A tropic response depends on the direction of the stimulus, and a nastic response does not. Ask one question to classify any plant movement: does the direction of the stimulus decide the direction of the movement?
This lesson opens the SPM Biology plant responses chapter.
How do tropic and nastic responses compare?
| Criterion | Tropic response | Nastic response |
|---|---|---|
| Direction of movement | Set by the direction of the stimulus | Independent of the direction of the stimulus |
| Usual mechanism | Uneven growth, cells elongating more on one side | Usually a change in water pressure in cells |
| Example | Shoot bends toward light (phototropism) | Leaflets of the sensitive plant fold when touched |
| Name pattern | Positive or negative | Not positive or negative |
The common tropisms are phototropism (light), geotropism or gravitropism (gravity), thigmotropism (touch) and hydrotropism (water).
Worked example: classify each response
Three observations are given. Classify each and justify it.
Observation 1. A bean shoot grows toward a lamp placed on its right. The movement follows the light direction, so it is a tropic response. The shoot grows toward the stimulus, so it is positive phototropism.
Observation 2. A sensitive plant folds its leaflets when a student touches it from any side. The folding happens the same way from every direction, so it is a nastic response (thigmonasty).
Observation 3. A pea tendril coils around a wire. The coiling starts on the side touching the wire, so the direction depends on the stimulus. It is a tropic response (thigmotropism).
Observations 2 and 3 use the same stimulus, touch. The difference is in the test question, which shows why you classify by direction and not by the stimulus name.
The mistake that costs marks
The common slip is to write “tropism” for every plant movement, or to write “positive nastic response”. The first ignores the direction test, and the second mixes the two naming systems.
| Wrong | Right | |
|---|---|---|
| Sensitive plant folds | Positive thigmotropism | Thigmonasty, a nastic response |
| Root grows downward | Positive phototropism | Positive geotropism (gravitropism) |
| Shoot grows upward | Negative phototropism | Negative geotropism, and positive if toward light |
Check yourself
A student places a potted plant on its side in the dark. After some days, the shoot curves upward and the root curves downward. Name each response and say whether it is positive or negative.
Answer
The shoot curves upward, away from gravity, so it shows negative geotropism (gravitropism).
The root curves downward, toward gravity, so it shows positive geotropism.
Both are tropic responses, because the direction of bending depends on the direction of gravity.
What to study next
The next lesson, explaining growth responses to stimuli, shows how auxin produces the bending. If you want to compare plant and animal responses, read coordination and response.
The biological data interpretation trainer offers more observations to classify, and the mistake log and paper-error review records your slips. For a teacher to check your reasoning, see online one-to-one Biology tuition.