Question

To test the effect of an auxin on plant growth, the auxin was dissolved in a gelatin block. Several plant seedlings were prepared and growth was measured every five days.

Plant	Growth after 5 days	Growth after 10 days
1	15 mm	29 mm
2	4 mm	9 mm
3	5 mm	10 mm
4	14 mm	31 mm
5	8 mm	20 mm

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 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

Answer 1

The correct option is D Auxin stimulated cell division on the same side of the plant.

If the light is coming from right the plant will bend to the right and cells on the left side of the plant must grow faster than cells on the right. If the light is coming from left then the plant will bend to the left and the opposite must occur; cells must grow faster on the right than on the left. 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.