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.
Alkalies.—The caustic alkalies must usually be present in from five to ten times as great concentrations as those of the mineral acids, in order to produce the same injurious effects upon the rootlets of common plants. The so-called "alkali" of soils is not alkali at all, but is neutral soluble salts present in sufficient concentration to exert a toxic effect.
Salts of the heavy metals are especially toxic to rootlets of plants. Salts of copper, mercury, and silver, have been found to kill the roots of seedlings immersed in them for twenty-four hours when present in proportions of less than three parts per ten million, while salts of many other heavy metals are toxic when present in concentrations of less than one part per million. The salts of the alkali metals are considerable less injurious than are those of the heavy metals, but even these exert their familiar injurious effect if present in concentrations which, measured by the ordinary standards, would still be regarded as very dilute solutions.
Illuminating gas, and similar hydrocarbon gases, kill plants when present in the atmosphere in as little as one part per million. Leaves, buds, and roots are all alike sensitive to this toxic effect, the nature of which is not yet understood.
Formalin, or formaldehyde, is a penetrating toxic agent for nearly all plant cells, and is commonly used as a fungicide for the destruction of parasitic fungi. It probably affects the colloidal condition in some way similar to its hardening effect upon gelatin, etc.
The toxic effect of many different organic compounds is so varied in its nature and extent that it is impossible to give any satisfactory brief review of its manifestations. Recent investigations appear to indicate that organic products of decomposition of plant residues in the soil may exert powerfully toxic effects upon succeeding generations of the same, or of different, plants growing on the land. But the experimental data and conclusions concerning these matters are not yet accepted without question by all students of plant science or of the problems of the productivity of the soil. In fact, it is yet an open question whether toxic soil constituents are really an important factor in the so-called "unproductivity" of certain soils.
Alkaloids, and even the amino-acids which are produced in the tissues of some species of plants, while not toxic to the plants or organs which elaborate them, sometimes exhibit strikingly toxic action upon other plant organs with which they are brought into contact. There is, as yet, no satisfactory explanation of this difference in behavior between plant tissues toward various organic toxic substances.
In fact, the whole subject of the toxic action of various substances upon plants needs much more study before it is brought to the point where it will afford definite knowledge of either the physiological problems involved or of their practical applications in questions of soil productivity, etc.