(a)(i) Define tropic and nastic movements. (ii) List four differences between tropic and nastic movements. (b) Describe an experiment to show that auxins ar...

Answer Details

(a)(i) Definitions

Tropic movement (tropism): a growth movement of a plant or part of a plant in response to an external stimulus that comes from a definite direction, the direction of the movement being determined by the direction of the stimulus.

Nastic movement (nasty): a non-directional movement of a plant part in response to a diffuse (non-directional) stimulus, where the direction of the response is independent of the direction of the stimulus.

(a)(ii) Four differences between tropic and nastic movements

(b) Experiment to show that auxins are produced in the apical cells (tip) of a shoot

• Take several similar germinating oat (or maize) seedlings (coleoptiles).
• Leave one seedling intact as a control.
• From a second seedling, cut off the tip of the coleoptile.
• On a third seedling, cut off the tip and then replace it on the stump.
• On a fourth, cut off the tip and place it back with a thin block of agar jelly between the tip and the stump.
• Keep all seedlings under the same conditions and illuminate them from one side.

Observation: the intact seedling and the ones with the tip replaced (directly or through agar) continue to grow and bend towards the light. The seedling whose tip was removed stops growing and does not bend.

Conclusion: because growth and curvature occur only when the tip is present, and the influence passes through the agar block, the growth substance (auxin) is produced in the apical cells of the shoot.

(c)(i) Two plant hormones other than auxins

1. Gibberellin (gibberellic acid).
2. Cytokinin (kinin).

Abscisic acid and ethylene are also acceptable.

(c)(ii) Two examples of tropism

1. Phototropism - shoot bending towards light.
2. Geotropism - root growing downwards towards gravity.