Hordeum distichum 24·5 _days_ Polygonum Fagopyrum 40 „ „ Persecaria 42 „ Various experiments have been carried on at Rothamsted with peas and barley. With arsenious acid on barley a depressing influence is manifest even at a concentration of 1/10,000,000, while no growth at all is possible with 1/10,000 and upwards. Apparently the toxic action on the root ceases at a higher strength than on the shoot, as with 1/1,000,000 and less the dry weight of the root remains practically constant. At this same strength the shoots look better than the controls, but this is not apparent in the dry weights (Figs. 9 and 10). With peas the depression is again evident to 1/10,000,000, but the plants are more sensitive to the higher concentrations, as no growth can take place in the presence of 1/250,000 arsenious acid (Fig. 11). A striking difference is observed with arsenic acid on barley, as apparently this does not act as a toxic even with such comparatively great concentrations as 1/100,000, though possibly the shoot is slightly depressed by this strength (Fig. 12). [Illustration: Fig. 9. Photograph showing the action of arsenious acid on barley in the presence of nutrient salts. (March 16th–May 9th, 1911.) 1. Control. 2*. 1/50,000 arsenious acid. 2. 1/100,000 „ „ 3. 1/150,000 „ „ 4. 1/200,000 „ „ 5. 1/250,000 „ „ 6. 1/500,000 „ „ 7. 1/1,000,000 „ „ 8. 1/5,000,000 „ „ 9. 1/10,000,000 „ „ 10. 1/25,000,000 „ „ 11. 1/50,000,000 „ „ ] [Illustration: Fig. 10. Curve showing the mean value of the dry weights of ten series of barley plants grown in the presence of arsenious acid and nutrient salts. (March 16th–May 9th, 1911.)] [Illustration: Fig. 11. Curve showing the mean value of the dry weights of ten series of pea plants grown in the presence of arsenious acid and nutrient salts. (June 8th–July 21st, 1910.)] [Illustration: Fig. 12. Curve showing the mean value of the dry weights of ten series of barley plants grown in the presence of arsenic acid and nutrient salts. (Feb. 28th–April 24th, 1911.)]