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.)