THE NATURAL CONDITIONS OF PLANTS. 23 
We must, therefore, look for another and more 
efficient cause of depression, and this, I believe, 
is to be found in the sooty particles diffused 
through the air, interfering with the respiratory 
functions of the leaves. It is well known, that, 
cceteris paribus, plants with smooth leaves suffer 
less in London than those which have leaves that 
are hairy or covered with viscid or resinous secre- 
tions. Hence the miserable appearance of most 
of the coniferce in London, although these are 
plants, many species of which flourish in the 
driest sands. 
In my letter to Sir W. J. Hooker, published 
in the “ Companion to the Botanical Magazine ” 
for May 1836, I expressed an opinion that the 
depressing influence of the air of large towns 
upon vegetation depended almost entirely upon 
the fuliginous matter with which such an atmo- 
sphere is impregnated, and which produces similar 
effects upon the leaves of plants as upon the 
lungs of animals. This opinion has been ques- 
tioned by the late Mr. Ellis, in an admirable 
London, but in every brick-field around it; in Loddiges’ Orcbideous 
bouse, and in my own large fern-bouse, where the temperature 
frequently rises to 120°; and I possess specimens in m j herbarium 
from all parts of the world — from Egypt, the Cape of Good Hope, the 
East and West Indies, New Zealand, New Holland, &c. The 
peristome of this moss is one of our most beautiful microscopic 
objects® 
