Ono found that Aspergillus and Penicillium are retarded in growth in the higher concentrations of copper sulphate, but that they are stimulated by weaker strengths. The range of stimulating concentrations is given as from ·0015%–·012%, the biggest crop being obtained with both moulds in the strongest of these solutions. Hattori gives the optimum as being considerably lower for the two fungi mentioned, Penicillium being at its best in a solution of ·008% and Aspergillus in ·004%. A. Richter (1901) opposes this absolutely so far as _Aspergillus niger_ is concerned. In his experiments copper appears invariably as a depressant, all concentrations from 1/150 to 1/150,000,000 giving growth below the normal, no stimulative action ever being observed. Zinc however proved to be a definite stimulant and in a mixture of copper and zinc salts in appropriate concentrations the toxic effect of the copper was completely paralysed by the stimulating action of the zinc, 1/200,000 zinc salt paralysing or overcoming the copper salt at 1/1125. Ono states that the optimal quantity of such poisons as copper salts is lower for algae than for fungi, copper failing to stimulate algae at dilutions which were the most favourable to the growth of fungi. Bokorny indicates that silver and copper salts work harm in unusually dilute solutions. Attempts have been made to utilise the poisonous action of copper on algae in clearing ponds of those plants. Lindsay (1913) describes experiments carried on in a reservoir infested with Spirogyra. A quantity of copper sulphate sufficient to make a solution of 1/50,000,000 was found necessary to kill off the Spirogyra, but it is suggested that the solution was probably weaker before it reached the algae, owing to the currents of fresh water. Anaboena needed 1/10,000,000 before it was killed off, while Oscillatoria is less sensitive still, 1/5,000,000 usually representing the mortal dose, though 1/4,000,000 was necessary in some instances. Algae seem to be peculiarly sensitive to the copper sulphate, far more so than the higher plants, as _Nuphar lutea_, _Menyanthes trifoliata_, and _Polygonum amphibium_ grew in the water unharmed by the addition of the poisonous substance. For some unexplained reason it seems that “the concentration of copper sulphate necessary to kill off the algae in the laboratory is five to twenty times as great as that needed to destroy the same species in its natural habitat.” _Conclusion._ Altogether, after looking at the question from many points of view, one is forced to the conclusion that under most typical circumstances copper compounds act as poisons to the higher plants, and that it is only under particular and peculiar conditions and in very great dilutions that any stimulative action on their part can be clearly demonstrated. CHAPTER IV EFFECT OF ZINC COMPOUNDS I. PRESENCE OF ZINC IN PLANTS.