THE INJURIOUS EFFECTS OF SMOKE ON TREES. 123 



serious problem, with which it is difficult to grapple. It is even 

 questionable if it can be absolutely remedied. The damage 

 directly done by smoke and its allies is of a threefold nature. 

 First, there is the killing of the living matter in the green parts 

 of plants, particularly in the leaves, caused by acid in the smoke ; 

 second, the clogging-up of the stomata of certain plants by soot, 

 and the general coating-over with the same of all their parts ; 

 and third, the effect of the smoky atmosphere in obscuring the 

 sun. It is difficult, or even quite impossible, to define exactly 

 the amount of damage done by each cause, because their action is 

 always combined. Yet it may be as well to consider separately, 

 as far as practicable, how the plants are affected by each of the 

 causes detailed above, afterwards looking at the effect of their 

 combined action. 



Effects of Acid. — The gaseous acid in the smoke enters into 

 the leaves during the process of carbon assimilation, at once 

 attacking the living matter in the cells, and destroying it by 

 oxidising or bleaching it, therefore rendering the affected portion 

 absolutely useless. A leaf affected by acid, if held up to a strong 

 light, shows little clear spots wherever the action has begun. 

 Through time, these spots usually become brown, almost black in 

 some plants, and they might readily be taken for an attack by 

 a fungus. On a closer examination, however, a clear margin 

 is seea round the coloured part, which distinguishes it from any 

 disease. This form of injury is generally attributed to sulphurous- 

 acid gas, which is always given off in the burning of coal and 

 particularly dross, in varying quantities, according to the quality 

 of the article being consumed, and there is little reason to doubt 

 but that it is the chief factor in the mischief, although there are 

 other gases given off in limited quantities from various works 

 which assist in the injury to vegetation. The effect of acid may 

 be seen more or less in almost all species of broad-leaved trees 

 growing in any smoky district, and is very easily recognised in 

 its early stages on trees with coloured leaves. The various trees, 

 however, are not all affected to the same extent, neither is any 

 one species affected alike during all the growing season. It seems 

 to be worst when the leaves are at their most active stage, very 

 young leaves not being so readily injured as those that are fully 

 developed, and are, so to say, in full swing at the process of 

 carbon assimilation. Neither are leaves so readily affected when 

 the season has well advanced and their working power has begun 

 to wane. On Pollok Estate, this form of mischief is particularly 



