[10] In the present state of our knowledge such a concentration seems relatively strong! The results show very clearly how impossible it is to draw any general conclusions with regard to the action of arsenic compounds on plants, as they emphasise the strong individuality of the species in their reaction. 3. _Do arsenic compounds stimulate higher plants?_ The question of stimulation due to arsenic does not seem to have engaged the attention of investigators to any extent. Water culture experiments at Rothamsted have so far yielded negative results, and no stimulation has yet been obtained with any plant, with the possible exception of white lupin with sodium arsenite. In a single series a stimulus was suggested, beginning to make itself felt at 1/500,000, rising to an optimum at 1/10,000,000. No stress can be laid on this result, as it is never safe to draw any certain conclusions without several repetitions of the same experiment. With arsenic acid on barley a possible stimulus is sometimes indicated to the eye, the plants being fine and of a particularly healthy dark colour, but this is not corroborated by the dry weights. Additional tests were made with peas and barley, treated with sodium arsenite and arsenate, the dilutions being carried down to 1/250,000,000, but no evidence of stimulus was obtained, so that it hardly seems possible that arsenic can act as a stimulative agent for these two plants when grown in water cultures. It had been thought that the failure to find a stimulation point hitherto might be due to the too great concentration of the toxic substance rather than to the actual inability of the poison to stimulate, but this hypothesis must now be dismissed so far as these plants are concerned. III. EFFECT OF ARSENIC COMPOUNDS ON CERTAIN OF THE LOWER PLANTS. 1. _Algae._