A student used a potometer to investigate how temperature affects the rate of transpiration in a leafy shoot. A heater was used to increase the temperature.
The table shows the results.
| Time (minutes) | Volume of water absorbed (cm3\text{cm}^3cm3) - Heater switched on after 3 minutes | Volume of water absorbed (cm3\text{cm}^3cm3) - Heater not switched on |
|---|---|---|
| 0 | 0.0 | 0.0 |
| 1 | 0.2 | 0.2 |
| 2 | 0.4 | 0.4 |
| 3 | 0.6 | 0.6 |
| 4 | 0.9 | 0.8 |
| 5 | 1.3 | 1.0 |
| 6 | 1.6 | 1.2 |
| 7 | 2.0 | 1.4 |
| 8 | 2.4 | 1.6 |
| 9 | 2.7 | 1.8 |
| 10 | 3.1 | 2.0 |
Give one reason why the volume of water absorbed by the shoot was also measured when the heater was not switched on.
