[Loew (1883)] was sceptical concerning the specific toxicity of arsenic for plant protoplasm. He was convinced that arsenic and arsenious acid were poisonous to algae, not because of their specific character as arsenical compounds, but because of their acid nature, algae being peculiarly sensitive to any acid, and he maintained that these substances were not more poisonous than vinegar or citric acid. He placed various species of Spirogyra in solutions of ·2 gm. potassium arsenate per litre water (1/5000), and found that the algae grew well without making any abnormal growth in a fortnight, showing hardly one dead thread. Some of this alga was then transferred to a 1/1000 solution of potassium arsenate. This suited it excellently and it increased and the appearance under the microscope was very fresh and strong, which was attributed more to the potash than to the arsenic acid. Loew maintained that for the lower animals and for many of the lower plants arsenic in the form of neutral salts is not a poison. When the differentiation of the protoplasm into certain organs reaches a specific degree in the higher plants, then the poisonous action of the arsenic compounds comes into play.
[Knop (1884)] found that certain unicellular green algae grew luxuriantly in a neutral solution supplied with potassium arsenate. [Bouilhac (1894)] concerned himself chiefly with the possibility of the replacement of phosphates by arsenates. He recognised that the influence of arsenic is not the same on all species of plants, so he confined his attention to certain of the algae. Stichococcus bacillaris Naegeli was found to live and reproduce itself in a mineral solution containing arsenic acid. Even in the presence of phosphoric acid the arsenic acid favours growth, the best dose being about 1/1000. The arsenic acid is capable of partly replacing phosphoric acid. Other species of algae, Protococcus infusionum, Ulothrix tenerrima, and Phormidium Valderianum invaded the original culture of Stichococcus from the atmosphere, but with no arsenic or phosphoric acid their development was poor. The jars with arsenic compounds were invaded by still more species which grew strongly. Under these conditions it is evident that these algae are capable of assimilating arsenic, and the addition of arsenic acid to a solution free from phosphoric acid is sufficient to enable these algae to live satisfactorily, the arsenates in this case replacing the phosphates. [Ono (1900)] found that algae are favourably influenced by small doses of poisons, the optimal quantity for algae being lower than that for fungi. Protococcus showed a possible stimulus when grown in concentrations of potassium arsenate varying from ·00002–·0005%. This possible stimulus is interesting in view of the failure to observe stimulation in higher plants by minute traces of arsenic.
2. Fungi.
The effect of arsenic on fungi is of special interest in that it has a direct bearing upon hygienic and commercial interests. Gosio ([1892], [1897], [1901]) found that certain of the fungi, Mucor mucedo and Aspergillus glaucum, will grow on various arsenic compounds and exercise a reducing influence on them. These moulds attack all oxygen compounds of arsenic including copper arsenite, and develope arsenical gases. Sulphur compounds of arsenic are not influenced by these fungi. The same moulds would, if cultivated in soil containing arsenic, develope hydrogen arsenide. Penicillium glaucum has such a strong and definite action on arsenic compounds that he states that there is no doubt of the possibility of poisoning by arsenical gas in a room hung with paper containing arsenic. The compounds are so extraordinarily potent that if a mouse is placed in a vessel in which the mould is strongly developed in the presence of arsenic, it dies in a few seconds. Penicillium brevicaule uses the element in its development as a food substance. If material containing arsenic is placed in contact with dead fungi no reaction occurs. The life activity of the mould is evidently necessary for the reaction by which the arsenic-containing gases are liberated. [Csapodi (1894)] put forward the earlier results of Gosio and noted that the so-called arsenical fungicides do not only fail to kill the mould fungi but actually favour their development. This action explains why wallpaper containing arsenic is so disadvantageous in a room. [Abba (1898)] severely tested Gosio’s method of detecting arsenic by means of growths of Penicillium brevicaule, whereby arsenic gases are liberated, vindicating the method completely, and establishing the test as an exceptionally delicate one. [Segale (1904)] applied the same method to the detection of the presence of arsenic in animal tissues.
[Ono (1900)] grew Penicillium cultures with solutions of potassium arsenate and found no important differences either of depression or stimulation. [Orlowski (1902–3)] stated that small doses of arsenic (1/1000–1/100% Sodium arsen—[11]) stimulate the growth of Aspergillus niger, larger doses up to 1⁄8% retard growth, while 1⁄6% kills. Spores of the fungus taken from soil containing arsenic are said to possess an immunity against arsenic, in that they germinate in the presence of an arsenic content which rapidly kills control fungi. This immunity is not specific for arsenic, but extends also to other poisons. The chemical composition and water content are not altered.
Conclusion.
The toxic effect of arsenic upon higher plants is much more marked with arsenious acid and its compounds than with arsenic acid and its derivatives. No definite evidence of stimulation has yet been obtained with any arsenic compound, however great the dilution at which it is applied. With certain algae a stimulus may occur, and it is possible that arsenic acid is capable of replacing phosphoric acid to some extent under certain conditions. With fungi the toxic effect of great concentrations is marked with certain species, but there are others which are capable of living happily on arsenical compounds and of liberating highly poisonous arsenic gas.