The poisonous action of arsenic on plants has long been recognised. [Chatin (1845)] gave accounts of tissues poisoned by strong arsenical 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 (As2O3) 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, interrupted 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 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.
| 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]).
