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a compound produced by plants in small quantities to stimulate growth or formation. Phytohormones are diverse in structure and effect. They include auxins, gibberellins, kinins, and florigen, the composite hormonal agent that promotes flowering. Phytohormones regulate many life processes of plants, including germination, growth, the differentiation of tissues and organs, flowering, and the ripening of fruits.
Phytohormones are usually formed in one organ and transported to another organ or a different part of the same organ. In contrast to animals, plants do not have special organs for the synthesis of hormones. Certain organs are more highly saturated with particular hormones than others. Thus, the apical meristem of the stem is richer in auxins, while the leaves are richer in gibberellins and florigen, and the roots and maturing seeds are richer in kinins. Phytohormones produce a great variety of effects. Auxins, for example, can stimulate the dilation of cell walls and also cell division. Phytohormones act in a definite order, with kinins and gibberellins predominating in the early stages of development and auxins in later stages. They interact closely with one another; for example, a change in the concentration of one phytohormone influences the reaction caused by another. Natural growth inhibitors, such as abscisic acid, constitute a special group of phytohormones.
The effects of phytohormones on growth may be rapid, in which case the membrane system is involved; the phytohormones take part in processes that influence the structure of the plasma membrane. When the effects are slow, nucleic acids and proteins are involved.
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V. I. KEFELI