780 TRANSACTIONS OP SECTION M. 



A further comparison was made of ordinary milking, taking the teats in 

 pairs, and simultaneous milking, by two milkers, of all four quarters. A 

 difference of 2 per cent, in milk-yield and 6 per cent, in fat-yield in favour of 

 the latter method was indicated, despite the occasional disturbance of the 

 cow inevitable with this mode of milking. Further tests with the milking- 

 machine are proposed. 



6. The Plant as an hulex of Smoke -pollution. By Professor C. 

 Ceowther, M.A., Ph.D., and A. G. Euston, B.A., B.Sc. 



Eesults obtained in several years' experiments and observations in and 

 around Leeds are summarised under the following heads : — 



1. The general type of vegetation in smoke-infested areas. 



2. The general appearance of individual smoke-damaged plants. 



3. The specific effects of smoke-damage in plants. 



1. Trees and shrubs make only a stunted grovvth ; dead and dying trees are 

 common objects. Conifers are particularly sensitive. Plants with thick, tough 

 leaf-cuticle {e.g., iris, auricula) are more resistant than those with thin cuticle 

 and crinkled, hairy surface {e.g., primrose). Most bulbous and seed-bearing 

 plants are sensitive to smoke. In areas badly infested with smoke, coarse 

 grasses (bent, couch, Yorkshire fog) and weeds (dock, sorrel, plantain) 

 monopolise the meadows and lawns, leguminous plants rapidly disappear, whilst 

 the hawthorn hedge barely persists. Rhubarb does not seem to be affected, 

 and tiie elder can be successfully grown. 



2. Destruction of young shoots and buds is noticeable. Characteristic dis- 

 colouration of leaves takes place apart from the inevitable coating of tarry soot, 

 and the fall of the leaf is greatly accelerated. The colours of flowers fall 

 short of their normal intensity, blues and reds tending to run to white, and 

 bronzes to pale yellow. 



3. Previous observations as to the choking of stomata by soot particles have 

 been confirmed. Tintcmetric measurements show a certain degree of correlation 

 between depth of tint of flowers and atmospheric-pollution. 



The correlation between plant-growth and atmospheric-pollution previously 

 established by measurements of rate of assimilation has been confirmed by 

 actual vegetation tests in the garden and in the field. In the garden tests within 

 the city boundaries the produce (three crops) obtained in the ' cleanest ' garden 

 was fully three times greater than that obtained in the ' dirtiest ' area. In 

 the field tests made outside the city the difference between extremes (one 

 season's crops) was fully 20 per cent. In soils long exposed to smoke-pollution 

 there is a marked failure of root-development, root-hairs and fibrous roots being 

 few or absent. 



The chemical composition of plants grown in polluted areas shows a relatively 

 high content of sulphur, chlorine, and arsenic — an abnormally high proportion 

 of the sulphur being in non-protein forms. 



In the case of seed-bearing plants smoke-pollution has been found to cause 

 a decrease in size and weight of seed, and a lowering of germination-capacity 

 and germination-energy. 



Inhibitory effects upon enzyme-activity have been demonstrated by com- 

 parative measurements of oxidase-, catalase-, lipase-, and emulsin-activity. 



