Agulhon (1910 a) worked with sterile nutrient solutions, and found that the higher strengths of boric acid hindered growth, 200 mg. boric acid per litre rendering growth impossible. He supported Hotter’s idea that the toxic action affects the roots and the formation of chlorophyll, and he stated that the plants are less green as the dose of boron increases, plants growing in doses of above 10 mg. per litre being yellowish. In other experiments he found that at 100 mg. boric acid per litre life seems impossible for the plant. The roots seem to be more adversely affected by toxic doses than do the shoots. In control plants Agulhon determined the stem/root ratio as 6, with a little boron as 7, while the ratio rose to 13 as the dose of the poison increased to 50–100 mg. boron per litre. The Rothamsted experiments show that boric acid is definitely poisonous to barley down to a strength of 1/250,000 (Fig. 15), the depressing effect frequently being evident at much smaller concentrations, while peas can withstand far more of the poison, the limit of toxicity being about 1/25–1/50 thousand (Fig. 16). With the greater strengths of poison the lower leaves of both barley and peas are badly damaged. In barley the leaves turn yellow with big brown spots, giving the leaves a curious, mottled appearance, while with peas the poisoning seems to begin at the tip and edge of the leaves, spreading inwards, without, however, showing the large spots as in barley. So far as chemical tests go at present, it is very probable that boron is deposited in the leaves in the same way as manganese, and that this is the cause of the degeneration. As with manganese, the lower leaves are attacked first, and the trouble spreads upwards, one leaf after another being involved. These observations fit in very well with those of Hotter, and the hypothesis of direct boron poisoning gains support from the fact that in dilutions which produce stimulation of the shoot the leaves show hardly any sign of dying off, even after prolonged growth in the solutions. With barley the effects of boron can be seen in the leaves in concentrations as low as 1/2,500,000, and it may be significant that this is the point at which the depressant action of boric acid entirely ceases in many cases. [Illustration: Fig. 15. Curve showing the mean value of the dry weights of ten series of barley plants grown in the presence of boric acid and nutrient salts. (May 1st–June 20th, 1911.)] [Illustration: Fig. 16. Photograph showing the action of boric acid on pea plants in the presence of nutrient salts. (Sept. 30th–Dec. 20th, 1912.) 1. Control. 2. 1/5,000 boric acid. 3. 1/10,000 „ „ 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 „ „ ]