Plant Hormones The growth and development of many plants are regulated by the activity of plant hormones. Hormones are biochemical substances produced in one part of a plant and transported to a different part where they exert a particular effect. An example of a plant hormone is a series of substances called auxins . Auxins increase the length of most plant cells and thereby contribute to the growth and elongation of the plant. Another plant hormone is abscisic acid , which is produced in mature leaves and inhibits growth in developing leaves and germinating seeds. The inhibition occurs during the winter, contributing to the plant's dormancy. Another hormone, ethylene , encourages ripening and the dropping of leaves and fruits from the trees. Slight pressure permits the fruit to break loose from the stem. Two important growth-regulating hormones are the gibberellins and the cytokinins.
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