Theory question 104
Plant Responses and Hormones(a) (i) Define tropic and nastic movements. (ii) List four differences between tropic and nastic movements.
(b) Describe an experiment to show that auxins are produced in the apical cells of a shoot.
(c) (i) Name two plant hormones, other than auxins. (ii) Give two examples of tropism.
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(a)(i) A tropic movement (tropism) is a growth movement of part of a plant in response to a stimulus coming from one direction. A nastic movement is a non-directional movement of part of a plant in response to a stimulus, not related to the direction the stimulus comes from.
(ii) Tropic vs nastic: direction of response related to the stimulus vs not related; growth movement vs often turgor (non-growth) movement; slow vs usually fast; usually irreversible vs reversible; involves auxins vs usually changes in turgor; e.g. stems bending to light vs closing of Mimosa leaves.
(b) Use three similar seedlings (e.g. maize coleoptiles) A, B and C. Leave A intact (control). Cut off the tip of B and leave it. Cut off the tip of C, place it on an agar block for some hours, then put the agar block on the cut stump of C. After a few days A and C grow, B does not. So a growth substance (auxin) made in the tip passed into the agar and caused growth.
(c)(i) Any two: gibberellins; cytokinins; abscisic acid; ethylene.
(ii) Any two: phototropism; geotropism; hydrotropism; thigmotropism (haptotropism); chemotropism.
Tropic = direction matters; nastic = direction does not matter.