substance produced by a plant that. in low concentrations. affects physiological processes such as growth and development. often at a distance from the substance’s point of origin (see plant growth regulator).
Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, moved to other locations, in other functional parts of the plant. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit. Plants, unlike animals, lack glands that produce and secrete hormones. Instead, each cell is capable of producing hormones. Plant hormones shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves, and fruits. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant longevity, and even plant death. Hormones are vital to plant growth, and, lacking them, plants would be mostly a mass of undifferentiated cells. So they are also known as growth factors or growth hormones. The term 'Phytohormone' was coined by Thimann in 1948[clarification needed].
Phytohormones are found not only in higher plants but in algae , showing similar functions, and in microorganisms, such as unicellular fungi and bacteria, but in these cases they play no hormonal or other immediate physiological role in the producing organism and can, thus, be regarded as secondary metabolites.