146 BIOLOGICAL PROBLEMS 



bourhood of cities, where, incidentally, the conditions 

 are prejudicial to the health of the plant. Everyone 

 who has visited the Chelsea Physic Garden a few 

 weeks after the young growth has unfolded in spring 

 will have noticed a fine black carbonaceous film coat- 

 ing the foliage. The yellowing of the potatoes grown 

 in front of Buckingham Palace, which had commenced 

 before the spraying operation in July, was attributed 

 in part to the petrol fumes and exhaust products dis- 

 charged from the numerous motor-vehicles passing 

 within a few yards of the potato plots. 



The injury caused to plants by aphides is often 

 emphasized by spraying. Perhaps soluble copper is 

 formed through the action of the saliva or excrement, 

 and this may penetrate the epidermal and subjacent 

 tissues through the punctures made by the insect's 

 proboscis. It must be remembered, however, that 

 unsprayed plants attacked by aphides will frequently 

 succumb after a few days' heavy rain. 



Some curious cases of injury through spraying 

 were reported by the Lancashire County Council 

 authorities in 1914. It was thought that certain con- 

 ditions which tend to check vigorous growth predis- 

 pose the plants to spraying injury. 



The whole question is very obscure. It should 

 be studied experimentally by trials conducted in the 

 neighbourhood of factories, on sites where aphides are 

 prevalent, and on farms where the effect of spraying 

 under diverse conditions could be carefully investi- 

 gated. 



Burgundy mixture should be regarded in the light 

 of an invention which needs to be perfected, just as 

 we may improve and refine other articles of com- 

 merce, aiming at securing increased economy in 

 chemicals, labour, and cost, without in any way 

 impairing the fungicidal power. 



