7. Explain the following referring to plant growth hormones (e.g. auxin):
(a) shoots grow towards light
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(b) roots show negative phototropism
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(c) roots show positive geotropism
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Answers
Explanation:
(a) Auxin stimulates the growth of cells on the shady side of the plant which causes bending of the plant to the other side. This gives the appearance that the stem of the plant bends in the direction of light.
(b) Auxin moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism. Auxin also plays a role in maintaining apical dominance.
(c) In both roots and stems, auxin accumulates towards the gravity vector on the lower side. In roots, this results in the inhibition of cell expansion on the lower side and the concomitant curvature of the roots towards gravity (positive gravitropism).