Nastic movement

Where it happens

Nastic movements happen in specialised structures such as the pulvinus, a swelling at the base of each leaflet of Mimosa pudica (semalu), and the response depends on the strength of the stimulus, not the direction it comes from.

The pulvinus is a cushion of thin-walled parenchyma cells wrapped around a central bundle of vascular tissue. The cells on the lower side of the pulvinus (the extensor cells) are normally fully turgid and hold the leaflet open; the cells on the upper side (the flexor cells) are smaller.

Nastic movements are also seen in other plants: the leaves of the rain tree and of legumes such as the tamarind fold at dusk (a sleep movement, or nyctinasty), tulip and crocus flowers open in warmth and close in cold (thermonasty), and the trap of the Venus flytrap snaps shut when trigger hairs are touched (thigmonasty). In each case the stimulus sets off the response, but the direction of the movement is fixed by the structure of the plant, not by where the stimulus comes from.

Inputs and outputs

• Input: a stimulus such as touch, heat, vibration, or a change in light intensity, applied from any direction.
• Input: an electrical signal that spreads from the stimulated point through the leaf to the pulvinus, similar in principle to a nerve impulse but far slower.
• Input: potassium and chloride ions inside the pulvinus cells, which are pumped out across the membrane when the signal arrives.
• Output: water leaving the extensor cells by osmosis, so they shrink and become flaccid.
• Output: leaflets that fold together and a petiole that droops, in the same way regardless of where the touch came from.
• Output: a slow recovery as ions are pumped back into the cells by active transport, water re-enters by osmosis and turgor is restored.

The steps

1. A stimulus, such as touch, is applied to a leaflet of Mimosa pudica.
2. Receptor cells at the point of contact generate an electrical signal that travels through the leaf tissue to the pulvinus at the base of the leaflet.
3. When the signal reaches the pulvinus, potassium and chloride ions move rapidly out of the extensor cells on the lower side.
4. Water follows the ions out of these cells by osmosis, so the cells lose turgor pressure and become flaccid.
5. The flaccid extensor cells can no longer support the leaflet, while the flexor cells on the upper side keep their turgor, so the leaflets fold upward and together within seconds.
6. If the stimulus is strong, the signal spreads to the pulvinus at the base of the petiole and the whole leaf droops.
7. Over the next few minutes, active transport pumps the ions back into the extensor cells, water re-enters by osmosis, turgor is restored and the leaflets reopen.

Tropism versus nastic movement

Nastic movements and tropisms are both responses to stimuli, but they differ in how the direction of the response is determined.

Why it matters and how it is controlled

Nastic movements matter to the plant as protection. The sudden folding of Mimosa leaflets startles grazing insects and reduces the leaf area exposed to a herbivore or to heavy rain and wind.

The movement is controlled by the strength of the stimulus: a light touch folds only the leaflets nearest the point of contact, while a strong shake folds every leaflet and drops the petiole. Recovery uses energy, because ions must be pumped back by active transport, so repeated stimulation eventually leaves the plant unresponsive until it has restored its ion balance.

How it is examined

You may be asked to explain why touching Mimosa pudica from any direction produces the same folding response, to describe the role of turgor pressure and potassium ions, or to distinguish a nastic movement from a tropism using a table like the one above.

Experiment-based questions describe touching the plant at different points and timing the fold and the recovery. You are expected to identify the manipulated variable (the point of contact or the strength of the stimulus), the responding variable (the time taken to fold or the number of leaflets folded), and to explain why the response is classified as nastic.

Questions also pair Mimosa with a seedling bending toward light and ask you to state two differences in a table, so keep the three comparison points, direction, mechanism and speed, ready.

Common misconceptions

Worked exam-style question

Question. A student touched a single leaflet of a Mimosa pudica plant from the left side, and then, after the plant had recovered, touched the same leaflet from the right side. In both cases the leaflets folded upward and together in about two seconds.

(a) Name the type of response shown. (b) Explain, with reference to the pulvinus, how the folding occurs.

(c) State two ways in which this response differs from the bending of a shoot toward light.

Model answer. (a) A nastic movement (thigmonasty). (b) Touch triggers a signal that reaches the pulvinus at the base of the leaflet.

Potassium ions move out of the cells on the lower side of the pulvinus, and water follows by osmosis. These cells lose turgor pressure and become flaccid, so they can no longer hold the leaflet up, and the leaflet folds.

The response is the same in both trials because the direction of the fold depends on the structure of the pulvinus, not on the direction of the stimulus. (c) The bending of a shoot toward light is a tropism: its direction depends on the direction of the stimulus, and it is caused by unequal growth (cell elongation) controlled by auxin, so it is slow, whereas the Mimosa response is a turgor change that is fast and involves no growth.

Source:SRC-DSKP-EN

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