58 rOEESTS, WOODS, AND TREES 



each belt having a certain form or forms of plant life, since certain s})ecies 

 are more susceptible to smoke injury than others. 



Crowther and Ruston summarise the results of several years' experiments 

 and observations on the effect of smoke on plants in and around Leeds in 

 Brit. Assoc. lieport, 1915, p. 780 (1916), from which we may c^uote : 

 "Trees and shrubs make only a stunted growth ; dead and dying trees are 

 common objects. Conifers are particularly sensitive. Destruction of young 

 shoots and buds is noticeable. Cliaracteristic discoloration of leaves takes 

 place apart from the inevitable coating of tarry soot, and the fall of the leaf 

 is greatly accelerated. The colours of flowers fall short of their normal 

 intensity. Previous observations on the choking of stomata by soot particles 

 have been continued. In the garden tests within the city boundaries, the 

 produce (three crops) obtained in the ' cleanest ' garden was fully three times 

 greater than that obtained in the ' dirtiest ' area. In soils long exposed to 

 smoke pollution, there is a marked failure of root-development, root hairs 

 and fibrous roots being few or absent." See Cohen and Ruston, Smoke, a, 

 Study of Toicn Air (1912) ; Ruston, Report of Smoke Abatement League of 

 Great Britain, pp. 44-53 (1911); Pierce in Popular Science Monthly, 1911, 

 p. 332. 



4. The chief cause of injury to vegetation, due to tlie .smoke of copper 

 smelting works, is sulpliur dioxide, according to Haywood in U.S. Dept, 

 Agric. Bur. Cltem. Bull. 113 (1908). In deciduous trees the injury, when 

 acute, consists of irregular reddened areas of dead leaf-tissue between the 

 veins. The region in tlie immediate vicinity of the midrib and principal 

 veins is last to be attected. Repeated acute injury of this sort kills the twigs 

 and finally brings al)out the death of the whole tree. Deciduous trees in 

 S.E. Tennessee appear to be more resistant than conifers, and are resistant 

 in the following order, the most highly resistant being named first : Qaercas 

 alba, Acer rubrum, Poimlus deltoides, Lirioclemlron tulipifera, Bobinia 

 Pseudacacia, Platanus occiclcn.talis, and Ulmus crcossa. See G. G. Hedgcock 

 in Journ. Wash. Acad, of Sciences, iv. (1914) p. 70. 



The vegetation at Foyers, Inverness-shire, is affected by the dust of the 

 cryolite carried by the wind from the British Aluminium Works. Coniferous 

 trees are being killed, whilst deciduous leaved trees remain uninjured. See 

 G. West in Proc. Roy. Soc. Edinbiirgfi, vol. 25, pt. 2, p. 995, fig. 25 (1906). 



5. An investigation into the injurious effects of tarred roads upon 

 neighbouring trees has been made at Milan by U. Brizi. The trees most 

 susceptible are the common horse-chestnut {Acsculus Hippocastcmum) and 

 the red-flowering horse-chestnut (A. ca/riiea), tlie leaves of which, at the 

 beginning of summer, turn rusty at the edge and curl up slightly, while 

 their surface is covered with numerous small spots which at first are yellow 

 and look as if covered with a thin layer of shining varnish. The leaf then 

 curls up more and more, dries, and is blown away. Another sensitive tree is 

 the beech. The damage is caused almost entirely by the very fine dust due 

 to the passage of motor-cars. The dust settles slowly and is most abundant 

 on the lower branches of the trees. Once deposited, the small particles of 

 tar give off injurious vapours when strongly heated by the sun. The parts 

 not directly reached by the sun never show this damage. The best remedy 

 is to keep down the dust by the regular and abundant watering of the roads. 

 Scientific American, 2nd Nov. 1918, p. 351. 



6. See A. D. Richardson in Trans. Scot. Hoii. Assoc, vol. ii. part i. pp. 

 80-82, Plates I., II., III. (1913). 



7. See Elwes and Henrv, Trees of Great Britain, vol. vii. pp. 1869-1871 

 (1913). 



