Botanists recognize five major groups of phytohormones (plant hormones): auxins, gibberellins, ethylene, cytokinins, and abscisic acid.
Auxins
Auxins are hormones involved in cell elongation, apical dominance, and rooting. The auxin, indoleacetic acid or IIA, is produced in actively growing shoot and root tips and in developing fruits. IIA triggers an increase in the plasticity, or stretchability of cells walls allowing elongation to occur. Developing seeds also produce IIA, which stimulates the development of fleshy fruits. For example, if the seeds are carefully removed from a tiny strawberry that is just beginning to develop, the fruit will never get any larger. However, if the seeds are removed from a tiny strawberry and then this seedless strawberry is treated with IIA, the fruit will enlarge to a normal size.
Auxins, produced in the apical meristem of a stem, causes the stem to grow toward light and away from the pull of gravity. If the source of light is to one side of the stem, however, auxin will move away from the light to the shaded side of the stem. The higher concentration of auxin in the shaded side of the stem will stimulate cells in that side to elongate more than cells in the lighted side. This will cause the stem to bend toward the light.
