A student investigated the effect of different concentrations of gibberellic acid (a plant hormone) on the stem elongation of dwarf pea plants (Pisum sativum).
To find the optimal concentration for promoting growth, she grew six groups of dwarf pea plants in identical pots containing the same compost. Each group contained 15 plants.
She measured and recorded the initial mean stem height of the plants in each group.
She then prepared five different concentrations of gibberellic acid solution, leaving one group sprayed with distilled water as a control. She sprayed each group with their respective treatment.
After two weeks of growth in a greenhouse, she recorded the final mean stem height of the plants in each group.
The results are shown in the table below:
| Concentration of gibberellic acid (ppm) | Mean stem height (cm) | |
|---|---|---|
| Before applying treatment | Two weeks after applying treatment | |
| 0 | 4.2 | 5.1 |
| 10 | 4.1 | 6.8 |
| 50 | 4.3 | 9.5 |
| 100 | 4.0 | 12.3 |
| 200 | 4.2 | 15.1 |
| 500 | 4.1 | 18.2 |
Give one variable, other than the type of compost or pot, that the student should control in this investigation.
