Tropism is the locomotive response of a biological organism in response to an external stimulus. Generally a term referring to plants, their are numerous manifestations of tropism, ranging from gravitropism, growing downwards contingent on gravity, to heliotropism, developing in a way affected by sunlight. Two broad types of tropism are generally distinguished, taxis: the directional growth/movement of the organism in question(the type of heliotropism featured in the above photo),and kinesis: non-directional tropism. The plant above has altered its stem growth to directionally develop towards sunlight, a form of photo or heliotropism.

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