SAT Subject Biology Practice Question 424: Answer and Explanation
14. The following experiment was performed to test the effect of an auxin on plant growth. The auxin was dissolved in a gelatin block; gelatin does not affect the biological activity of the auxin. Several plant seedlings were prepared as described below and growth was measured every five days.
In a separate experiment, an auxin/gelatin block applied to only half the cut edge of the tip caused the plant to grow and bend in the opposite direction. For example, if the auxin/gelatin block was applied to the left side of the cut edge, the plant grew and bent toward the right. Which of the following is the most likely explanation for this observation?
A. Sunlight caused the plant to bend.
B. The plant exhibited gravitropism.
C. Auxin stimulated cell division on the opposite side of the plant.
D. Auxin stimulated cell division on the same side of the plant.
E. Auxin stimulated cell growth toward a light source.
Correct Answer: D
D Because light is not mentioned in the question at all, we can eliminate choices that have light as the reason for growth, such as choices A and E. We can also eliminate choice B, because movement toward the earth is not described. For a plant to bend to the right, cells on the left side of the plant must grow faster than cells on the right. For a plant to bend to the left, the opposite must occur; cells must grow faster on the right than on the left. In the question, auxin applied to half the plant causes the plant to bend in the opposite direction; for example, auxin on the left stimulates bending to the right. Bending to the right means growth on the left; therefore, auxin must stimulate growth on the same side of the plant to which it is attached.