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._