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 Krüger (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 stronger in some degree, while their life was lengthened. Apparently, treatment increased the chlorophyll content, and, correlated with this, was a rise in the assimilatory capacity, more starch being produced. Rise in transpiration was also observed. While the leaves were the organs most affected, a subsidiary stimulation occurred in the tubers, since the greater quantity of starch produced required more accommodation for its storage. In different varieties the ratio of tuber formation on treated and untreated plants was 19:17 and 17:16. In discussing the meaning of this stimulation these writers, following the custom then in vogue, were inclined to hold that it was due to a catalytic rather than to a purely chemical action, an idea similar to one which later on came much into prominence in connection with the work of Bertrand’s school on manganese, boron and other substances. The imputed increase in photo-synthesis seems to have met with approval and acceptance, but nevertheless it did not pass unchallenged. Ewert (1905) brought forward a detailed discussion and criticism of the assumption that green plants when treated with Bordeaux mixture attain a higher assimilation activity than untreated plants. His experiments were made to test the effects of differing conditions of life on plants treated in various ways, and his conclusions lead him to assert that “instead of the organic life of the plant being stimulated by treatment with Bordeaux mixture it is rather hindered.”