The Rothamsted experiments go to uphold Kanda’s statements as to the failure of copper sulphate to stimulate plants grown in water cultures. Peas are perhaps slightly more resistant to the greater strengths of copper sulphate than are barley and buckwheat, for while 1/100,000 proves mortal to the latter, peas will struggle on and fruit in 1/50,000, though this strength is very near the limit beyond which no growth can occur (Fig. 4). As a general rule, with barley the depression caused by the poison is still evident with 1/5,000,000 and 1/10,000,000, though occasionally these doses act as indifferent doses, no sign of stimulation appearing in any single instance. With peas again, even 1/20,000,000 copper sulphate is poisonous, although to the eye there is little to choose between the control plants and those receiving poison up to a concentration of one part in 2½ million (Fig. 5). In the case of buckwheat the matter is still undecided, as in some experiments apparent stimulation is obtained with 1 in 2½ or 1 in 5 million copper sulphate, while in others a consistent depression is evident, even when the dilution is carried considerably below this limit. The reason for the variation with this particular plant is so far unexplained. [Illustration: Fig. 4. Photograph showing the action of copper sulphate on pea plants in the presence of nutrient salts. (Oct. 3rd–Dec. 20th, 1912.) 1. Control. 2. 1/50,000 copper sulphate. 3. 1/100,000 „ „ 4. 1/250,000 „ „ 5. 1/500,000 „ „ 6. 1/1,000,000 „ „ 7. 1/2,500,000 „ „ 8. 1/5,000,000 „ „ 9. 1/10,000,000 „ „ 10. 1/20,000,000 „ „ ] [Illustration: Fig. 5. Curve showing the mean values of the dry weights of four series of pea plants grown in the presence of copper sulphate and nutrient salts. (Oct. 3rd–Dec. 20th, 1912.)] Yet, in spite of all the accumulated evidence as to the consistent toxicity of copper salts in great dilution, the possibility still remains that the limit of toxicity has not yet been reached, and that a stimulating concentration does exist, so that it is still uncertain whether beyond the limits of toxicity copper salts act as indifferent or stimulative agents. 4. _Action of copper on organs other than roots._