Passage I

Scientists have observed that if mature cocklebur plants (Xanthium strumarium) are maintained on a 16L, 8D light-dark cycle (a light-dark cycle with 16 hr of light, L, then 8 hr of darkness, D), they do not flower. However, if the plants are maintained on a 14L, 10D light-dark cycle, they do flower. Three hypotheses were proposed to explain these observations.

Hypothesis 1

Mature cocklebur plants flower if, and only if, their leaves are exposed to a total of 10 or more hours of darkness each day.

Hypothesis 2

Mature cocklebur plants flower if, and only if, their leaves are exposed to a block of 10 or more continuous hours of darkness each day. If daily exposure to darkness exceeds a total of 10 hr, but is not continuous for 10 or more hours, the plants do not flower.

Hypothesis 3

Mature cocklebur plants flower if, and only if, their stems are exposed to a total of 9 or more hours of darkness each day. Leaves are not necessary for flowering. A mature cocklebur plant without leaves will flower if its stems are exposed to a total of 9 or more hours of darkness each day.

Experiments

Figure 1 shows the results of flowering experiments performed with mature cocklebur plants maintained on various 24 hr light-dark cycles. Figure 2 shows the results of similar experiments with mature cocklebur plants having various levels of defoliation (leaf removal). In each experiment involving plants with no defoliation or with partial defoliation, during each light-dark cycle, the amount of light received by the stems was equal to the amount of light received by the leaves. (Note: In the figures, a white box indicates hours of exposure to light and a black box indicates hours of exposure to darkness.)

6. Both Hypothesis 2 and Hypothesis 3 would have predicted that flowering would occur in which of the following experiments?

Answer and Explanation

Your Answer is

Correct Answer is G

Explanation

Because hypothesis 3 is specifically aimed at the stem stems of plants, see experiments 12 & 14 in figure 2. These two experiments are complete defoliations, both of which are stems in the dark, and have nothing to do with leaves, that is, with Hypothesis 2 is irrelevant and should be excluded;

The dark duration of experiment 1 in Figure 1 is less than 9 hours, so hypotheses 2 & 3 will predict no flowering, so it is also excluded. Experiment 10 should be selected