58 FORESTS, WOODS, AND TREES 
each belt having a certain form or forms of plant life, since certain species 
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. Report, 1915, p. 780 (1916), from which we may quote: 
‘*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. Characteristic 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 confirmed. 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 Town Air (1912); Ruston, Report of Smoke Abatement League of 
Great Britain, pp. 44-58 (1911); Pierce in Popular Science Monthly, 1911, 
p. 332. 
4. The chief cause of injury to vegetation, due to the smoke of copper 
smelting works, is sulphur dioxide, according to Haywood in U.S. Dept. 
Agric. Bur, Chem. 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 the immediate vicinity of the midrib and principal 
veins is last to be affected. Repeated acute injury of this sort kills the twigs 
and finally brings about 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: Quercus 
alba, Acer rubrum, Populus deltoides, Liriodendron tulipifera, Robinia 
Pseudacacia, Platanus occidentalis, and Ulinus crassa. See G. G. Hedgecock 
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. Edinburgh, 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 Hippocastanum) and 
the red-flowering horse-chestnut (A. cavnea), the 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 Noy, 1918, p. 351. 
6. See A. D. Richardson in 7rans, Scot. Hort. Assoc, vol. ii. ‘part i. pp, 
80-82, Plates I., II., III. (1913). 
7. See Elwes and Henry, 7'rees of Great Britain, vol. vii. pp. 1869-1871 
(1913). 
