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Ch. 39 - Plant Responses to Internal and External Signals
Chapter 39, Problem 3

Charles and Francis Darwin discovered that a. auxin is responsible for phototropic curvature. b. red light is most effective in shoot phototropism. c. light destroys auxin. d. light is perceived by the tips of coleoptiles.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Auxin and Phototropism

Auxin is a plant hormone that plays a crucial role in regulating growth and response to light, known as phototropism. When light is directed towards one side of a plant, auxin accumulates on the shaded side, promoting cell elongation and causing the plant to bend towards the light source.
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Red Light and Plant Growth

Red light is particularly effective in influencing plant growth and development, especially in processes like phototropism. It is absorbed by photoreceptors in plants, which trigger responses such as stem elongation and leaf expansion, making it a key factor in optimizing light conditions for growth.
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Coleoptile and Light Perception

The coleoptile is a protective sheath covering the emerging shoot of monocotyledonous plants, such as grasses. It is sensitive to light and plays a vital role in directing the growth of the plant towards light sources, as the tips of coleoptiles are where light perception occurs, influencing the distribution of auxin.
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Steps of the Light Reactions