Obnoxious Gases 69 
function of the leaves, while a still more serious injury 
comes from the sulphurous acid which accompanies the 
smoke of most coals, and poisons the foliage. A small 
amount of this acid taken up by the leaves suffices to do the 
mischief. 
In most cases, however, the movement of the air, and 
the chemical change of sulphurous acid into innocuous sul- 
phuric acid produced by contact with the humid air, prevent 
the worst effect; but, if conditions are favorable, the smoke 
of a brick yard, for instance, may suffice to kill the foliage. 
Even the smoke from locomotives may become injurious 
in moist climates, or in certain locations, such as in narrow 
valleys, when it remains stationary for some time. We may 
mention such smoke-resistant trees as Sycamore, Poplar, 
Ash, Alder, Maple, and Elm. 
The duration of the poisonous condition of the air rather 
than the concentration of the poison is of moment. The 
first visible signs of poisoning are characteristic pale spots 
on the blades of leaves, which turn brown, while the veins of 
the leaves remain green; the normal water movement in the 
leaves is interrupted and the green tissues are disorganized, 
the tips and margins of the leaves being first affected and 
drying up. The effect is similar to that of drouth and sun- 
burn. If the trouble continues, the entire leaf becomes yellow 
or brown, except in the case of conifers, when the leaf turns 
red. Finally the leaf rolls up and drops. The drier the 
air, the higher the temperature and the more intense the 
light, in other words, the more active the foliage, the more 
quickly are these effects produced. The effect is therefore 
greater in the day and less at night. The poison does not 
penetrate the tree, but remains in the foliage, and hence 
the effect is only that of defoliation in general, namely, 
reduction or loss of food material. 
