The mixture of copper sulphate and lime universally known as Bordeaux mixture has been in common use as a fungicide in many countries for some time; but it is still uncertain by what means such a mixture, containing no soluble copper, is effective. It has undoubtedly a physiological effect upon plants on to which it is sprayed, and may increase their powers of resistance to the attack of parasites; but in addition to this, there appears to be a direct fungicidal action which requires explanation.