ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 23 



enclosing cell-wall. In those cells which contain them, the. 

 chlorophyll-corpuscles exhibit a corresponding crowding to- 

 gether. The chlorophyll-corpuscles themselves are not at once 

 disintegrated, nor are their margins ragged ; but they are fre- 

 quently bent and distorted by the contraction of the enveloping 

 protoplasm. The pigment itself is profoundly altered. The 

 body of each corpuscle appears yellow or brownish, never 

 bright green, as in uninjured tissues. Very frequently small 

 granules of a brown colouring matter are found attached to the 

 corpuscle or embedded in its substance. If these do not appear 

 within a few hours of performing the experiment, they can 

 generally be relied on to do so if the leaves be kept for twenty- 

 four or forty-eight hours. These are droplets of acid-chlorophyll 

 or chlorophyllan, a product which is produced from chlorophyll by 

 the action of acids. As the chlorophyllan appears, the substance 

 of the chlorophyll-corpuscles gradually loses colour, becoming in 

 many cases colourless, and then exhibiting some degree of dis- 

 integration. 



I was a good deal interested in ascertaining the precise mode 

 of attack of the sulphurous acid on foliage — that is to say, 

 whether the acid passed directly through the cuticle of the sur- 

 face, or whether it insinuated itself into the lacunar system of 

 the leaf by the stomata. After a very long series of experiments, 

 I have been able to convince myself that the latter is the usual, 

 or at any rate the most important, means of entry. In the first 

 instance, I coated the surfaces of leaves with a mixture of vaseline 

 and wax — some on the upper, others on the under surface — and 

 exposed them to the action of a vapour of sulphurous acid. But 

 this plan I soon abandoned for the following. Two circular discs 

 of wood about eight inches in diameter were taken, and in each 

 two round holes were cut, in such a manner that when the discs 

 were laid together the apertures in question exactly coincided. 

 These holes were about an inch in diameter. Leaves for experi- 

 ment were placed between the discs, so that the apertures in 

 question were closed by the leaves as with diaphragms. The 

 leaves being inserted in reversed positions, there would be exposed 

 in one direction the upper surface of one leaf and the lower sur- 

 face of the other. The discs were then clamped on to a vessel 

 of appropriate dimensions, into which the acid vapour was 

 introduced. In this manner the comparative effects of the same 



