ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 25 



has easy access to the elongated sides of these cells, the with- 

 drawal of the protoplasm from below is never so well marked. 



Where the upper surface is the exposed one — taking the case 

 of a plant with delicate cuticle and no stomata on this surface — 

 in due time the acid penetrates the cuticle, destroys the epidermal 

 cells, and then attacks the palisade layer. In this instance, the 

 direction of attack being reversed, it is a common observation to 

 find the protoplasm of the palisade cells slightly withdrawn from 

 the end walls which abut upon the epidermis. That this should 

 be so might be anticipated from what has been narrated above. 



Amongst the specimens examined spectroscopically for me by 

 Dr. Ed. Schunck were two which had been exposed to a vapour 

 of sulphurous acid, of strength about two milligrammes to the 

 cubic foot, for two hours. They were leafy shoots of Bouvardia 

 and Hydrangea. The alcoholic chlorophyll- extract of each 

 showed considerable alteration, the change being that which 

 would be produced by a small quantity of acid. The Bouvardia 

 extract, though considerably altered, was by no means so much 

 so as that obtained from the same plant treated with hydrochloric 

 acid. The extract of Hydrangea leaves, on standing, deposited 

 regular colourless crystals, most likely inorganic in their nature, 

 accompanied by small dark-coloured rosettes, probably a chloro- 

 phyll product. Later, there was a further formation of these 

 rosettes, the residual fluid becoming almost colourless. These 

 results show that, at any rate when considerable amounts of 

 sulphurous acid are present, not only is the general cell-protoplasm 

 affected, but that the chlorophyll-corpuscles are reached and the 

 pigment altered. 



When the atmosphere contains only small quantities of the 

 acid — about *2 milligrammes per cubic foot— no such rapid action 

 as that described is noted. Ultimately the leaves change colour 

 in blotches ; or in some cases, as in Poinsettia, a gradual change 

 of the whole area takes place. This gradual change, which in 

 Poinsettia may be continued over more than a fortnight, 

 resembles generally the changes which are exhibited when the 

 leaves die normally. It is only when the amount of sulphurous 

 acid present is kept strictly within the limits defined above that 

 this gradual change is brought about. If this limit is over- 

 stepped the leaves will be killed rapidly and the characters 

 shown by the tissues will be the same as those already described 



