DISEASES OF SHADE. AND FOREST TKEES. 117 



haves followed by slow dying, reduction in the rate of 

 uTiwth of the twigs and, in fact, of the tree generally. 

 I. utually the twigs die and finally the branches and trunks. 

 The different kinds of trees show varying degrees of re- 

 sistance and, therefore, trees in the vicinity of furnaces, 

 smelter?, mills, etc., will not show an equal degree of injury 

 from the central point ; some species of trees at considerable 

 distances from the course of smoke and gas may be killed 

 while other species very near may continue to live for many 

 years. ''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 

 rui'ida Mill.), Chestnut oak (Quercus prinus L.), Hickory 

 i Hlcoria sp.), Black-jack (Quercus marilandica Muench.), 

 White oak (Quercus alba L.), Post oak (Quercus minor 

 i Marsh.') Sargent), Chestnut (Castanea dentata (Marsh.) 

 Bnrkh. i. Spanish oak (Quercus digitata (Marsh.) Sud- 

 worth). Scarlet oak (Quercus coccinea Muench.), Tulip pop- 

 lar (Liriodendron tulipifera L.), Maple (Acer sp.), Black 

 gum ( Xyssa syh'atica Marsh.)." 



Treatment. 



The injuries are greatest to leeward of the prevailing 

 winds. They can be overcome in a great measure by the 

 eon-traction of tall smokestacks which will carry the in- 

 jurious gases into the higher strata of air. Devices for con- 

 densing the gases, such as passing through water, have been 

 used with some degree of success. There is no satisfactory 

 treatment so long as trees are exposed to the abnormal en- 

 virMnments. The causes must be removed. 



Dust from cement and other establishments has also proved 

 ri be the cause of some injuries. When cement dust settles 



*Taken from Bulletin No. 149, Bureau of Plant Industry, TL S. Department 

 of Agriculture. Diseases of Deciduous Forest Trees. Herman Von Shrenk 

 and Parley Spaulding. 



