196 ON THE EFFECTS PRODUCED ON TREES AND SHRUBS 



found to thrive better than other species do when placed by the 

 I'orce of circumstances in situations inimical to their nature, or 

 under conditions which injuriously affect plant life generally. 



Coniferous trees and shrubs will not thrive — the phrase must 

 be taken as comparative — in any polluted atmosphere ; for as we 

 have shown that trees and shrubs breathe through both the bark 

 and the leaf, such species as exude or secrete gum or resin in any 

 quantity are peculiarly liable to suffer from .polluted air. In the 

 case of such trees, soot and such-like substances cliug to them, 

 and when their resinous exudations become coated with carbon, 

 the rain has no effect in removing it, but rather the reverse, as it 

 washes down aqueous volumes of soot-impregnated water upon 

 their branches and bark. In like manner, hard-wooded trees, 

 possessing gummy or sticky buds and leaf sheaths, suffer in a 

 similar way ; and should not be planted in the near proximity 

 of large public works, or in the open spaces in the midst of 

 densely populated towns. 



The varieties of trees and shrubs which will be found to thrive 

 best (again using the phrase in a comparative sense) in smoky 

 atmospheres, are deciduous hardwoods with smooth leaf-surfaces 

 and scaly barks ; as for example, trees like the plane or the maple- 

 From the smooth upper surface of the leaf, much of the coating 

 deposit which settles there is easily washed off by rain, while the 

 constantly peeling off habit of the scales of the outer bark keeps 

 up a recuperative process in the cells and pores communicating 

 with the alburnum and inner bark surface for the oxygenation of 

 the sap. 



IMany evergreens also possess remarkable vitality and power 

 of resisting the baneful inHuences of atmospheric pollution. As 

 a rule, such plants as possess a thick leathery leaf with smooth 

 surface endure the effects of smoke with greatest impunity; such, 

 for example, as the common ivy and Aucuba japonica, whose 

 glossy leaves are easily cleansed, and consequently the epidermis 

 is kept in a more healtliy condition from the action of the 

 rain, than is the case with those plants and bushes wdth a 

 downy or hairy foliage. But although some evergreens may 

 succeed well in smoky atmospheres, there seems no doubt that, 

 owing to the complete renewal annually of their foliage, deciduous 

 trees and shrubs have a marked advantage in those situations over 

 the evergreen species ; and where it is intended to plant under 

 such trying circumstances, deciduous trees and shrubs should be 

 preferred, and such varieties of them as we have indicated as 

 l^ossessing special qualifications for withstanding the injurious 

 effects of an impure atmosphere should be selected. 



The following list comprises, such trees and shrubs as seem 

 most adapted for culture in these situations : — Platanus occiden- 

 talis, Acer eriocarpum, Populus balsamifera, Populus fastigiata, 



