(a) (i) What is the effect of gravity on plant growth? (ii) Describe an experiment to demonstrate the effect of gravity on the roots of plants. (b) Explain ...

(a)(i) Effect of gravity on plant growth

Gravity determines the direction of growth of plant organs. The root grows downwards towards the pull of gravity (it is positively geotropic), while the shoot grows upwards away from gravity (it is negatively geotropic). This response is called geotropism (gravitropism).

(a)(ii) Experiment to demonstrate the effect of gravity on roots

1. Soak and germinate several bean seeds until the radicle (root) appears.
2. Pin two or three germinating seeds to a cork so that their roots point in different directions (some horizontal, some upward).
3. Place the cork inside a bell jar or wide jar containing moist cotton wool to keep the air humid, and keep it in the dark to remove the effect of light.
4. Set up a control in which an identical cork with seeds is fixed to a slowly rotating clinostat (this nullifies the one-sided pull of gravity).
5. Leave both set-ups for two to three days and observe.

Observation: in the stationary set-up all the roots curve downwards, whatever their starting direction; on the clinostat the roots grow straight. Conclusion: gravity causes roots to grow downwards (positive geotropism).

(b) How bitter taste is detected in humans

Bitter substances in food dissolve in the saliva and stimulate the taste buds located mainly at the back of the tongue. The chemical stimulates the receptor (gustatory) cells, which generate nerve impulses. The impulses are carried by sensory nerves to the brain (cerebral cortex), where they are interpreted as a bitter taste.

(c) Endocrine glands and their functions