30 MISSOURI BOTANICAL GARDEN BULLETIN 
away from the deleterious effects of the smoke-laden 
atmosphere. 
Information concerning the effect of smoke on plant life is 
abundantly available. As early as 1866, at the international 
horticultural and botanical conference held in London, 
results of experiments on the effect of coal smoke on plants 
were presented. At this time, while soot was regarded as 
distinctly unfavorable to plants, it was not believed in itself 
to be toxic. The chief injurious agent generally was sul- 
phurous acid, the presence of which in the atmosphere in an 
amount of one part to five hundred thousand parts of air 
was sufficient to produce spots and discolorations on the 
foliage and eventually to cause the leaves to fall. Still later 
English botanists, investigating the same effects, came to the 
conclusion that sulphuric acid, which, of course, is readily 
obtained from sulphurous acid in the presence of moisture, 
is, because of its cumulative action, the principal cause of 
injury to trees and shrubs. It was demonstrated at that 
time that, as the result of burning coal containing pyrites 
and sulphur in other forms, sulphuric acid from the air was 
deposited on the plants and acted as a persistent and gradual 
caustic, eating into the tissues not only of the leaves but of 
more tender twigs and branches. Sulphurous acid was 
thought to act in an entirely different way, entering into the 
inter-cellular passages and acting directly upon the proto- 
plasm. Its injurious effect seemed to be more manifest in 
herbaceous and the so-called soft-wooded plants. However, 
sulphurous acid is by no means the only toxie substance 
present in smoke. There are various other products of the 
destructive distillation of coal which are injurious to vegeta- 
tion, but the effect is more obscure. 
Added to these toxic substances, the deposit of a coating 
of finely divided carbon mixed with other materials upon 
the surface of the plant may be either directly injurious or 
so reduce its general vitality as to make it more susceptible 
to the attack of insect or fungous pests. While the plant 
may not be immediately killed by being subjected to the 
deleterious agents present in smoke, the effect upon its rate 
of growth and its ability to produce flowers is often most 
marked. Many trees and shrubs growing in St. Louis are, 
of course, able to exist after a fashion under present condi- 
tions, but when a comparison is made between these plants 
