Effect of Arsenic Compounds 53 



solutions. Nobbe, Baessler and Will (1884) carried on water culture 

 experiments with buckwheat, oats, maize and alder, and found that 

 arsenic was a particularly strong poison for these plants. When small 

 quantities of arsenious acid (As 2 3 ) were added to the food solutions, 

 growth was measurably hindered by a concentration of 1/1,000,000 As 

 (reckoned as As). The element only appears in plants in very small 

 quantity and can never be detected in notable quantities. The aerial 

 organs show the effect of arsenical poisoning by intense withering, inter- 

 rupted by periods of recovery, but eventually followed by death. It was 

 also found that if plant roots were exposed to the action of arsenical 

 solutions for a short period, say ten minutes, and then were transferred 

 to normal food solutions, the action of the poison was delayed, but 

 eventually hindering of growth or death occurred, according to the 

 strength of the poison used in the first solution. 



At the same time that Nobbe, Baessler and Will were establishing 

 the great toxicity of the lower oxide of arsenic, Knop (1884) was carry- 

 ing 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 potas- 

 sium arsenate proved to be harmful to young maize seedlings if the 

 solutions contained '05 1 gm. arsenic acid per litre (= I/ 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 indi- 

 cated 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 phos- 

 phoric 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 



