A student investigated the effect of light intensity on the rate of water uptake by a leafy shoot using a potometer.
They measured the volume of water taken up by the shoot every minute for 8 minutes under two different conditions:
Their results are shown in the table below.
| Time (minutes) | Volume of water taken up (mm3\text{mm}^3mm3) - Lamp switched off | Volume of water taken up (mm3\text{mm}^3mm3) - Lamp switched on after 2 minutes |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 3 | 3 |
| 2 | 6 | 6 |
| 3 | 9 | 11 |
| 4 | 12 | 16 |
| 5 | 15 | 21 |
| 6 | 18 | 26 |
| 7 | 21 | 31 |
| 8 | 24 | 36 |
Give one reason why the volume of water taken up by the shoot was also measured when the lamp was switched off.
