noun
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that which urges, provokes or prompts (someone or something) into action or accelerates the process; an impelling or influencing person, thing of force; a driving agent.
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a substance formed in an organ of the body, as in the adrenal glands, the pituitary, etc., and carried by a body fluid to another organ or tissue, where it has a specific effect; a hormone.
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the indoleacetic acid, produced in fruits, seeds, leaves or stem tips, to promote longitudinal growth and to control bud growth, root formation, leaf abscission, etc., auxin.
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a synthetically produced thing which acts similar to a hormone.