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the ability of the organs of plants to assume a certain position under the influence of the earth’s gravity. Geotropism causes the plant’s axial organs to grow vertically: the main root is directed straight downward (positive geotropism), and the main stem is directed straight upward (negative geotropism). If the main stem of the plant is displaced from its typical vertical position because of some external influence, for example, if it is bent or lodged by the wind, the young, still growing part begins to curve and its upper part rises and is again correctly oriented. The tip of the main root, when displaced from its vertical position, curves downward. Geotropic bending is closely linked to growth, and it occurs because in stems that have been displaced from a vertical state the lower side begins to grow more rapidly and the upper side slows down its growth. The unequal rate of growth of the upper and lower sides of horizontal stems is linked with the shifting of auxins to the lower side of the stem or root under the influence of gravity. The parts of plants that have completed their growth are not capable of geotropic bending. Therefore, when plants have been lodged by wind or rain, only the young, growing top of the stem rises. The grasses are an apparent exception: their ganglia retain the ability to renew growth for a long time. When lodging occurs, the lower side of the lower nodes begins to proliferate and raises the part of the stem located above.
In addition to stems growing vertically under the influence of geotropism (orthotropic), there are also horizontally growing stems (plagiotropic). For the most part these are rhizomes and runners (tendrils). A change in the geotropic reaction may occur because of external influences, such as a lower temperature, causing shoots to press close to the ground in alpine and polar plants. The geotropic reaction may also be altered under the influence of certain gases, such as ethylene.