A technician investigates phototropism using two trays containing equal numbers of similar oat seedlings on damp cotton wool. One tray is placed in a lightproof box with an opening facing a lamp. The other tray is placed in a box with light entering evenly from above. All other conditions are kept the same.
After 48 hours, the technician measures the curvature of every shoot. The technician then excludes every result that does not match the expected response before comparing the trays. The technician concludes that one-sided light causes oat shoots to curve towards the opening.
Which change would provide more valid evidence for this conclusion?
discard the smallest curvature measurement from each tray before comparing the results
repeat measurements on the same selected seedling in each tray
use only the tallest seedlings from each tray when comparing curvature
identify damaged seedlings before starting and use all other curvature measurements
105 exam-style questions on Edexcel GCSE Biology 6.3 Plant hormones, covering 6.3.1 Plant hormones and tropisms and 6.3.2 Commercial uses of plant hormones. Each one has a worked solution and a mark scheme showing where the marks go.