194 SCIENCE AND FRUIT GROWING 



a small increase in effect. This explains why attempts to deter- 

 mine the most efficient strengths of copper fungicides so often 

 lead to contradictory results ; for a doubling of the strength 

 produces such a small increase in action, that this may be entirely 

 counterbalanced by chance differences in the conditions attend- 

 ing the spraying. 



Experiments made by the dipping method gave results similar 

 to those made by the immersion method, except that the values 

 throughout were only about one quarter as large, a consequence, 

 of course, of there being a smaller amount of substance available 

 on- the leaf ; and in this case the progress of the scorching could 

 be followed throughout nine days, and was found to proceed at 

 a uniform rate of n to 14 per cent, per diem. 



It was proved that lime in water exercised a decided scorching 

 effect on leaves, and presumably, therefore, would act as a 

 fungicide, so that part of the efficacy of ordinary Bordeaux 

 mixture must be attributed to the excess of lime in it. Such an 

 action was further manifested in the fact that when older leaves 

 were taken, lime had, just as in the case of copper, a preservative, 

 or anti-scorching, effect (XI, 151). Water alone appeared to 

 have a scorching effect in the case of older leaves, due, no doubt, 

 to the moisture favouring the development of the destructive 

 organisms present. 



In one respect the scorching and fungicidal effect did not 

 appear to go together : Clark found that the presence of neutral 

 salts, such as potassium sulphate, increased the fungicidal action 

 of copper (p. 186), whereas it was found at Woburn that scorching 

 action was reduced by the presence of this salt. There is, however, 

 no difficulty in reconciling these opposing results provided the 

 action of the salt be explained on physical grounds, instead of 

 on the chemical grounds (quite unacceptable) suggested by 

 Clark for vegetable cells of a different character behave very 

 differently as regards exosmosis, and such differences probably 

 exist between those of leaves and those of fungus spores. 



A considerable amount of work was done in an endeavour to 

 determine the nature of the action of electronegative copper on 

 the leaf, and presumably, therefore, on the fungus. As copper 

 in this condition passes spontaneously, or by the action of the 

 air, into the ordinary form, it would be impossible to recognise 

 any difference in the action of the two forms, unless the electro- 

 negative copper was the more active ; all the experiments showed 

 that this was not the case. The probability, indeed, was that 

 electronegative copper, as such, had no scorching or fungicidal 



