A tropism is explained by uneven growth, and uneven growth is explained by uneven auxin. Write the explanation as a chain, so each sentence causes the next one.
This lesson follows distinguishing tropic and nastic responses in the plant responses chapter.
What is the chain for any tropism?
- Stimulus: one side of the plant receives more of the stimulus.
- Auxin moves: auxin becomes unevenly distributed across the shoot or root.
- Cells respond: cells on one side elongate more than on the other.
- Result: the plant bends toward or away from the stimulus.
Worked example: a shoot bending toward light
Question: explain why a seedling placed near a window bends toward the light. [4 marks]
A full answer:
- The light comes from one side, so the shoot tip receives more light on one side than the other.
- Auxin moves from the lit side to the shaded side of the shoot.
- The higher concentration of auxin on the shaded side makes those cells elongate more than the cells on the lit side.
- The shaded side grows longer, so the shoot bends toward the light. This is positive phototropism.
Each step names the cause of the next one. Skipping step 2 is the commonest slip, because the answer jumps from light to bending.
Why does the same hormone act differently in a root?
The auxin effect depends on the concentration and the organ. In a shoot, a higher concentration of auxin makes cells elongate more. In a root, the same higher concentration slows elongation.
In a root lying on its side, auxin gathers on the lower side, and the lower side elongates less than the upper side. The upper side grows longer, so the root curves downward, which is positive geotropism.
The mistake that costs marks
The common slip is to write “auxin makes the plant grow toward the light” without saying where auxin is and what it does to the cells.
| Wrong | Right | |
|---|---|---|
| Shoot bending | Auxin makes the shoot grow to the light | Auxin moves to the shaded side and makes those cells elongate more |
| Root bending | High auxin makes the root grow more | High auxin on the lower side slows elongation there |
| Cause | The plant knows where the light is | Uneven distribution of auxin causes uneven growth |
Check yourself
Explain why a potted seedling laid on its side in the dark has a shoot that curves upward.
Answer
Gravity causes auxin to gather on the lower side of the horizontal shoot.
In a shoot, the higher concentration of auxin makes the cells on the lower side elongate more, so the lower side grows longer.
The shoot therefore curves upward, away from gravity. This is negative geotropism.
What to study next
Use the chain on experiment diagrams in interpreting plant-hormone investigations. The claim, evidence and explanation revision desk gives you practice at linking claims to evidence.
Record chain gaps in the mistake log and paper-error review. For a teacher to go through your written chains, see online one-to-one Biology tuition.