Many dwarf plants exhibit a decrease in height because they contain low levels of the stem elongation hormones called gibberellins. These hormones, of which more than thirty have been identified, increase the amount of water taken up by the cells of stems. As the individual cells swell from the increased water content, the stem grows longer. In addition to stem elongation, the gibberellins elicit a number of other plant responses.
The seeds of numerous plant species exhibit dormancy, which can be broken by gibberellins. The length of the daylight period (photoperiod) is crucial to the flowering response in many plants. Fall- and winter-flowering plants require short days, while the plants that flower in the spring and summer must be exposed to long days. Some plants must also be subjected to a prolonged period of cold before flowering can occur. The gibberellins can substitute for the long day or the cold requirement in many plants. Additionally, the gibberellins can produce thicker stem growth in certain woody plants and increase the number of fruits that develop in some species.
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