ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 43 



These quotations clearly show that we must look in many 

 cases to some other source of injury than sulphurous acid for an 

 explanation of the observed effects. Elsewhere in this report I 

 have dealt with pyridine, phenol, and other coal-tar products. 

 When these are applied, although the leaves exhibit often an 

 extreme discoloration, the chlorophyll itself can frequently be 

 extracted in a very pure condition. One case which was very 

 striking may be taken as typical. Fronds of Phegoptcris 

 trichoides commenced to turn brown at once when the vapour of 

 phenol was introduced, and in a short space of time they were of 

 a uniform deep brown hue. The alcoholic extract of the browned 

 fronds, after it had been filtered, gave a very pure chlorophyll 

 spectrum, and retained its bright green colour for a long time. 

 So that it is quite possible for a marked change in colour to be 

 produced without any direct co-operation on the part of the 

 chlorophyll. Indeed sometimes with phenol and other tarry 

 products the chlorophyll-corpuscles are found to contain brown 

 granules which almost completely mask the green pigment. 

 Nevertheless an alcoholic extract often shows (as in the case just 

 mentioned) that the chlorophyll itself has been little altered. In 

 these cases something is present in the chlorophyll-corpuscles — in 

 addition to chlorophyll — and this substance it is which gives the 

 deeply coloured body with phenol, &c. In other cases the chloro- 

 phyll appears to be slightly altered, a result perhaps due to the 

 action of the acids of the cell- sap. As Dr. Schunck puts it — 

 " When poisonous substances, such as you have employed, are 

 allowed to act on leaves, two processes, in my opinion, go on side 

 by side ; one is what may be called the spontaneous decompo- 

 sition of the chlorophyll ; the other, the action on pyridine, 

 phenol, or whatever it be of something contained in the cells, the 

 nature of which is unknown. In using pyridine or any other 

 weak base the former process is arrested in consequence of the 

 neutralisation of the acid present, and the colouring matter there- 

 fore remains intact, whereas phenol, being practically neutral, has 

 no such effect." In this view I entirely concur. 



