Plant physiologists investigated the lateral transport of auxin in response to unilateral light. Coleoptile tips were harvested from genetically identical oat seedlings grown under uniform conditions. These tips were then placed onto agar blocks as shown in the experimental diagrams below.
State two control variables that the physiologists must keep constant in this experiment to ensure valid results.
A physiologist proposed the following hypothesis: "One-sided light causes auxin to migrate laterally from the illuminated side of the coleoptile tip to the shaded side."
Explain how the results from Setup Y and Setup Z support this hypothesis.
Another researcher proposed that "one-sided light inactivates or breaks down auxin on the illuminated side of the shoot."
Explain how the quantitative data from this experiment show that this alternative hypothesis is incorrect.