While all the inorganic substances examined in this monograph are toxic in high concentrations, some lead to increased growth in lower concentrations, while others apparently have no effect. In this sense all substances could be classed as toxins, even the nutrients. Thus the old distinction between toxin and nutrient has now lost its sharpness, but it does not lose all its significance. The old “nutrients” had certain definite characters in common, in that they were essential to plant growth, the growth being in a great degree proportional to the supply, a relatively large amount of the nutrients being not only tolerated but necessary. The substances dealt with more particularly in this book have none of these characters. Even those that cause increased growth do not appear to be essential, at any rate not in the quantities that potassium, phosphorus, nitrogen, &c., are essential, while there is no evidence that growth is proportional to supply. The substances fall into two groups: (1) Those that apparently become indifferent in high dilutions and never produce any increase in plant growth. (2) Those that cause a small, but quite distinct, increased growth when applied in quantities sufficiently small. The former group may be legitimately regarded as toxins; the latter present more difficulty and even now their function is not settled. It is not clear whether they stimulate the protoplasm or in some way hasten the metabolic processes in the plant, whether they help the roots in their absorbent work, or whether they are simple nutrients needed only in infinitesimal quantities. The two groups, however, cannot be sharply separated from one another. Indeed a substance may be put into one of these classes on the basis of experiments made with one plant alone and into another when a different plant is used, while it is quite conceivable that further experiments with other plants may abolish the division between the two groups altogether. It is even impossible to speak rigidly of toxicity. The addition of the inorganic food salts to solutions of a poison reduces the toxicity of the latter, so that the plant makes good growth in the presence of far more poison than it can withstand in the absence of the nutrients. This masking effect of the inorganic food salts upon the toxicity of inorganic plant poisons is paralleled by a similar action on organic toxic agents. Schreiner and Reed (1908) found that the addition of a second solute to a solution decreases the toxicity of that solution; further the plant itself may exercise a modifying influence upon the toxic agent. Water culture experiments were made upon the toxicity of certain organic compounds, with and without the addition of other inorganic salts. Arbutin, vanillin, and cumarin were definitely toxic and the toxicity decidedly fell off after the addition of sodium nitrate and calcium carbonate, especially with the weaker solutions of the toxins.