Any of various hormones or similar substances that promote and regulate the growth and development of plants. Auxins are produced in the meristem of shoot tips and move down the plant, causing various effects. Auxins cause the cells below the shoot apex to expand or elongate, and this (rather than cell division) is what causes the plant to increase in height. In woody plants, auxins also stimulate cell division in the cambium, which produces vascular tissue. Auxins inhibit the growth of lateral buds so that the plant grows upwards more than outwards. They can be produced artificially in laboratories for such purposes as speeding plant growth and regulating how fast fruit will ripen.