geotropism
geotropism (gravitropism) The growth of plant organs in response to gravity. A main root is positively geotropic and a main stem negatively geotropic, growing downwards and upwards respectively, irrespective of the positions in which they are placed. For example, if a stem is placed in a horizontal position it will still grow upwards. It is thought that mobile membrane-bound clusters of starch grains (amyloplasts) sink to the bottom of a plant cell, under the influence of gravity, and somehow create a physiological asymmetry within the cell. This results in differential growth of the upper and lower sides of the cell, and hence upward or downward curvature of the organ concerned, depending on whether it is a root or a shoot. The biochemical mechanisms of this differential growth remain largely unknown, although auxins are required for geotropism in shoots, and also possibly in roots, and there is evidence that calcium ions play a role too. See tropism.
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geotropism
