Storp (1883) experimented to determine the changes in the various characters of the soil by the action of zinc salts on it, and he makes the remarkable statement that in some soils the presence of zinc generates free sulphuric acid, which is particularly injurious to plant life. Grasses, young oaks and figs showed a decrease in dry weight, nitrogen and fat, as the quantity of zinc compounds increased in the water added to the soil. Both the quality and the quantity of the crop were adversely affected. This decrease in the dry weight due to the presence of zinc was confirmed by Jensch later on, and also by Nobbe, Baessler and Will (1884), who state that both lead and zinc compounds work disadvantageously to vegetation even when they are present in such small quantities that the plants are outwardly sound, the harmful action appearing in the decrease of dry weight. Contrary to Baumann’s opinion, zinc carbonate is said to be one of the salts that exercises this insidious poisonous action. Storp (1883) noticed that the direct poisonous action of zinc compounds is largely destroyed by their admixture with soil, but he suggests that a secondary cause of harm is introduced by the accumulation of insoluble zinc salts, so that the fertility of the soil is impaired to the detriment of the vegetation. Ehrenberg (1908) throws out a suggestion that zinc is specially harmful to plant life when it occurs in conjunction with ammonia, but no further evidence has come to light. (_d_) _Mode of action of zinc on plants._ The reason for the toxicity of zinc salts when present in soil forced itself upon the attention of some of the early investigators in this field. Freytag (1868) put forward the hypothesis that the zinc oxide is partly or exclusively absorbed by the roots on account of the cell walls of the root being corroded by the very thin layer of zinc salts lying in contact with it--the same theory as has been held with regard to copper. He stated also that the quantity of zinc oxide taken up by the plant through its roots is strictly limited, not being proportional to the quantity occurring in the soil, but varying between narrow limits. Krauch (1882) found himself unable to accept another hypothesis which at one time found favour, i.e. that the zinc salts kill the plants by coagulating the protoplasm. If this were so, he argued, no plants at all could grow upon soils containing zinc, and he was content to leave the cause as one yet to be explained. Even at the present time, thirty years after, we know very little more about the physiological cause of the toxicity of zinc. 2. _Effect of zinc compounds on germination._