ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 7 



phenol. The experiments with each substance (except in the 

 case of hydrochloric acid and ammonia) were continued over 

 long periods, and in some instances renewed. With the few 

 experimental chambers at my command, the investigation has, 

 of course, been a protracted one. But even with unlimited 

 resources of this kind, the necessarily concurrent examination 

 of the tissues of these plants would have been more than we 

 could have coped with successfully. 



I propose to deal with the material at my disposal in the 

 following manner. First, a brief resume of the salient features 

 of the injuries caused by fog, deferring for the present any 

 exhaustive list of the plants in which these have been found to 

 occur. Then, an account of the results obtained by exposing 

 plants to the various substances present in fog. Finally, a dis- 

 cussion of the whole matter. 



I. The Effects of Fog. 

 It is a matter of common observation that a plant may 

 exhibit a gradual yellowing of its leaves, progressing from below 

 upwards, followed by the dropping of those leaves in the order 

 in which they showed the change in colour. Indeed, this 

 is what generally happens when a plant " goes wrong." This 

 effect — which is, broadly speaking, the same in all cases — may be 

 due to a variety of causes. It may be due to over-watering or to 

 under-watering ; to a check caused by removal from a stove to a 

 greenhouse, or from a humid to a dry atmosphere, or the 

 reverse. Or, again, it may be caused by the attack of some 

 worm or insect upon the subterranean parts of the plants ; or to 

 continuous deficient illumination, or to the lack of nutritive 

 matters generally. Sometimes the phenomenon is very rapid in 

 its development, as in the following instance. I had acquired, a few 

 summers ago, a fine healthy plant of the common India-rubber 

 Fig, intending to make certain physiological experiments upon it. 

 One morning, early, it was syringed, and whilst still wet the un- 

 tempered rays of a powerful sun shone full upon it for several 

 hours. Before evening five or six of the lowest (and oldest) 

 leaves had turned yellow, whilst those above showed blotches of 

 the same colour. Next day these also succumbed ; at a touch 

 they all disarticulated perfectly, leaving the bare stem with its 

 terminal bud. Thick, filth-laden fogs produce much the same 



