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]
An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Attempts have been made to explain what controls the flowering.
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Answer Choices
(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.
选项
答案A,C,F
解析 文章总结题。A项“日中性植物完全成熟后就会开花，但是短夜植物和长夜植物开花取决于持续黑暗的时长”是对第3段第2句及第4句的整合。C项“植物通过内置的生物钟检测黑暗，当光敏色素探测到某种类型的光时，内置的生物钟会重置”是对第5段第2句Virtually all organisms have an internal biological clock that measures the time，以及第3句的particularly light, can reset the clock的整合。F项 “一旦开花植物开始开花，其接触到的光的颜色和时长就很重要，冈为有些光会阻碍开花，有些则无此影响”概括了第6段讲述的开花受制因素“光照暴露”(第2—5句)与“光色”(第6、7句)这两个要点。本文探讨控制植物开花的因素：第3段开始进入正题(前两段作铺垫)，该段主要讲述“植物根据其开花受制因素如何分类”和“开花受持续黑暗时长影响的植物”这两个要点；第4段是对上一段的拓展，补充了，关于诱导开花信号的作用过程的信息；第5段具体解释光照如何影响植物开花；第6段补充光照在什么条件(包括时长和颜色)下可以影响开花。A、C、F三项涵盖了本文主体段落的信息要点，B项“科学家不知道哪种化学物质或化学物质组合会诱发开花信号，但是科学家知道该信号从叶子传到了花蕾”出自第4段提及的“多种尚未确认的植物激素之间的相互作用可能刺激或抑制开花”和 “促使开花的信号从叶子传到了花蕾”，但它并非文中重要信息。第3段提到了短夜植物开花的前提是黑暗时长短于物种特定的临界暗期。长夜植物开花的前提是黑暗时长长于物种特定的临界暗期，D项“短夜植物开花的前提是植物的任何一个部位都接触到不间断的黑暗，而长夜植物只要一片叶子找到一束光就不会开花”没有依据。第6段首句提到“植物似乎将光敏色素系统和内部的生物钟相结合来检测黑暗持续的时间”，E项“日中植物利用光敏色素系统来探测环境状况，而不是探测会影响植物的花和种子生存的光”颠倒原文信息。
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