MARTIN SHIELDS / SCIENCE PHOTO LIBRARY MARTIN SHIELDS / SCIENCE PHOTO LIBRARY
Gravitropism (sometimes referred to as geotropism) is a plant response to earth's gravitational field. Plant stems (shoots) exhibit negative gravitropism because they respond by growing away from the gravitational attraction. Gravitropism is initiated when starch filled plastids called amyloplasts sink towards the gravitational field. This stimulates the release of the growth hormone auxin. This bean plant, Phaseolus vulgaris, shows a turn in its stem which occurred after its pot was tipped over. The response also occurs in the dark showing that it is not phototropism.
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