The same substance is sometimes much more injurious to the tissues of one part of a plant than it is to those of another part of the same plant. Thus, the rootlets of a young growing plant are much more susceptible to injury by many mineral salts than are the vegetative parts of the same plants; while anæsthetics of various kinds generally exhibit their greatest injurious effects upon the leaves, or synthetizing cells. Again, the mycelia of fungi are much more easily killed by toxic agents used as fungicides than are the spores of the same fungi. Some of these observed differences in toxicity may be due to differences in the physiological effect of the substance upon the protoplasm of the tissues which it enters, and others may be due to differences in the resistance of the protoplasm, or of its protective coverings, to penetration by the toxic material. Indeed, the possibilities of different types of toxic action, and of resistance to it by individual plants and species, are so varied that it is not possible to divide toxic agents into specific groups according to the nature of their injurious action upon the plant cell. They are, therefore, more commonly grouped into classes according to their chemical nature and economic significance as fungicides, as follows: inorganic and organic acids; caustic alkalies; salts of the heavy metals; hydrocarbon gases; formaldehyde; alcohols and anæsthetics; nitrogenous organic compounds; and miscellaneous decomposition productions of organic origin. The following brief review of some of the results of the experimental studies of the toxicity of different compounds belonging to these several groups will serve to indicate the general trend of the investigations of these matters which have thus far been made. =Acids.=--The common inorganic acids (hydrochloric, nitric, and sulfuric) kill the rootlets of common farm crops when the latter are immersed for twenty to twenty-four hours in solutions of these acids containing from three to five parts per million of free acid. Acetic acid must be about five times as concentrated as this, and other organic acids may be much more concentrated still before they produce the same injurious effects. The toxic effect of all these acids is greatly reduced in soil cultures, or if particles of sand, graphite, clay, filter paper, etc., are suspended in the solutions containing the acids, the reduction in toxic effect being probably due to the adsorption of the acids upon the solid particles. Hence, the concentrations which limit the toxic effects of these acids in water solutions cannot be taken as representing the condition with which the same plant will have to contend when growing under normal cultural conditions.