BY SMOKE FROM PUBLIC WORKS. 195 



and the direct action of the sun's rays has more power and oppor- 

 tunity to cause undue evaporation. And with a decreased healthy 

 action of foliage, or with a scant crop of leaves induced by the 

 effects of such substances as have been referred to, another very 

 important agency in sanitary economy is interfered with ; for 

 while leaves, doubtless, in tlie first instance, by the healthy 

 action of their various functions, act for their own benefit ; they, 

 at the same time materially influence and conduce to the purity 

 of the atmosphere itself, by their absorption of carbonic acid gas, 

 which in excess is so injurious and fatal to animal life. Under 

 the influence of light, leaves in a healthy state, and young 

 green twigs and shoots of shrubs and trees, decompose this dele- 

 terious gas, and so contribute largely to the preservation of the 

 purity of the air for man's own breathing. 



But not only upon the leaves of trees and shrubs are the effects 

 of smoke and other deleterious substances apparent, nor is it 

 through their medium alone that the life of the plant is destroyed. 

 The lateral air-vessels which penetrate the bark and stems of 

 trees horizontally for the purpose of " oxygenating," as it may be 

 called, the juices and secretions, form another channel by which 

 the injurious and subtle poisons are conveyed into the system of 

 the plant. The plugging-up of the mouths of these surface- 

 absorbents prevents the inhalation by the bark of the moisture 

 of the atmosphere, and in this way, acts prejudicially upon the 

 growth and development of the alburnum or sap-wood, as well 

 as upon the expansion of the new buds, and consequently upon 

 the ultimate growth of the tree itself. A familiar instance may 

 be given of the injury which accrues to a tree by the adherence 

 of a foreign substance to the surface of the bark, if we notice 

 the effects produced by smearing the stem with coal-tar, oil, or 

 paint, so as to completely stop up the sjnracida or pores of the 

 bark for some considerable length up the trunk from the ground. 

 The result will be found to correspond precisely with what has 

 been stated with regard to smoke clogging the bark from 

 atmospheric causes. The new buds will first be destroyed, and 

 finally the tree itself. 



Upon the various descriptions of trees and shrubs the effects 

 of smoke or a polluted atmosphere are not always equally severe 

 or rapid. Doubtless plants, like animals, may individually vary 

 in regard to the delicacy of their constitution, and while some 

 are much more susceptible than others of the alterations in the 

 temperature and humidity of the atmosphere, it is quite possible 

 that some plants may be injuriously, if not even fatally, affected 

 by the presence of foreign inimical matter in the air, or soil, or 

 food, from which others of the same species, of a hardier and 

 more robust constitution, would scarcely suffer. But indepen- 

 dently of this, there are species of trees and shrubs which are 



