THE NATURAL CONDITIONS OF PLANTS. 25 
foot-stalk ; and both exerted this injurious ope- 
ration, when present in such minute proportions 
as to he wholly inappreciable by the animal 
senses. 
“ After having suffered much injury from these 
acid gases, the plants, if removed in time, will 
recover, but with the loss of their leaves. Hence, 
in vegetation carried on in smoky atmospheres, 
the plants are rarely killed altogether, but merely 
blighted for a season. Accordingly, in spring, 
vegetation recommences with its accustomed lux- 
uriance, and as in many situations there is, at 
that season, and during the summer, a consider- 
able diminution in the number of coal fires, 
there will be a proportionate decrease in the 
production of sulphurous acid gas ; and, con- 
sequently, less injury will be done to plants 
during that season. In winter, too, when coal 
fires mostly abound, and gas is most abundantly 
generated, deciduous plants are protected from 
its noxious operation by suspension of their ve- 
getating powers ; but the leaves of evergreens, 
which continue to grow through that season, 
are constantly exposed to its action when present 
in its greatest intensity. Accordingly, in many 
of the suburban districts round London, espe- 
cially in the course of the river where new manu- 
factories are constantly rising up, the atmo- 
c 
