absence or insufficiency of one essential element arrests or diminishes growth. This applies not only to those substances which are present in the greatest abundance, such as C, P, N, &c., but also to those elements like manganese, boron, and iodine, which only occur in traces. These elements are usually specialised in function, and for them the name “catalytic” elements was suggested, in view of the work they are held to do. As late as 1910 the rôle of manganese in the functioning of the oxidising enzymes was again insisted on. It was concluded that manganese intervenes as a catalytic agent in the material changes of which plants are the seat, and that it participates in an indirect manner in the building up of the tissues and in the production of organic matter. _Conclusion._ Manganese exerts a toxic influence upon the higher plants, if it is presented in high concentration, but, in the absence of great excess of the manganese compounds, the poisoning effect is overshadowed by a definite stimulation. As is the case with boron, manganese stimulates some species more than others, the action on barley being more evident than that on peas. It seems probable that manganese may prove to be an element essential to the economy of plant life, even though the quantity usually found in plants is very small. CHAPTER VIII CONCLUSIONS In the foregoing chapters a very limited number of plant poisons have been considered, yet there is sufficient evidence to show that even these few differ considerably in their action upon plant-life. This action is most variable, and it is impossible to foretell the effect of any substance upon vegetative growth without experiments. The degree of toxicity of the different poisons is not the same, and also one and the same poison varies in the intensity and nature of its action on different species of plants. While certain compounds of copper, zinc and arsenic are exceedingly poisonous, compounds of manganese and boron are far less deleterious, so that a plant can withstand the presence of far more of the latter substances than of the former. Again, the tested compounds of copper, zinc and arsenic do not seem to stimulate growth, even when they are applied in the smallest quantities, whereas very dilute solutions of manganese and boron compounds decidedly increase growth. But, differentiation occurs even in this stimulative action, for while manganese is the more effective in stimulating barley, boric acid is far more potent for peas, the shoots being particularly improved.