Experiments were carried on with barley, in which the plants were grown in the various grades of distilled water indicated above, both with and without the addition of nutrient salts. It was found that the presence of the nutrients exercises a very definite masking effect upon the action of the poisonous substance, so that the deleterious properties of the toxic substance are materially reduced. Later work, in which known quantities of such toxic salts as copper sulphate were added to pure distilled water showed that in the presence of nutrient salts a plant is able to withstand the action of a much greater concentration of poison. For instance, a concentration of 1:1,000,000 copper sulphate alone stops all growth in barley, but, if nutrient salts are present, a strength of 1:250,000 (at least four times as great) does not prevent growth, though the retarding action is very considerable (Figs. 2 and 3). [Illustration: Fig. 2. Photograph showing the action of copper sulphate on barley in the presence of nutrient salts. (March 5th–April 19th, 1907.) 1. Glass distilled water. 2. Copper distilled water. 3. 1/12,500 copper sulphate. 4. 1/25,000 „ „ 5. 1/50,000 „ „ 6. 1/100,000 „ „ 7. 1/250,000 „ „ 8. 1/500,000 „ „ 9. 1/1,000,000 „ „ ] [Illustration: Fig. 3. Curve showing the dry weights of a series of barley plants grown in the presence of copper sulphate and nutrient salts. (March 13th–May 3rd, 1907.) NOTE. In each scale of concentrations represented in the curves a convenient intermediate strength is selected as a unit, and all other concentrations in the series are expressed in terms of that unit. Thus, with 1/1,000,000 as the unit a scale of concentrations might run thus: 10 1/100,000 4 1/250,000 2 1/500,000 1 1/1,000,000 0·5 1/2,000,000 0·1 1/10,000,000 0·05 1/20,000,000 0· Control. ]