Question
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The movement of plant organs in response to the force of gravity is called
A. Phototropism
B. Thigmotropism
C. Geotropism
D. Chemotropism

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Hint: The effect of gravity on a plant's growth is an extremely important factor for the proper development of the plant. The effect of gravity can be easily studied in any potted plant. Sir Charles Darwin was one of the few notable scientists who studied geotropism and published his results on the same.

Complete answer: Geotropism or gravitropism is an important coordinated property possessed by plants, which deals with the differential growth pattern displayed by a plant or fungus in response to gravity. Gravity may be either “artificial gravity” or natural gravity. It is a general feature of all higher as well as lower plants to show such behavior. Roots are positively geotropic (i.e. they grow in the same direction as gravity) and shoots are considered to show negative geotropism (i.e. they grow opposite to the direction of gravity).

Additional information: Geotropism is caused by an unequal distribution of auxin. In a root placed horizontal condition, the bottom side consists more auxin and grows less - causing the root to grow in the direction of the force of gravity.

So, the correct answer is option C. Geotropism.

Note: It is important to note that certain mutants with altered responses to gravity have been reported. one such example is the plant species Arabidopsis thaliana. These mutants have alterations in either negative gravitropism in hypocotyls and/or shoots, or positive gravitropism in roots, or in both.
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