1913] KNIGHT & CROCKER—TOXICITY OF SMOKE 365 
may produce gases very toxic to plants. Whether they are likely 
to reach sufficient concentration to do injury is not determined. 
Injuries from coal smoke are generally attributed to tars and 
oxides of sulphur (3, 4, 7, 40,), while reduced carbon-bearing gases 
have never been considered as a factor. According to HEMPEL 
(14, p. 257), these gases, especially the heavy hydrocarbons, exist 
in such small quantities, if at all, even when the oxygen supply is 
very little more than enough to produce complete oxidation, that 
they cannot be detected by gas analysis methods. This does not 
mean that they can be neglected as a source of injury to vegetation, 
for, as we have shown, growth rate is reduced in the pea epicotyl 
in 0.0003—0.0005, the least concentration of ethylene detectable 
by gas analysis methods. In short, while the gas analysis methods 
are quite adequate for guarding against considerable energy loss 
due to incomplete combustion of heavy hydrocarbons in furnaces, 
the only way to make sure that they are not in sufficient concen- 
tration to do injury to vegetation is to use a more delicate test, such 
as the pea epicotyl. 
One factor that favors the effectiveness of the oxides of sulphur 
as plant poisons in the open as against heavy hydrocarbons is their 
great solubility in the plant cell, which would lead to their accumu- 
lation even under great variation in the atmospheric concentration, 
whereas the heavy hydrocarbons will accumulate to a far less degree, 
and variations in concentration greatly reduce their injurious effects. 
It is probable that smoke from the beehive coke oven is much 
richer in heavy hydrocarbons than furnace smoke, especially in the 
early firing (2). Part of the destruction of vegetation about these 
may be due to the carbon-bearing gases, though here, as in furnace 
smoke, there is an abundance of sulphur dioxide and tars. The 
economic loss through injury to vegetation is probably rather slight, 
because of the nature of the region in which this industry is carried 
on. It certainly is inconsiderable beside the $44,000,000 worth 
of products this wasteful method of coking is pouring into the 
atmosphere annually in the United States alone (19). 
Artificial illuminating gas is a source of great economic loss 
through injury to plants. A large number of cases of injury to 
greenhouse stock in different parts of the country have been called 
