Auxin
Auxin, any of a group of plant hormones, substances naturally produced in actively growing parts of plants that regulate many aspects of plant growth and development. These substances affect the growth of the stem tip, leaves, and roots, and the development of side branches and fruit. Auxins affect the growth of these plant parts by stimulating certain cells to elongate, or lengthen, and by inhibiting the growth of other cells, depending on the amount and distribution of auxin in the plant tissue. The effect of auxin on plant cells is important in controlling plant functions called tropisms. A tropism is a plant’s response to external stimuli that causes a change in the direction of the plant’s growth, such as bending, turning, or curving. When an indoor plant is placed in a sunlit window, the plant appears to bend or grow toward the sun. This response to the stimulus of light is called phototropism. It is believed that light destroys auxin where it strikes the stem, causing an imbalan...