In a hormone investigation, each change to the set-up answers one question. To interpret it, match every change to what it tests and state what the result proves.
This lesson follows explaining growth responses to stimuli in the plant responses chapter.
What does each change test?
- Tip removed: tests whether the tip is needed.
- Tip covered with an opaque cap: tests whether the tip detects the light.
- Agar block with auxin on a stump: tests whether auxin alone causes growth.
- Agar block without auxin: acts as a control for the block itself.
Worked example: four coleoptile set-ups
Four oat coleoptiles are left for two days. A, B and C receive light from the right, and D is kept in the dark. The results are below.
| Set-up | Treatment | Result |
|---|---|---|
| A | Intact coleoptile | Bends toward the light |
| B | Tip removed | No growth and no bending |
| C | Tip covered with an opaque cap | Grows straight upward |
| D | Tip removed, agar block with auxin placed off-centre on the left of the stump, no light | Bends to the right, away from the block |
A is the control and shows the normal response.
B shows the tip is needed for growth, because growth stops when it is removed.
C grows straight up, so the tip must be where light is detected. Without light reaching the tip, there is no bending.
D shows auxin alone can cause the growth and bending. The block side receives more auxin, so the left side elongates more and the coleoptile curves to the right.
Conclusion. The tip makes auxin and detects light, and auxin moves down to make the cells below elongate. One-sided light moves auxin to the shaded side, which causes bending toward the light.
The mistake that costs marks
The common slip is to read D’s result as “it bends toward the block”. The side with more auxin elongates more, so it becomes the longer, outer side of the curve.
| Wrong | Right | |
|---|---|---|
| D bends | Toward the auxin block | Away from the block |
| C result | The tip makes no auxin | The tip cannot detect light, so no bending |
| Conclusion | The tip makes the plant grow | Auxin from the tip makes cells below elongate |
Always check which side has more auxin and ask which side becomes longer.
Check yourself
A fifth coleoptile has its tip removed, and a plain agar block without auxin is placed on the stump. Light comes from one side. Predict the result and state its purpose.
Answer
The coleoptile shows no growth and no bending, because the block has no auxin.
This is a control. It shows that the agar block itself does not cause the growth, so the result in set-up D comes from the auxin.
What to study next
Use these skills to forecast new results in predicting responses using supplied evidence. The biological data interpretation trainer gives more diagrams and tables, and the mistake log and paper-error review records which set-up you matched wrongly.
For a teacher to go through experiment diagrams with you, see online one-to-one Biology tuition.