2. 外语
  3. WHAT CONTROLS FLOWERING (1) The timing of flowering and seed production is precisely tuned to a plant’s physiology and the r

WHAT CONTROLS FLOWERING (1) The timing of flowering and seed production is precisely tuned to a plant’s physiology and the r

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问题                                         WHAT CONTROLS FLOWERING
    (1) The timing of flowering and seed production is precisely tuned to a plant’s physiology and the rigors of its environment. In temperate climates, plants lost flowers early enough so that their seeds can mature before the deadly winds of fall. Depending on how quickly the seeds and food develop, flowering may occur in spring as it does in oaks; in summer as in lettuces; or even in fall as in asters.
    (2) What environmental cues do plants use to determine the seasons? Most cues such as temperature or water availability are quite variable: Fall can be warm; a late snow could fall in spring; also summer might be unusually cool and wet. So the only reliable cue is day length: longer days always mean that spring and summer are coming; shorter days foretell the onset of fall and winter.
    (3) With respect to flowering, botanists classify plants as day-neutral, long-day or short-day. A day-neutral plant flowers as soon as it has sufficiently grown and developed regardless of the length of day. The neutral plants include tomatoes, corn, snapdragons, and roses. Although the naming is traditional, long-day and short-day plants are better described as short-night and long-night plants because their flowering actually depends on the duration of continuous darkness rather than on day length. Short-night plants (which include lettuces, spinach, irises, clover and petunias) flower when the length of darkness is shorter than a species’ specific critical dark period. Long-night plants (including asters, potatoes, soybeans, goldenrod, and cockleburs) flower when the length of uninterrupted darkness is longer than the species’ specific critical dark period. Thus spinach is classified as a short-night plant because it flowers only if the night is shorter than 1I hours (its critical dark period), and the cocklebur is a long-night plant because it flowers only if an uninterrupted darkness lasts more than 8.5 hours. Both of these plants will flower with 10-hour nights.
    (4) Plant scientists can induce flowering in the cocklebur by exposing leaves to long nights (longer than its 8.5-hour critical dark period) in a special chamber, while the rest of the plant continues to experience short nights. Clearly, a signal that induces flowering transmitted from the leave to the flowering bud. Plant physiologists have been attempting for decades to isolate these elusive signaling molecules often called florigen (literally, flowering maker). Some researchers believe they are close to demonstrating a flower’s stimulating substance for specific type of plant. Using genetic manipulation, it is likely, however, that interactions among multiple and yet unidentified plant hormones stimulate or inhibit flowering, and that these chemicals may differ among plant species. Researchers have had more success in determining how plants measure the length of uninterrupted darkness, which is a crucial stimulus for producing whatever substance controls flowering.
    (5) To measure continuous darkness, a plant needs two things: some sort of metabolic clock to measure time (the duration of darkness) and a light detecting system to set the clock. Virtually all organisms have an internal biological clock that measures the time even without environmental cues. In most organisms including plants, the biological clock is poorly understood, but we know that the environmental cues, particularly light, can reset the clock. How do plants detect light? The light detecting system of plants is a pigment in leaves called phytochrome (literally, plant color).
    (6) Plants seem to use the phytochrome system in combination with their internal biological clocks to detect the duration of continuous darkness. Cockleburs, for example, flower under the schedule of 16 hours of darkness and 8 hours of light. However, interrupting the middle of the dark period with just a minute or two of light prevents flowering. [A] Thus their flowering is controlled by the length of continuous darkness. [B] It is evident that even brief exposure to sunlight or white light will reset their biological clocks. [C] The color of the light used for the light exposure is also important. A nighttime flash of pure red light inhibits flowering, while a flash of light at the far-red end of the spectrum has no effect on flowering, as if no light were detected. [D]
According to paragraph 3, why is spinach considered a short-night plant?
选项 A、It will not flower if it experiences too many hours of uninterrupted darkness.
B、It flowers after certain number of short nights.
C、It flowers as soon as the nights become shorter than the days.
D、It flowers for only a short period of time usually at night.
答案A
解析 本题询问为什么菠菜被认为是一种短夜植物，属于事实信息题。第3段倒数第2句提到，菠菜只有在黑夜的时间短于11小时(它的临界暗期)时才会开花，换言之，它暴露于黑暗的时长超过11小时则不开花，故A项“如果菠菜经历太长时间的持续黑暗，它就不会开花”是该句原文的同义转述。C项“当黑夜短于白昼时，菠菜就会开花”，假设黑夜为11.5小时，而白昼为12.5小时，属昼长夜短的情况，按C项的说法，菠菜会开花，但这就跟第3段倒数第2句提到的“菠菜只有在黑夜的时间短于11小时 (它的临界暗期)时才会开花”的说法相悖。B项“菠菜在经历若干个短夜之后会开花”和D项“菠菜通常在夜间短暂开花”均无依据。
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