Scientists investigated the effect of gravity on the distribution of the plant hormone auxin in seedling shoot tips. They grew seedlings in uniform conditions, cut off the shoot tips, and placed them in different orientations on agar blocks as shown in the diagram below.
State two control variables that scientists must keep constant in this investigation to ensure valid results.
A scientist proposed the following hypothesis: "Gravity causes auxin to migrate laterally from the upper side of a horizontally oriented shoot tip to the lower side." Explain how the evidence from Setup Y and Setup Z supports this hypothesis.
Another scientist hypothesized: "Gravity destroys or breaks down auxin on the upper side of a horizontally oriented shoot tip." Explain how the data from the experiment show that this hypothesis is incorrect.