30 Effect of Copper Compounds 



stimulation appearing in any single instance. With peas again, even 

 1/20,000,000 copper sulphate is poisonous, although to the eye there is 

 little to choose between the control plants and those receiving poison 

 up to a concentration of one part in 2 million (Fig. 5). In the case 

 of buckwheat the matter is still undecided, as in some experiments 

 apparent stimulation is obtained with 1 in 2 J or 1 in 5 million copper 

 sulphate, while in others a consistent depression is evident, even when 

 the dilution is carried considerably below this limit. The reason for 

 the variation with this particular plant is so far unexplained. 



Yet, in spite of all the accumulated evidence as to the consistent 

 toxicity of copper salts in great dilution, the possibility still remains 

 that the limit of toxicity has not yet been reached, and that a stimu- 

 lating concentration does exist, so that it is still uncertain whether 

 beyond the limits of toxicity copper salts act as indifferent or stimulative 

 agents. 



4. Action of copper on organs other than roots. 



The bulk of the work on the relations of copper with the life- 

 processes of plants has dealt with those cases in which the metal has 

 been supplied to the roots in some form or other, and many of the 

 results may be said to apply more strictly to the theoretical, or rather 

 to the purely scientific aspects of the matter, than to the practical 

 everyday life of the community. This statement is hardly correct, in 

 that the two lines of work are so inextricably interwoven that the one 

 could not be satisfactorily followed up without a parallel march of progress 

 along the other. In practice, copper has proved remarkably efficient as 

 a fungicide when applied as sprays in the form of Bordeaux mixture to 

 infested plants and trees. Observations on the action of the fungicide 

 have shown that the physiological processes of the treated plants are 

 also affected to some degree, and a number of interesting theories and 

 results have been put forward. 



(a) Effect of copper sprays on leaves. 



Frank and Kriiger (1894) treated potato plants with a 2 % Bordeaux 

 mixture, and obtained a definite improvement in growth, which they 

 attributed to the direct action of the Bordeaux mixture upon the 

 activities of the plant. The effect of the copper was most marked in 

 the leaves, and was chiefly indicated by increase in physiological activity 

 rather than by morphological changes. The structure of the sprayed 

 leaves was not fundamentally changed but they were thicker and 



