THE INFLUENCE OF MAN. 



337 



presence, probably, to the extraordinary power which its 

 leaves possess of resisting pressure from above. The 

 others are all hardy and quick-growing weeds, which 

 delight in rich soil, and multiply very rapidly. Thus 

 there are at least four groups of " weeds," or artificial 



Fig. 43. — Tree In-jured by F^ngine Smoke. The foliage surface is nearly 

 straight on the injured side. Photographed from the railway bridge. The 

 distant trees show injury by wind. 



floras, which owe their existence to civilised man. The 

 arable and pasture weeds, those of shale heaps, of rail- 

 way tracks, and of the rich nitrogeneous waste lands by 

 roads and human dwellings. 



These have not, however, the melancholy interest 

 which attaches to the destructive effects of smoke, and 

 to river pollution. The prevalent winds in the West of 

 Scotland are from the south-west, and for two miles or 

 more to the north-east of a factory, the trees, and even 



(B7) Y 



