substances produced in plant cells that stimulate the growth processes (growth of roots in cuttings, elongation of cells in stem segments, cell division in plant tissue culture); a group of plant hormones. At low concentrations auxins accelerate plant growth, but at high concentrations they inhibit it. Judging by their structure, auxins are probably monobasic hydroxy acids. Beta-indolylacetic acid (called heteroauxin), which is most widespread in plants, and some of its derivatives which are formed in plants during oxidative deamination of tryptophan are similar to auxins. Auxins are formed in young, vigorously growing parts of higher plants—growing points in stems, in root tips, in young leaves and buds, in fungi, and so on. The high content of auxins in growing organs stimulates the inflow of nutrients from other parts of the plant. Auxins are capable of moving down the stem and up the root. Growth movements in the plant and various tropisms are due to the uneven distribution of auxins in the axial organs. Auxins are found in free or bound states in plant tissues; only free auxins are biologically active. Auxins interact both with other plant hormones (gibberellins and kinins) and with metabolic products. Their mechanism of action is varied and still not completely elucidated. Auxins are assumed to stimulate the biosynthesis of some protein-enzymes involved in the formation of the structural components of cell walls or to form unstable complexes with ribonucleic acid, thereby regulating cell division. Many synthetic organic substances (for example, betaindolebutyric acid) have the same action as auxins. Auxins are used in plant-growing to hasten the rooting of cuttings and the like.