9 min readConcept sheet1-mark MCQ and assertion-reason; 2 or 3-mark questions on tropisms, the touch-me-not plant and plant hormones; sometimes a case-based question on an experiment

Coordination in plants and plant hormones

Plants respond without nerves. The touch-me-not folds its leaves at once, while shoots and roots slowly grow towards or away from a stimulus, guided by hormones like auxin.

TropismGrowth of a plant part towards (positive) or away from (negative) a stimulus

AuxinMade at the shoot tip; moves to the shady side and makes cells there grow longer

Growth inhibitorAbscisic acid; causes wilting of leaves

Touch-me-notMimosa pudica; leaves fold as cells change their water content

Immediate response vs growth movement

Immediate response (touch-me-not)Leaves of Mimosa pudica fold as soon as they are touched. There is no growth: cells pass the message by electrical-chemical means and change shape by quickly gaining or losing water.

Growth movement (tropism)Slow and directional, caused by unequal growth. A pea tendril grows faster on the side away from a support than on the side touching it, so it coils around the support.

Tropic movements

Tropism	Stimulus	What happens
Phototropism	Light	Shoot bends towards light (positive); root grows away from it (negative)
Geotropism	Gravity	Root grows down (positive); shoot grows up (negative)
Hydrotropism	Water	Roots grow towards water
Chemotropism	Chemicals	Pollen tube grows towards the ovule

Plant hormones

Hormone	Effect
Auxin	Made at the shoot tip; makes cells grow longer; makes the shoot bend towards light
Gibberellins	Help the growth of the stem
Cytokinins	Promote cell division; found in large amounts where cells divide fast, such as fruits and seeds
Abscisic acid	Inhibits growth; causes wilting of leaves

How auxin bends a shoot towards light

1
Light falls on the shoot from one side.
2
Auxin, made at the shoot tip, moves towards the shady side.
3
On the shady side, auxin makes the cells grow longer.
4★
The shady side grows faster than the lit side, so the shoot bends towards the light.

★ key step

Activity: showing phototropism

1
Fill a conical flask with water and cover its mouth with a wire mesh.
2
Place two or three freshly germinated bean seeds on the mesh.
3
Put the flask in a cardboard box with one side open, facing light from a window.
4★
After 2 or 3 days, observe: the shoots bend towards the light and the roots grow away from it.
5
Now turn the flask so the shoots face away from the light. After a few days, the old parts stay as they were, but the new growth bends towards the light again.

★ key step

In real life

A money plant on a windowsill leans towards the light: that is phototropism. Touch a chhui-mui (touch-me-not) plant in a garden and its leaves fold at once.

Exam angle

2 marks: “How does the touch-me-not plant respond to touch? How is this different from the movement of a pea tendril?” Its leaves fold at once as cells lose water and change shape, with no growth. A tendril coils slowly by unequal growth, faster on the side away from the support.

3 marks: “Name the plant hormone that (a) helps the stem grow, (b) promotes cell division, (c) inhibits growth. Which hormone bends a shoot towards light?” (a) Gibberellins, (b) cytokinins, (c) abscisic acid. Auxin bends the shoot towards light.

3 marks: “Describe an activity to show that shoots bend towards light and roots grow away from it.” Germinated bean seeds on a wire mesh over a flask of water, in a box open on one side; shoots bend to light, roots away; turning the flask makes new growth bend to light again.

Remember it

Auxin runs Away from light; Gibberellins make stems Grow; Cytokinins make Cells divide; Abscisic acid Arrests growth.