A consideration of the experimental work that has been done on this subject of poisoning and stimulation leads one to the inevitable conclusion that it is not true to maintain the hypothesis that _all_ inorganic plant poisons act as stimulants when they are present in very small quantities, for while some poisons do increase plant growth under such conditions, others fail to do so in any circumstances. It is probable that what has been found true with the few substances tested would prove to be similarly true over a much wider range of poisons, and at any rate the hypothesis must be dismissed in its universal application. A more accurate statement would be that some inorganic poisons act as stimulants when present in small amounts, the stimulating concentrations varying both with the poisons used and the plants on which they act. It is quite possible for a stimulation in one respect to be correlated with a retardation in another. In the Rothamsted experiments on the action of manganese sulphate on barley the weaker concentrations of the salt improved the vegetative growth, as was shown by the increase in the dry weights, but with the same strengths of the poison the ripening of the grain was retarded, so that, while certain of the physiological functions were expedited, others were hindered by the action of the poison. Thus it is evident that it is exceedingly difficult sharply to characterise either toxic or stimulant action. In neither case is the reaction simple--many factors may come into play and many processes are concerned, while the effect of a so-called poison may vary in respect of each of the functions and processes concerned. If the poison is presented in great strength the toxic action is dominant, and probably affects many functions in the same sense, so that the action is, so to speak, cumulative. Lower down in the scale of concentration differentiation of action may set in, and while some processes may still be hindered, others may be stimulated. If the two actions balance one another an apparent indifference may be manifested, so that it seems that such strengths of the poison have no effect on growth, either harmful or beneficial. At still lower concentrations, with certain plants and certain poisons, the stimulative action overpowers the toxic effect, so that in some respect or other improvement occurs in growth. It is quite conceivable, however, that some poisons are truly indifferent in weak concentrations, as no stimulation makes itself evident under any circumstances. In these cases one is inclined to suspect that the action is somewhat more simple, in that the toxic effects gradually diminish until no poisonous action is manifest at very weak concentrations, and as no stimulation is present to bring the growth above the normal with these very weak concentrations the plant is similar to those grown without any addition of the poison.