55 



and ^lanchester is by no means the worst town in this 

 respect. It is gratifying to know that efforts are being 

 made in many of our industrial centres to understand, and 

 let us hope, also to cope with this problem of air pollution, 

 and all such movements deserve the hearty support and 

 co-operation of gardeners and ilower-lovers. 



From what has been said in earlier chapters, it will be 

 clear that bright sunshine is the most important factor 

 in the nutrition of plants, as it is only in the presence 

 of light that the green chlorophyll of the lea\'es is able 

 to form starch, the food so essential for the further growth 

 of the plant. A murky atmosphere therefore lowers con- 

 siderably this prime nutritive activity of the leaves. If 

 in addition to that the leaf, as is always the case with 

 town evergreens, has become coated Vv'ith a more or less 

 opaque layer of sooty matter, the normal power of leaf 

 nutrition is still further reduced. What wonder then that 

 our town and suburban gardens suffer many serious losses. 

 It is only hardy plants which can survive these adverse 

 conditions. Conifers are usually regarded as fairly 

 resistant plants, and so they are to drought or cold, but 

 smoke is most harmful to them as their breathing pores or 

 stomata are situated at the base of a depression on the 

 surface of the leaf, and this pit becomes partiallv filled 

 with soot and the pore is thus blocked. In ordinary leaves 

 these pores are usually on the protected under surface and 

 are therefore not endangered by the smoke, but m the case 

 of the needle-shaped leaves of the conifers they are freely 

 exposed to smoke and fog. As a consequence we nnd 

 that conifers do not thrive in the neighbourhood of our 

 industrial towns. A blackening of leaves is, however, not 

 the only noticeable effect of air pollution on vegetation. 

 We observe, particularly on evergreens, that many of the 

 leaves have brown spots or dead margins, and in both cases 

 this is traceable to the acidity of the atmosphere, which 

 kills the tissues, particularly those near the stomata or 

 breathing pores. It is particularly in ivinier fogs that 

 the air becomes very acid, owing to the sulphur contained 

 in the coal, and our own sense organs enable us easily to 

 detect the presence of the sulphuric acid. Even in summer 

 the acidity of the atmosphere causes the discolouration 

 and the early fall of the leaves of trees in our parks and 

 town gardens. 



