Some students investigate the effect of a plant growth regulator, gibberellin, on the elongation of wheat seedling shoots. One petri dish contains distilled water, while another contains a test solution of gibberellin. After 4 days, the students measure the shoot lengths of the seedlings. The mean shoot lengths recorded are shown in the table below:
| Treatment | Mean shoot length (mm) |
|---|---|
| Test solution (gibberellin) | 24.6 |
| Water control | 18.8 |
Explain the purpose of using distilled water instead of the gibberellin solution.
Calculate the percentage change in mean shoot length in the test solution compared with the water control. Give your answer to 2 significant figures.
