The physiological action of arsenic compounds on plant life early attracted the attention of investigators. Chatin (1845) put forward some rather curious and unexpected considerations with regard to this action. He stated that the effect of arsenic on plant growth is determined more by the constitution and temperament of individual plants than by their age, and that apparently difference in the sex of plants is of no significance. The chief determining agent, however, is the species, and Chatin found that as a general rule Cryptogams are more sensitive than Phanerogams, and Monocotyledons than Dicotyledons, as is shown by the fact that under treatment the former perish first. Some extreme exceptions exist, though, as _Mucor mucedo_ and _Penicillium glaucum_ will grow on moist arsenious acid, whereas leguminous plants are killed by an arsenical solution in a few hours. Chatin held the view that elimination of the poison succeeded its absorption, and that this elimination is complete if the plant lives long enough. Here again the species exerts a great influence on the excretory functions of the plants. Lupins and Phaseolus are presumably able to eliminate in six weeks all the arsenious acid they can absorb without dying. Most Dicotyledons need 3–5 months, while Monocotyledons retain traces of poison for six months after its absorption. Lichens are said to eliminate it more slowly still. Again, woody species are longer in freeing themselves than herbaceous, and young plants carry out the elimination more easily than old plants. The excretory function is influenced by other physiological factors such as dryness and season. The toxic effects and elimination are supposed to act inversely and parallel, the absorbed arsenious acid combining with alkaline bases, making a very soluble salt which is excreted by the roots. Calcium chloride is given as the antidote to arsenious acid, all soluble acid being “neutralised” by it. This view of the elimination of arsenic apparently did not gain much support, as no further references to the matter have so far come to light. In view of the work of some modern investigators (Wilfarth, Römer and Wimmer) on the excretion of salts by plant roots, the idea may prove of fresh interest. Chatin also found that moving or still air influenced the working of the poison, indicating that the external physical conditions affect the toxic action considerably. Nearly forty years later Nobbe, Baessler and Will found that, if transpiration were hindered by placing plants in a dark or moist room, it was possible to keep the plants turgescent in arsenic solutions for a long time without thereby increasing the toxic effect later on. The poisonous action proceeds from the roots, of which the protoplasm is disorganised and the osmotic action hindered. Finally, in the presence of sufficient of the poison, the root dies without growth.