Question

3.6 an experiment was set up to investigate a plant growth response to a stimulus. a seedling has a radicle (young root) and a plumule (young stem). this seedling was place horizontally in a dark place and a growth response was observed after seven days as shown in the diagram below. 3.6.1 name the: (a) growth response observed after seven days. (1) (b) plant hormone responsible for the growth response name in question 3.6.1 (a) (1) 3.6.2 explain the growth response observed in the root of the seedling. (5) 3.6.3 explain how a control set - up will be different from the above set - up. (2)

Explanation:

(a) The growth response observed is geotropism (gravitropism), where roots grow downwards and shoots grow upwards in response to gravity.
(b) The plant hormone responsible is auxin. Auxin accumulates on the lower side of the horizontally - placed root due to gravity. In roots, high auxin concentration inhibits cell elongation, causing the upper side of the root to grow faster and the root to curve downwards.
3.6.2: When the seedling is placed horizontally, gravity causes auxin to accumulate on the lower side of the root. In roots, high auxin concentration inhibits cell elongation. So, the cells on the upper side of the root elongate more than those on the lower side, causing the root to curve downwards towards the gravitational pull.
3.6.3: A control setup would have the seedling placed vertically. This would eliminate the effect of gravity as a directional stimulus, and the root would grow straight down and the shoot straight up due to normal growth processes without the influence of the horizontal orientation.

Answer:

3.6.1 (a) Geotropism (gravitropism)
3.6.1 (b) Auxin
3.6.2: Gravity causes auxin accumulation on lower side of root. High auxin inhibits lower - side cell elongation, upper side grows faster, root curves down.
3.6.3: Place the seedling vertically instead of horizontally.