ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 13 



and also upon the following additional ones : Justicia carnea, 

 Buellia macrantha, Hydrangea hortensis, Conoclinmmianthinum, 

 Eupatorium odoratum, and Impatiens Sultani. I first applied 

 a very strong solution, 5 per cent., painting the right-hand halves 

 of several leaves of each. Within four hours of applying the 

 acid all the leaves were profoundly affected. The upper epi- 

 dermal cells and all the cells of the mesophyll and of the lower 

 epidermis as well, showed a very strong plasmolysis and dis- 

 coloration of their chlorophyll-corpuscles. No general brown- 

 ing of the protoplasm occurred till the next day, when several 

 of the leaves readily disarticulated. The amount of acid applied 

 had powerfully stimulated the leaves, affecting cells in regions 

 not immediately below the painted area of the surface. The 

 same plants similarly treated with a *5 per cent, solution exhibited 

 the same characters within twelve hours ; and here again several 

 of the leaves shortly fell away. With a *1 per cent, solution the 

 action was the same, though in some cases (Centropogon, 

 Cucurbita, Begonia sanguinea, and Hydrangea) the acid took 

 longer to penetrate the epidermis. With a # 05 per cent, solution 

 similar results were obtained, though in many cases four or five 

 days elapsed before any manifest injury became apparent.* 



I think it undeniable that a thin coating of sulphuric acid, 

 such as actually occurs in foggy weather, is sufficient to 

 cause such results. Whether, and to what extent, the other 

 substances of the deposit co-operate in this action there is no 

 evidence to show. The metallic iron alone did, as we found in 

 every case, eat into the leaf ; still the amount of this present in 

 fog deposits is so minute that its influence in the matter can be 

 neglected. Nor have I any observations upon the hydrocarbons 

 in the deposits. My experiments with them have been in the 

 form of vapours only. Were the corrosive action of the deposits 

 upon the leaves through the epidermis the only mode in which 

 the impurities of fog attacked the plant, there would be little 

 cause for grumbling among horticulturists. The injuries 

 arising from this cause alone are trifling when compared with 

 those which I have assembled together in the next paragraph. 



These would seem to constitute the main effect of fog upon 

 plants ; and I shall consequently devote considerable space to 

 their elucidation. 



* With -1 per cent, and under there was no disarticulation of foliage. 



