| When L. gibba and L. polyrhiza are grown together in a tank, L. polyrhiza dies out. A student notices that L. gibba has air-filled sacs that seem to make it float higher in the water. |  |
The following is a multiple choice question. Read over the answers carefully and decide which is the best answer. Then for each of the answers, you must write ACCEPT next to the best answer and REJECT next to the other answers. In addition, write a short, logical argument that justifies your rejection or acceptance of the answer. 12 points: 2 points for best answer; 10 points for arguments.
Lemna is a tiny duckweed that floats on the surface of ponds.
| When L. gibba and L. polyrhiza are grown together in a tank, L. polyrhiza dies out. A student notices that L. gibba has air-filled sacs that seem to make it float higher in the water. | |
He interprets these observations in the following way: The air-filled sacs make L. gibba float higher in the water. As a result, L. gibba has a better chance to intercept light. This makes L. gibba a better competitor than L. polyrhiza and explains the exclusion of L. polyrhiza.
Which of the following new observations would add the most support to his hypothesis? [For each answer you reject, clearly state what the observation suggests about the most reasonable explanation for the interaction of these duckweeds.]
| a. L. polyrhiza does not survive in water from a tank where L. gibba is growing. |  |
| b. Both species survive in a tank that is illuminated from the bottom. |  |
| c. L. gibba dies out when both species are grown in a tank illuminated from the bottom. | |
| d. L. gibba dies out if the water in the tank is less than 15 oC. |  |
| e. A plant of L. polyrhiza is heavier than one of L. gibba. |  |