hormone



hormone defined in 1951 year

hormone - hormone;
hormone - Organic substance produced in minute quantity in one part of an organism and transported to other parts where it exerts a profound effect. Hormones (phytohormones) play a prominent role in plant growth. See also: Abscisic acid, Auxins, Cytokinins, Gibberellin, Florigen. Method of transport within plant not fully understood. Movement of auxins (e.g. in young stems) is active, i.e. it is under control of living cells and is not merely a simple diffusion process. Auxin transport is unidirectional, away from stem tip (basipetal). If applied to cut base of stem auxin will not move to the tip (unless e.g. plants have been treated with the antiauxin 2, 3, 5-tri iodobenzoic acid). An important recent concept is that of action and interaction of different types of phytohormones, e.g. relative amount of leaf and root formation in tissue culture can be varied by altering ratios of levels of kinetin and IAA. Hormone-like substances, liberated into water or air, have also been demonstrated to play an important role in sexual reproduction of certain fungi and algae. In animals hormones are usually secreted by endocrine glands directly from gland cells into blood stream. Various hormones of different composition and activities are produced in different glands, e.g. See also: Adrenal, Insulin, Pituitary, Thyroid, Secretin. Occur conspicuously in vertebrates, but known in some invertebrates, e.g. See also: Corpora allata.

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