While Frank and Krüger indicated a rise in transpiration when copper compounds were applied to the _leaves_ as sprays, Hattori (1901) attributed part of the toxic effect of copper salts, when applied to the _roots_, to a weakening action on the transpiration stream, and he maintained that the toxic effect of the copper salts is therefore connected with the humidity of the air. No further confirmation or refutation of this statement has so far come to light. In certain plants the application of cupric solutions as sprays causes a slight increase in the quantity of sugar present in the matured fruits. Chuard and Porchet (1902, 1903) consider that such a modification in the ripe fruit during the process of maturation occurs in all plants which ripen their fruits before leaf-fall begins. Injection of solutions of copper salts into the tissues of such plants as the vine causes more vigorous growth, more intense colour and greater persistence of the leaves; in other words the copper acts as a stimulant to all the cells of the organism. A similar effect is produced by other metals such as iron or cadmium. By injecting small quantities of cupric salts into the branches of currants an acceleration of the maturation of the fruits was caused, identical with that obtained by the application of Bordeaux mixture to the leaves. If the quantity of copper introduced into the vegetable organism was augmented, the toxic action of the metal began to come into play. These investigators attributed the stimulus, as shown by the earlier maturation of the fruits, to a greater activity of all the cells of the organism and not to an excitation exercised only on the chlorophyll functions. (_b_) _Effect of solutions of copper salts on leaves._ Treboux (1903) demonstrated the harmful action of solutions of copper salts on leaves by means of experiments on shoots of _Elodea canadensis_. The activity of photo-synthesis was measured by the rate of emission of bubbles of oxygen. On placing the shoots first in water, then in _N_/1,000,000 copper sulphate (·0000159%), there was a reduction from 20 to 15 or 16 bubbles in 5 minutes. On replacing in water there was an increase to 18, but not to 20, indicating a permanent injury. With _N_/10,000,000 copper sulphate there was little or no reduction in the number of bubbles. This experiment had an interesting side issue in that it was noticed that not only the concentration, but also the quantity of fluid was concerned in the toxic action, indicating that both the proportion and the actual amount of poison available play their part. For instance, with a shoot 10 cm. long in 100 c.c. solution the plants were only slightly affected by ·000015% copper sulphate, but in 500 c.c. solution the shoots were killed after some days in ·0000015% copper sulphate, a concentration only one-tenth as great.