TOEFL Reading: ETS-TOEFL阅读机经 - 5WT98D35536F3732L$

Attempts have been made to explain what controls the flowering. A. Day neutral plants flower as soon as they have sufficiently developed but the flowering of short night and long night plants is dependent on the duration of continuous darkness. B. Scientists do not yet know what chemical or combination of chemicals is responsible for the signal to flower, but they do know that the signal travels from leaf to bud. C. Plants measure darkness through an internal biological clock which is reset when the plant's phytochrome detects certain types of light. D. Short night plants flower if any part of the plant is exposed to continuous darkness, but long night plants will not flower if even one leaf experiences a flash of light. E. Day neutral plants use the phytochrome system to detect environmental conditions other than light, which may affect the viability of their flowers and seeds. F. Once a flower has started to bloom, the color and duration of light it is exposed to is important, because some kind of light will prevent flowering while others will have no effect.