Relations of Plants to Minute Doses of Inhibitive Substances.

It is generally recognized that minute doses of substances which are toxic in larger quantities may act in a stimulatory fashion, that is, they may cause the plant to accelerate one or more of its activities in excess of any stoichiometric relationship. Minute traces of mineral salts (2, 4) and -auxins, bioses, vitamins, and other growth accessories (1, 3, 7) which are of biological origin, all seem to show this action and, above certain specific coiicentrations, the toxic action of representatives of most of these classes of materials has been demonstrated. The action of these substances, however, is much more complicated than the just mentioned facts would indicate. Living organisms not only react by stimulation or retardation of their individual activities, but also ultimately become accommodated to increased or decreased quantities of physical and chemical stimulative factors. This accommodation or acclimatization may become evidenit in various ways. THIMANN (6) has briefly called attention to a type of adjustment which is of very great significance in physiological studies. He says: "If roots are treated with auxin so that they are inhibited, and the auxin afterwards removed, their subsequent growth is accelerated. . . . The higher the concentration of auxin used, the longer it takes for this acceleration to appear, but on the whole the greater the acceleration is when it does come." Apparently referring to the same work WENT and THIMANN (8, p. 144) suggest that this reaction "presumably, is due to the rapid disappearance of auxin from the root, so that its concentration eventually reaches the accelerating level. " Some recent observations incidental to another investigation, upon rooting of oleander cuttings in solutions of hetero-auxin indicated the same type of reaction. Three sets of cuttings were made on April 18, some cuttings being placed in tap water (set C), some in 0.0013 per cent. hetero-auxin in water (set B), and some in 0.02 per cent. hetero-auxin in water (set A). Those in set B were given fresh solutions twice in the next three weeks, at the end of which time they were placed in tap water. They had developed callus tissue, but no root initials were externally visible. On April 20 the cuttings of set A were washed off and placed in tap water. The first cutting to show root development was from set C; and by May 9, 9 out of the 10 in this set had developed roots. On the other hand, by this date all of the cuttings of sets A and B had developed more callus and overgrowth of tissue than those in set C, but nd roots. Those of set A had the greatest amount of overgrowth. By May 16 numerous roots were showing on all 3 cuttings in