A) To investigate the effect of gravity on the growth of the stem, one needs to compare a setup where gravity's directional effect is present with one where it is nullified.

5.1

a) To investigate the effect of gravity on the growth of the stem, one needs to compare a setup where gravity's directional effect is present with one where it is nullified.

• In Diagram B, the plant is horizontal and stationary, allowing gravity to continuously act on the lower side of the stem, leading to a gravitropic response.
• In Diagram A, the plant is horizontal but rotating on a clinostat. This rotation ensures that the gravitational stimulus is evenly distributed around the stem, effectively cancelling out any directional gravitropic response. By comparing the growth in Diagram A and Diagram B, the effect of gravity can be determined. The two diagrams are $\boxed{\text{A and B}}$.

b) To investigate the effect of auxins on the growth of the stem, one needs to compare a setup where auxins are present and functional with one where they are absent or their production is inhibited.

• In Diagram B, the plant is horizontal and stationary with its tip intact. The tip is the primary site of auxin production, so auxins are present and can mediate a gravitropic response.
• In Diagram C, the plant is also horizontal and stationary, but its tip has been cut. Removing the tip eliminates the primary source of auxins. By comparing the growth in Diagram B (auxins present) and Diagram C (auxins largely absent), the role of auxins in stem growth can be investigated. The two diagrams are $\boxed{\text{B and C}}$.