(a) Describe an experiment to demonstrate the effect of auxin on growth of a plant shoot. (b) List three uses of auxin in agriculture. (c) Describe the mechanism of transmission of impulses through a nerve fibre.
Explanation
(a)To demonstrate the effect of auxin on growth of a plant shoot, cut off the growing tip of coleoptile or shoot of three potted maize seedlings A,B, and C. Get three agar blocks A.B.C. Place one of the cut tips on an agar block A, so that the auxins in the tip can diffuse into the agar block. Then put three drops of IAA or Synthetic auxin on agar blockB. Put agar blocks A and B on one side or on top of the cut tip of the coleoptile or shoot A and B respectively. Then place the untreated agar block C on the side or tip of third cut coleoptile or shoot tip C as control experiment. Leave the three coleoptiles or shoots in unilateral light for 12 to 36 hours. The Coleoptiles or Shoots carrying agar block A and B show growth curvature while C shows no growth curvature. This shows that auxin promotes growth (b)The use of auxin in agriculture are: (i) Auxin helps to induce ripening of fruits. (ii) It also helps to produce lateral bud development in stem (iii) It helps in controlling weeds as well as controlling flowering and fruiting in plants. (v) It assists in breaking seed dormancy. (vi) It helps to initiate adventitious roots in stem cuttings and for parthenocarpic fruiting. (c) Mechanism of transmission of impulses through a nerve fibre is characterised by change in electrical, potential and ionic charge. When at rest, the outside of the nerve is positively charged while the inside is negatively charged. Chemically, the nerve fibres possess potassium ions which are concentrated inside the fibre, while sodium ions are concentrated more on the outside. The change in permeability of the cell membrane is brought about when the fibre is stimulated and sodium ions enter the nerve while potassium ions move to the outside. The change in permeability along the nerve or change in electrical potential and ionic change along the fibre cause the movement of the electrical current along the fibre. In this way, the impulse is propagated or transmitted while the original resting stage of the nerve is restored as soon as the impulse passes.
