Arthur G. Ruston 307 



the town, where the amount of smoke pollution has previously been 

 determined; using the soil for growing a succession of crops each suc- 

 cessive year and carefully weighing the produce. In selecting the 

 various stations care was taken that the altitude, exposure and all other 

 conditions other than atmospheric smoke pollution should be as nearly 

 as possible identical. Apart from a few slight irregularities the results 

 indicate a fairly close correlation between the relative degree of purity 

 of the atmosphere in the neighbourhood of the station, as determined 

 from earlier observations, and the actual amount of plant growth 

 obtainable. 



An examination of the roots of plants will give some indication of 

 the amount of smoke pollution; plants grown in soil that has been for 

 long exposed to such pollution being marked by an almost entire absence 

 of root hairs and fibrous roots. The differences in the root development 

 of the wallflowers grown in soil which had been exposed in the various 

 districts four years were most marked. Oats and barley were the 

 following year grown in the same districts in the native soils and the 

 differences in root development were still more marked. 



An examination of a felled tree will often give valuable information 

 as to the purity of the atmosphere in which it was grown. The tree 

 automatically keeps a record of its yearly growth, and the presence of 

 any inhibiting factor will make itself known by the narrowing of the 

 annual rings. This is well seen in the case of the section of the Scotch fir 

 shown in Fig. 7, PI. XXV, for which I am indebted to Mr D. W. Steuart. 

 The tree in question was grown at Broxburn, near the Roman Camp Shale 

 Works, which were opened 17 years before the tree was cut down. In 

 consequence of the acid fumes and smoke-contaminated atmosphere, 

 the diameter growth as measured by the annual ring was sharply 

 checked when the tree was 12 years old, though under normal circum- 

 stances it would be fairly constant up to 30 years old and then fall 

 steadily off. This sharply defined check in the growth of the tree, as re- 

 corded by itself, is coincident with the opening of the Shale Works in 1893. 



