• Plants usually do not change place, but their parts do move in response to stimuli.
• (a) Touch movement (nastic movement).

• The leaves of Mimosa pudica (the "sensitive plant") collapse and fold their leaflets when touched.
• The movement is a response to a stimulus whose direction does not determine the direction of the movement; it is caused by rapid loss of turgor in the cells of the swollen leaf base (pulvinus).
• (b) Sleeping movement.

• The leaves of Mimosa and of some legumes partly close in darkness and return to their open position in the light.
• This alternation of light and dark produces the daily rhythm of the leaves, similar to sleeping and waking in animals; so it is called the sleeping movement.
• (c) Tropisms.

• A tropism is a growth response of part of a plant to a directional stimulus — light (phototropism), water/humidity (hydrotropism), gravity (geotropism), touch/contact (haptotropism/thigmotropism).
• The response is positive if towards the stimulus and negative if away from it.
• Hormonal control by auxins is explained in Chapter 2.
• (d) Haptotropism (pulling/twining movement). A special kind of movement seen in the tendrils of the pea plant, and in the contractile (pulling) roots of corms and bulbs.

• Tendrils. A tendril rises into the air and, as it grows, makes contact with a solid object.
• It then twines round the object in a few spiral turns.
• Its length decreases, so the stem is drawn near the support and can grow upright.
• The tendril thickens and lays down a good deal of mechanical (supporting) tissue, becoming stronger.
• If a tendril finds no support it withers and dies.
• Cause of twining growth is slower on the side that touches the support and faster on the side away from the support.
• The longer outer side makes the tendril curl around the support.

• Pulling roots of corms and bulbs. Corms and bulbs have special roots below them.
• When these roots contract (shorten), they pull the corm or bulb down to a suitable depth in the soil.
• The underground stem stays at a suitable distance from the surface, which supports it and protects the aerial parts from wind.

• (e) Cytoplasmic streaming.

• Living cytoplasm is in continuous rotational movement inside the cell.
• It is easily seen in the leaf cells of Elodea (aquatic plant) under the high-power microscope: the cytoplasm forms a thin layer lining the inner side of the cell wall and flows in one direction.
• The movement is revealed by the chloroplasts carried along in the cytoplasm.
• It distributes materials inside the cell.

✍️ Worked examples

• Worked example 5.
• A student places a cucumber tendril against a pencil.
• After two hours the tendril has curled around it.
• Which cells grew faster?
• Those on the side away from the pencil, because the contact side grows more slowly; the faster growth of the outer side causes bending towards the support.
• Worked example 6.
• Why do Mimosa leaves open again some minutes after being touched?
• The cells in the pulvinus regain water, and turgor is restored.

• Why is cholinesterase important? If acetylcholine were not destroyed, the membrane would stay depolarised, the muscle would be stimulated continuously, and could not relax or respond again to a new impulse (spasm).
• Some poisons (organophosphate insecticides, nerve gases) inhibit cholinesterase and cause convulsions for this reason.