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 \'eins 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 fohage, 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 fohage, and hence 

 the effect is only that of defoliation in general, namely, 

 reduction or loss of food material. 



