ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 41 



to the formation of granules of " chlorophyllan," which were 

 microscopically recognisable in the corpuscles.* 



But the development of these decomposition-products of 

 chlorophyll in leaves injured by fog cannot be regarded as a sure 

 indication of the action of an acid present in the air ; for when- 

 ever a leaf which contains a relatively acid cell-sap is killed, as 

 by steaming or boiling, these products are formed. The leaf 

 itself possesses the essentials for the destruction of its own 

 chlorophyll ; and, as I have tried to show, there is good reason 

 to believe that the amount of acid in the leaf increases in dull 

 weather. But I will quote his own words from Dr. Schunck's 

 letter — written after he had examined certain batches of 

 injured leaves which I had transmitted to him. The first 

 extract refers to injured leaves in which the chlorophyll did not 

 appear to be alteredt : " . . . your specimens have puzzled me 

 a good deal, and though I have examined them chemically as- 

 well as I could, I am still at a loss to understand what is the 

 cause of the peculiar discoloration which they show. If spectro- 

 scopic examination in this field may be trusted — and I think it 

 may — then I should say that the discoloration of these leaves 

 was not caused by acid of any kind, but by some other agent the 

 nature of which remains to be determined. I took the leaves of 

 Aristolochia, Beaumontia, and Clerodendron, neglecting the 

 others, in which the effect would be essentially the same, only 

 differing in degree, and I extracted portions of each with boiling, 

 alcohol. To my surprise, I obtained solutions of chlorophyll as 

 pure as any I have ever seen, the band iv., which shows the 

 presence of products of decomposition with acids, being hardly 

 visible ; the solutions too retained their bright green colour in 

 the dark for a long time. The leaves after treatment with 

 alcohol appeared more or less brown, the Aristolochia leaves 

 being the darkest. I quite expected that by further treatment of the 

 brown leaves with alcohol or other solvents I should obtain extracts 

 showing bands due to products of decomposition of chlorophyll, but 

 not a trace of any such bands was to be seen ; hence I infer the 

 absence of any such products. I then made some experiments 



* Under Type 3, p. 16, the chlorophyll is undoubtedly broken up, and 

 its products may be spectroscopically recognised. These cases of slow 

 killing, which are on the whole exceptional, are excluded from discussion 

 in the present section. 



t I.e., as judged by spectroscopic examination of the extract. 



