ENVIRONMENTAL DISEASES. 11 



smelter can be condensed so as to form sulphuric acid, a readily salable 

 product. 



Different species of trees show a marked difference in susceptibility 

 to the action of sulphur gases. As a rule, conifers are killed much 

 more readily than deciduous trees. This may be explained by the 

 facts that gases are injurious only to the leaves of trees and that the 

 leaves of coniferous trees are exposed to the gases for several years, 

 while the leaves of broadleaf trees are renewed from year to year. 

 In Germany, where many detailed examinations have been made 

 during a number of years, Haselhoff and Lindau (36) state that the 

 oak is the most resistant tree, followed closely by the different species 

 of maple and ash. The elm, alder, poplar, and linden are more sus- 

 ceptible, and the birch and beech are most susceptible. Haywood 

 states that 3 ^ miles north of a smelter large numbers of trees, espe- 

 cially pines, were dead. 



A series of investigations has been conducted for several years by 

 the senior writer in another region where an extensive mixed forest 

 has been severely injured by sulphur gases emanating from large 

 copper smelters. The forest consists largely of oaks and pines grow- 

 ing on an extremely poor and sterile soil, and the general develop- 

 ment of the forest, even before the action of the sulphur gases, was 

 very poor. The different species of forest trees showed a marked 

 difference in susceptibility. The order of susceptibility, beginning 

 with the trees most easily killed, is as follows: 



White pine (Pinus strobus L.). 



Hemlock (Tsuga sp.). 



Scrub pine (Pinus virginiana Mill.). 



Pitch pine (Pinus rigida Mill.). 



Chestnut oak (Quercus prinus L.). 



Hickory (Hicoria sp.). 



Black-jack (Quercus marilandica Muench.). 



White oak (Quercus alba L.j. 



Post oak (Quercus minor (Marsh.) Sargent). 



Chestnut (Castanea dentata (Marsh.) Borkh.). 



Spanish oak (Quercus digitata (Marsh.) Sud worth). 



Scarlet oak (Quercus coccinea Muench.). 



Tulip poplar (Liriodendron tulipifera L.). 



Maple (Acer sp.). 



Black gum (Xyssa sylvatica Marsh.). 



This list pertains to trees of pole size. During the sapling stage a 

 somewhat different series can be established. The saplings of post 

 oak and white oak are less easily affected than those of Spanish oak 

 and scarlet oak. The degree of discoloration of the leaves varies, 

 and this should be considered an important factor in diagnosing 

 smoke or gas injury. The following shows the susceptibility to dis- 

 coloration in a number of different species. 



149 



