托福考试是申请美国留学的考生都要参加的考试。以下是考生分享的2013年1月12日首场托福阅读考试机经回忆的内容，即将参加托福考试的考生们可以借鉴参考一下试题，为下一次的新托福考试做好充足准备。希望对大家的美国留学托福阅读考试有所帮助。

考试时间：2013年1月12日

植物开花与日照的关系

【机经回忆】

什么因素影响花期

植物通过感受不同光照事件长短，决定播种，科学家通过对叶子遮光处理，分为三种：不受光影响，长日照和短日照，检测植物的信号。但是最后还是没有收获，但是日照长短对植物的影响是明显的

【重点词汇解析】

sunlight 日光

long-day 长日照

short day 短日照

plant 植物

seed 种子

signal 信号

obvious 明显的

【机经解析】

光照长短对植物生长的影响

Light duration or photoperiod refers to the amount of time that a plant is exposed to sunlight. When the concept of photoperiod was first recognized it was thought that the length of periods of light triggered flowering. The various categories of response were named according to the light length (i.e., short-day and long-day). It was then discovered that it is not the length of the light period but the length of uninterrupted dark periods that is critical to floral development. The ability of many plants to flower is controlled by photoperiod.

Plants can be classified into three categories, depending upon their flowering response to the duration of darkness. These are short-day, long-day, or day-neutral plants. Short-day, (long nights) plants form their flowers only when the day length is less than about 12 hours in duration. Short-day plants include many spring and fall flowering plants such as chrysanthemum and poinsettia. Long-day, (short nights) plants form flowers only when day lengths exceed 12 hours. They include almost all of the summer-flowering plants, as well as many vegetables including beet, radish, lettuce, spinach, and potato. Day-neutral plants form flowers regardless of day length. Some plants do not really fit into any category but may be responsive to combinations of day lengths. The petunia will flower regardless of day length, but flowers earlier and more profusely under long daylight. Since chrysanthemums flower under the short-day conditions of spring or fall the method for manipulating the plant into experiencing short days is very simple. If long days are predominant, a black plastic sheet is drawn over the chrysanthemum for 12 hours daily to block out light until flower buds are initiated. To bring a long-day plant into flower when sunlight is not present longer than 12 hours artificial light is added until flower buds are initiated.

Light Duration

Light duration refers to the amount of time that a plant is exposed to sunlight. Travelers to Alaska often marvel at the giant vegetables and flowers that grow under the long days of the arctic sun even with cool temperatures

When starting transplants indoors, generally give plants 12 to 14 hours of light per day. Plant are generally intolerant of continuous light for 24 hours

Photoperiod

The flowering response of many plants is controlled by the photoperiod (the length of uninterrupted darkness). Photoperiod response can be divided into three types.

Short day plants flower in response to long periods of night darkness. Examples include poinsettias, Christmas cactus, chrysanthemums, and single-crop strawberrie

Long day plants flower in response to short periods of night darkness. Examples include onions and spinach

Day neutral plants flower without regard to the length of the night, but typically flower earlier and more profusely under long daylight regimes. Day neutral strawberries provide summer long harvesting (except during heat extremes)

光照长短对花期的形成和长短的影响

Bougainvillea plants were exposed to artificially reduced light intensity to capture different Photosynthetic Photon Flux Density (PPFD), 0% (direct sunlight), 30, 50 and 75% shading and to observe the effects of shading on flower formation and longevity, leaf chlorophyll and sugar content and quality of plants. Plant attained the greatest leaf size with maximum branching when seedlings were grown in 30 and 50% shading, whereas 0% shading showed the lowest value. Growth parameters related to the plant characteristics such as branch initiation, branch growth and potassium content increased under high shading treatments. Plants exposed to 0% shade showed the highest sugar content and the sugar content decreased by increasing shading. The low light intensity that results in decreased sugar and chlorophyll contents may be attributed to the reduction of flower initiation and in turn to the acceleration of flower abscission. In addition, more than 30% shading led to stop up flower initiation completely. Hence, it is suggested that 0% shading can be applied to maintain frequent flower initiation and flower longevity.

以上就是2013年1月12日首场托福阅读考试机经回忆的全部内容，各位考生可以以此作为参考，做好复习。最后预祝各位考生取得理想的成绩!

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