ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 29 



in colour is a slight darkening of the green. Microscopic exam- 

 ination shows that all the cells of the leaf are strongly plasmolysed, 

 whilst in the epidermis there is, in addition, a slight browning 

 of the protoplasm. The chlorophyll-corpuscles, though they may 

 be distorted by the contraction of the protoplasm, do not show 

 any indication of breaking up, nor are they altered in colour. 

 The action, then, of pyridine is very different from that of sul- 

 phurous acid, which readily affects the corpuscles. Other plants r 

 similarly treated, are in general agreement with Bouvardia. 

 These included Justicia camca, Ccntropogon Lucianus, Dendro- 

 bium nobile, Begonia sanguined and B. Haageana, Conoclinium 

 ianthinum, Anona reticulata, Hydrangea Jiortensis, Poinsettia T 

 Asjndium sp. 



In one or two cases there was a slight yellowing of the 

 chlorophyll-corpuscles — e.g. Rhododendron sp. and Hydrangea 

 Jiortensis. In the latter case I attribute the change to the acids 

 of the cell-sap, which attack the corpuscles when the pyridine 

 has killed the protoplasm, the slightly basic pyridine being 

 insufficient in this instance to neutralise the acids of the 

 sap. Dr. Schunck kindly examined spectroscopically several 

 batches of leaves which I had exposed to pyridine in the 

 manner described. He made alcoholic extracts of the chlorophyll 

 and investigated the absorption spectra of these extracts. The 

 extract of Conoclinium leaves showed that the chlorophyll was only 

 very slightly altered, and retained its bright colour for several 

 days. In the Hydrangea extract a slightly greater change was 

 perceptible, whilst that in Bouvardia was still more considerable. 

 I was rather astonished at this result, as I had examined the 

 spectrum of a Bouvardia extract immediately after exposure, and 

 found the chlorophyll almost normal. The corpuscles also 

 appeared quite green and unchanged. It is possible that the 

 interval, of at least twenty-four hours, which elapsed between the 

 exposing and the extracting of the chlorophyll may have some- 

 thing to do with the changes occurring in such cases. 



The browning of the protoplasm which occurred in the 

 epidermis in some cases may be due to the presence there of 

 tannin. At any rate, I was generally able to verify, by applying 

 the ordinary micro-chemical tests, the presence of tannin in 

 most instances where a brown coloration was produced by 

 pyridine- This browning is very marked in the flower-buds of 



