While it is evident that copper sprays have a definite action upon green leaves, whether favourable or unfavourable, the question arises as to the means whereby the copper obtains access to the plant in order to take effect. Dandeno found that solutions of copper sulphate were absorbed by the leaves of Ampelopsis, forming a brown ring. Generally speaking inorganic salts in solution are absorbed through both surfaces of the leaves, whether the leaves are detached or not, provided the surrounding atmospheric conditions are favourable, the absorption being usually more ready through the lower surface. Dilute solutions applied in drops stimulate the leaf tissue in a ring, whereas if the solutions are concentrated the entire area covered by the drop is affected. Too concentrated solutions of copper sulphate applied to leaves caused scorching, but if this was avoided while the solution was still strong enough to cause a darkening of green colour after a time, Dandeno considered that the action was probably of the nature of a stimulus to growth, and produced a better development of chlorophyll and protoplasm in the region where the tissues appeared dark to the naked eye, a conclusion which tallies very closely with that of Frank and Krüger. Amos (1907–8) experimented to see whether the application of Bordeaux mixture affected the assimilation of carbon dioxide by the leaves of plants, and whether any stimulation was produced. Brown and Escombe’s methods and apparatus were used and the summarised results indicate that the application of Bordeaux mixture to the leaves of plants diminishes the assimilation of carbon dioxide by those leaves for a time. The effect gradually passes off, whatever the age of the leaves may be. The suggestion is made that the stomata are blocked by the Bordeaux mixture, so that less air diffuses into the intercellular spaces and less carbon dioxide comes into contact with the absorptive surfaces. If this hypothesis is correct, the physiological slackening of assimilation is not due to the toxic action of the copper in the Bordeaux mixture, but to a mechanical hindrance due to blocking of the stomata. III. EFFECT OF COPPER ON CERTAIN OF THE LOWER PLANTS.