338 PLANT LIFE. 



the mosses and lichens, clearly show the evil effect of the 

 sulphur and coaldust If trees are in a narrow valley, 

 so that the smoke blows over them, they are often quite 

 uninjured ; but upon a slope towards the factory, or in 

 the direction of the usual winds, they are greatly 

 injured. The damage seems to be done chiefly in spring, 

 especially during the few days in which the leaves are 

 unfolding, and the annual shoots are elongating. What 

 exactly causes the damage seems rather uncertain. It 

 is probably the sulphur fumes, or the closing of the 

 lenticels and stomata by coaldust, that destroys the 

 young leaves and twigs. The effect can be clearly seen 

 even in winter time. The usual dome-shape of the 

 twigs of a healthy tree is altered. On the side exposed 

 to smoke the outline of the branches is either a straight 

 line or a concave curve, and the whole growth becomes 

 greatly suppressed in that direction. Later on, the 

 twigs on the injured side die ; and eventually the 

 most sheltered ones are affected, and the whole tree 

 becomes attacked by beetles, and decays away. The 

 Rhododendron, Privet, Holly, Plane, Sycamore, and Lime 

 seem best able to withstand coal smoke. Mosses, 

 lichens, and liverworts, on the other hand, appear to be 

 exceedingly susceptible to it. 



The water flora also is subject to destruction. If 

 too much nitrogeneous matter enters a stream, the 

 diatoms and fresh-water algae are replaced by a 

 bacterial population. A fungus, Apodya lactea, also 

 often covers the entire bottom of a running stream 

 with a sort of scum or furred growth, i 8 micro-milli- 

 metres thick, of a peculiarly horrible appearance (Turn- 

 bull). These fungi and bacteria are of course engaged 

 in purifying the water, as is shown by the products 

 given off. There appears to be a regular succession. 



