Chemical Coordination in Plants — ICSE Class 10 Biology Important Questions

Answer: (c) Auxin.

Auxin, produced at the shoot tip, promotes cell elongation and causes the bending of shoots towards light (positive phototropism).

Answer: (b) Positive geotropism.

Roots grow towards gravity (downwards), which is positive geotropism; shoots grow away from gravity, which is negative geotropism.

Answer: (b) Ethylene.

Ethylene is a gaseous hormone that hastens the ripening of fruits and the ageing/abscission of leaves.

Answer: (b) Thigmotropism. It is a growth movement in response to touch (contact): the side of the tendril away from the support grows faster, so the tendril curls round it.

Answer: (a) The shoot shows positive phototropism. Auxin moves to the darker side, which elongates faster, bending the shoot towards the light. R correctly explains A.

A tropism is a growth movement of a plant part in response to a stimulus, in which the direction of growth depends on the direction of the stimulus.

Positive tropism: growth towards the stimulus, e.g. a root growing down towards gravity (positive geotropism).

Negative tropism: growth away from the stimulus, e.g. a shoot growing up, away from gravity (negative geotropism).

(i) Gibberellins — promote stem elongation and help break seed and bud dormancy (they also induce bolting).

(ii) Abscisic acid (ABA) — a growth inhibitor that closes stomata during water stress and promotes dormancy; it is called the 'stress hormone'.

Phototropism — growth movement in response to light. Example: a shoot bends towards light (positive phototropism).

Geotropism — growth movement in response to gravity. Example: a root grows downwards towards gravity (positive geotropism).

Hydrotropism — growth movement in response to water. Example: a root grows towards a source of moisture (positive hydrotropism).

In each case the response direction is set by the direction of the stimulus.

Auxins: promote cell elongation of the shoot, control apical dominance and cause phototropic bending.

Cytokinins: promote cell division (cytokinesis), delay ageing of leaves and help break dormancy of buds.

Abscisic acid (ABA): inhibits growth, promotes closure of stomata during water shortage and induces seed/bud dormancy.

When light falls on one side only, the shoot shows positive phototropism — it bends towards the light.

Auxin from the tip accumulates on the shaded side; those cells elongate more than the lit side, so the shoot curves towards the light.

(a) (i) Positive chemotropism (response to chemicals secreted by the ovule). (ii) Positive hydrotropism (response to water). (iii) Positive phototropism (response to light). (iv) Positive geotropism (response to gravity).

(b) Grow a young potted seedling inside a closed box that has a small opening on one side only, and keep it where light enters only through the opening. Keep a similar seedling in open light as a control. After a few days the shoot in the box has bent towards the opening, while the control grows straight up. This shows positive phototropism: the shoot grows towards the light because auxin collects on the shaded side and makes those cells elongate more.

Coordination without nerves: Plants have no nervous system, yet they respond to stimuli through these chemical messengers (phytohormones). The hormones are made in one region, diffuse or are transported to target tissues, and there they alter growth (e.g. rate of cell elongation or division). Because plant responses are mostly slow growth movements rather than rapid actions, this chemical coordination is sufficient for their needs.

(i) The stimulus is gravity; both responses are geotropism (gravitropism).

(ii) The shoot growing upward, away from gravity, is negatively geotropic; the root growing downward, towards gravity, is positively geotropic.

(iii) When the plant lies on its side, auxin accumulates along the lower side of both organs. In the shoot, this extra auxin makes the lower side elongate more, so the shoot bends upward. In the root, a high auxin concentration inhibits elongation, so the lower side grows less than the upper side and the root bends downward. The same hormone thus gives opposite effects because roots and shoots have different auxin sensitivities.

(iv) The shoot grows up towards light and air for photosynthesis, while the root grows down into the soil for anchorage, water and minerals — improving the plant's survival.