394 



Plant as an Index of Smoke Pollution 



annual solid deposit of 26 tons per square mile and no measurable acid 

 deposit, practically all were in full leaf up to the beginning of November. 

 Evidence of smoke pollution is available at any time in the stunted 

 growth of trees and shrubs, particularly of evergreens. The following 

 particulars with reference to the maximum growth of aucubas in different 

 parts of Leeds are striking. 



Annual deposit in tons 

 per sq. mile 



Differences of the same order have been found in the case of the 

 rhododendron and laurel. 



Even the colour of the flowers in the gardens and woods will give 

 some indication of the amount of smoke pollution. As a general rule 

 the greater the pollution the paler the tint, and the more blues and reds 

 will tend to run to white, and the bronzes to yellow. Intensely coloured 

 flowers will show patches, the scarlet of the geranium in a smoke- 

 infested district will be streaked with purple running to blue and even 

 to white. The blood-red wallflower will be broadly streaked with yellow, 

 and in a year or two most of the red will have disappeared. 



Certainly the colour of the leaves wall be more than suggestive, and 

 signs of smoke pollution are to be found in the black deposit of soot 

 and tar upon the leaf, and the absence of autumn tints. 



3. Though, as has been seen, the "General Type of Vegetation" in 

 a district and the "General Appearance of Individual Plants" will give 

 much information as to the amount of smoke pollution in that district, 

 most can be learnt by the "Detailed Analysis" of the plant. We may 

 notice in a general way that the leaves of plants grown in the town are 

 dirty. This can be readily seen from the photograph of the holly leaf 

 grown in industrial Leeds (Fig. 5, PI. XXV) and kindly placed at my 

 disposal by Professor Cohen. The upper part of the leaf in question had 

 first been cleaned, before dipping it in boiling water and extracting the 

 chlorophyll with alcohol. We can, however, get a better idea of their 

 relative cleanness or otherwise, by actually estimating the amount of 

 solid deposit upon a unit area of these leaves. This has been done in 



