Biology 2015 JAMB Past Questions - Question 42: The part marked I will contain a high concentration of

Correct Answer

C

Explanation

Auxin is growth hormone produced at the root (radicle) and shoot (plumule) tips. It promotes ceil division. The auxin found on the lower part of the horizontally placed germinating seedling migrates to the upper part, such that the concentration at the upper part of the radicle is higher that auxin concentration at the lower part. The upper part of the radicle divides morethan the lower part and so grows longer. This results in the bending of the radicle downwards towards the force of gravity. However the concentration of auxin on the upper part of the- plumule is higher than the concentration on the lower part, but high concentration of auxin inhibits cell division at the plumule. Consequently the lower portion of the plumule with low concentration of auxin divides more than the upper part with high concentration of auxin. The lower portion divides more and so grows more resulting in the bending of theplumule away from the force of gravity. The drawing of germinating seedling Klinostat serves as a control experiment. The klinostat rotates the seedling, such that every part of the seedling receives equal amount of auxin. There is no concentration' of auxin at the lower or upper parts of the radicle and plumule. Thus there is uniform division of the cells and this, prevents the bending of the radicle and plumule. The seedlinggrows straight. Ethylene is plant hormone thatmainly promotes the ripening of fruits. Abscisic acid, another hormone promotesthe formation of abscission zone in the leaves and fruits resulting in leavesand fruits fall. It also inhibits the growth of lateral buds. At the abscissionzone, the acid merely dissolves the cells of the xylem and phloem at the junction between the stem of the plant and the lower stalk or fruit stalk. This blocks the supply of soil solution to the flowers and fruits and also stops the supply of manufactured food to these parts. This weakens the affected parts and they fall off. Ascorbicacid is chemical name for vitamin c