At the same time that Nobbe, Baessler and Will were establishing the great toxicity of the lower oxide of arsenic, Knop (1884) was carrying the matter a step further by comparing the action of arsenious and arsenic acid and their derivatives on plant growth. He established the fact that while arsenious acid is a strong poison for maize plants, arsenic acid in small quantities is not toxic to the roots and that the plants can produce flowers and fruit in its presence. Arsenic acid applied as potassium arsenate proved to be harmful to young maize seedlings if the solutions contained ·05–·1 gm. arsenic acid per litre (= 1/–2/20,000 arsenic acid). If however the plants were allowed to form 10–15 leaves in a pure food solution and then when strongly rooted were transferred to a solution of ·05 gm. arsenic acid per litre, they were found to grow strongly and develope big healthy leaves. Careful measurements indicated that the development is unchecked by the addition of the poison, though arsenic was determined in the ash of the treated plants. Stoklasa (1896, 1898) tested the effect of arsenic compounds on plant growth with special attention to their comparative relation to phosphoric acid. He corroborated Knop’s statement as to the greater toxicity of arsenious acid and arsenites in comparison with arsenic acid and arsenates, stating that 1/100,000 mol. wt. arsenious acid per litre causes definite trouble in plants, while with arsenic acid 1/1000 mol. wt. per litre first shows a noticeable toxicity. Water culture experiments were made with and without phosphoric acid, in each case with and without the addition of arsenic and arsenious acid. It was found that the arsenic acid was unable to replace the phosphoric acid, the plants decaying in the flower in the absence of the latter. In the complete absence of phosphoric acid, arsenic acid causes a strong production of organic substances up to the flowering time. The following figures were obtained with maize:-- ·002 gm. As₂O₃ with P₂O₅ 2·84 gm. dry wt. ·005 gm. „ „ „ 2·37 „ ·01 gm. As₂O₅ „ „ 67·32 „ ·40 „ „ „ „ 64·13 „ ·03 „ As₂O₅ without P₂O₅ 39·98 „ ·07 „ „ „ „ 42·13 „ normal solution „ „ 12·93 „ „ „ with „ 65·84 „ Comparative experiments with the two arsenical oxides showed that varying times were required to kill different plants. Young seedlings were brought into solutions containing 1/10,000 mol. wt. arsenious acid (= ·019 gm. As₂O₃ per litre) and the plants died in a very short time. Hordeum distichum 46 _hours_ Polygonum Fagopyrum 84 „ „ Persecaria 90 „ With ten times the strength of arsenic acid (1/1000 mol. wt. = ·23 gm. per litre) the plants took much longer to kill.