in soil from a gas district, 100 per cent of ash, <>2 

 per cent of maple, 72 per cent of beech, 8 per cent of spruce, and no 

 oak trees were dead. 



The greatest distance at which the sulphur fumes injured trees was 

 about 3 miles, toward the north and northwest. 



The most extreme gas injuries usually occur close to the source of 

 gas production, and injury diminishes rapidly as the distance from 

 this point increases. The injury is furthermore greatest in the direc- 

 tion in which the prevailing winds blow. It is usually most extreme 

 where the source of gas production is in confined valleys or basins (37). 



Preventive measures with reference to gas injury can often be 

 carried out with much success. These consist in the construction 

 of tall smokestacks at the manufacturing plant where the injurioii- 

 es originate, so as to carry them into the higher strata of the air. 

 The same end may be obtained by the erection of the manufacturing 

 plant at the most elevated point in any given region. A second 

 method which has been used with more or less success in Kurope 

 consists in bringing about the condensation of the gases by pa>sing 

 the smoke from furnace- oi- kilns through water. 



r\FAVni:.\m.K SOU. roMMTION>. 



1'nder unfavorable soil conditions a largo number of di-ease- 

 cau>ing factors ;i re n-iully grouped, most of which are but imper- 

 fect Iv rccogni/ed so far as their specific action is concerned. Among 

 these may be mentioned the absence of a snilicieiit amount of oxygen 

 in the soil, the absence of necessary food substances, the absence of 



