WEEDS OF OUR FIELDS, WAYSIDES AND TOWNS. 13 



most of it off and the wind that dries the vegetation 

 quicker than the roadway, clears and lays the rest. 



But in larger towns man has vitiated the air and 

 poisoned the soil with smoke, and the behaviour of vege- 

 tation under this new condition is extremely interesting, 

 and may prove a source of valuable instruction. 



Finely divided carbon which is the principal visible 

 constituent of smoke, has remarkable antiseptic qualities 

 — that is to say, it is destructive of bacterial life, and is 

 probably injurious to plant life directly. Also when 

 present in any quantity in the air it seems to shut off 

 actinic vajs, a smoke fog being notoriously yellow. But 

 carbon is not the only constituent of smoke. There is 

 sulphurous acid and probably minute quantities of other 

 compounds even more deleterious, such as hydrochloric 

 acid. And these substances, in very small quantities as 

 compared to the atmosphere that dilutes them, may have 

 a very great cumulative effect on vegetation. The amount 

 of sulphur legally allowed to escape at the end of a 

 sulphuric acid process is one grain per cubic foot of air ; 

 and less than that amount of Hydric Chloride is serious 

 even when again diluted by the moving millions of cubic- 

 feet of fresh air. Add this to the effects of trampling and 

 the poisoning of the soil by impurities brought down by 

 rain, and a condition is arrived at under which all but a 

 very few plants fail to maintain existence. But those 

 that do must possess powers which are worthy of 

 investigation. 



Let me take a few of the principal species and see 

 what mere naked eye observation will show. 



First there is the coltsfoot. It can grow, as I men- 

 tioned, in soil in which organic matter is apparently 

 absent. Glacial moraine and railway cuttings or em- 

 bankments are mere triturated rock. Something in or 



