A few tests on yeasts made by Javillier showed that with vegetative yeasts zinc has a specific action, a consistent increase occurring in the amount of yeast formed and in the amount of sugar consumed as the quantity of zinc increased from 0–1/10,000,000–1/10,000. With ferment yeast, however, zinc exerted no appreciable action. These results lend force to the conclusion of Richards (1897) who carried out experiments on fungi with various nutritive media with the addition of certain salts of zinc, nickel, manganese, iron, &c. He considered that his general results showed that the fact of a chemical stimulation of certain metallic salts upon the growth of fungi is established, although it must not be considered without further investigations that all fungi react in the same degree to the same reagent. _Conclusion._ As matters stand at the present day, it appears that it is still uncertain whether higher plants grown in water cultures are susceptible to stimulation by zinc salts. If a stimulus does exist, it must be at exceedingly great dilutions, but further evidence is needed. In soil cultures, however, the fact of increased growth seems to be more firmly established, certain species responding to zinc salts when used as manure, though no increase has been obtained with other species. It must always be remembered that the action may be an indirect one. The soil is very complex in its constitution, and it is impossible to determine the exact action of the added poison upon it, so that a stimulating effect need not necessarily be due to a direct action of a substance upon the plant, but it may be the result of more favourable conditions for life induced by the action of the substance upon the soil. Among the fungi the stimulation of _Aspergillus niger_ by minute traces of zinc compounds seems to be well proved, though again it does not necessarily follow that all fungi will react in the same way to zinc. CHAPTER V EFFECT OF ARSENIC COMPOUNDS I. PRESENCE OF ARSENIC IN PLANTS.