A tropism is growth of a plant in response to a directional stimulus. To interpret an investigation, compare each group with the control, use the mean rather than a single value, and explain the result with auxin.
This skill is part of coordination and responses, and it connects to the idea in nervous and hormonal signalling that chemicals can coordinate a response.
What is the idea behind the experiment?
In a shoot, auxin is made at the tip and moves down. With light from one side, auxin collects on the shaded side. The shaded cells elongate more, so the shoot bends towards the light (positive phototropism).
If the tip is removed or blocked, the source of auxin or its movement is affected.
Worked example
The data below are invented for practice. Seedlings were grown in boxes with light from the right only. The angle of bend towards the light was measured after 24 hours.
| Group | Treatment | Bend (°) for five seedlings |
|---|---|---|
| A | Tip left alone (control) | 30, 36, 34, 40, 35 |
| B | Tip removed | 2, 0, 1, 3, 0 |
| C | Tip covered with foil | 4, 1, 2, 0, 3 |
Step 1, mean for A: 30 + 36 + 34 + 40 + 35 = 175, and 175 ÷ 5 = 35°.
Step 2, mean for B: 2 + 0 + 1 + 3 + 0 = 6, and 6 ÷ 5 = 1.2°.
Step 3, mean for C: 4 + 1 + 2 + 0 + 3 = 10, and 10 ÷ 5 = 2.0°.
Step 4, conclusion: the control bends by about 35°, but the seedlings without a tip, or with a covered tip, hardly bend. This suggests the tip detects the light and is needed for the bending.
Step 5, explanation: the tip makes auxin. With light from one side, auxin builds up on the shaded side, making those cells grow longer, so the shoot bends towards the light.
Limit: five seedlings per group is a small sample, so a teacher would want repeats before saying the result is reliable.
The mistake to watch for
A common conclusion is: “The light makes the shoot bend, so auxin is proven.” It goes beyond what the data show.
Mistaken answer: “Group A bends, so auxin made it bend.”
The table shows the tip matters. Auxin is the explanation from theory, not something the table measures.
The correction is to separate the evidence (“groups B and C barely bend compared with A”) from the explanation (“auxin from the tip is redistributed”). Use words such as “suggests” for what the data can support.
Check yourself
1. Why is group A called the control?
Show answer
It has the normal, untreated tip, so it shows what happens without the change being tested. The other groups are compared with it.
2. A fourth group, D, has bends of 28, 32, 30, 33 and 27. Calculate its mean.
Show answer
28 + 32 + 30 + 33 + 27 = 150, and 150 ÷ 5 = 30°.
3. Which side of the shoot has more auxin when light comes from the right, and what happens to the cells there?
Show answer
The left (shaded) side has more auxin. Those cells elongate more, so the shoot bends towards the right, towards the light.
Where this leads next
Try the whole set in the coordination and responses practice. The mistake log and retest queue helps you keep track of which step you missed.
A teacher in online one-to-one Biology tuition can also help you turn data into careful conclusion sentences.