55 
and Manchester is by no means the worst town in- this 
respect. It is gratifying to know that efforts are being 
inade 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 flower-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 leaves 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 ihat the leaf, as is always the case with 
town evergreens, has become coated with 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 in the case 
of the needle-shaped leaves of the conifers they are freely 
exposed to smoke and fog. As a consequence we find 
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 zz winter 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. 
