A student investigated the effect of auxin (IAA) concentration on the elongation of wheat coleoptile segments. They set up five petri dishes, each containing a different concentration of IAA (10−610^{-6}10−6, 10−510^{-5}10−5, 10−410^{-4}10−4, 10−310^{-3}10−3, and 10−2 mol dm−310^{-2}\text{ mol dm}^{-3}10−2 mol dm−3). The student placed five 10 mm wheat coleoptile segments into each dish. After 24 hours, they measured the final length of each segment to calculate the percentage change in length.
Explain one way the student could improve this investigation to obtain more reliable results.
