What Controls Flowering

Paragraph 1: The timing of flowering and seed production is precisely tuned to a plant’s physiology and the rigors of its environment. In temperate climate plants lost flower early enough so that their seeds can mature before the deadly winds of autumn. Depending on how quickly the seed and fruit develop, flowering may occur in spring as it does in oaks; in summer as in lettuces; or even in autumn as in asters. 

 

 

1. Paragraph 1 suggests which of the following about the seeds of asters in temperate climate?

 

O They mature a short time after the plant blooms. 

 

O They begin to develop in autumn and mature in winter. 

 

O They were more plentiful than those of oaks or lettuces. 

 

O They are not designed to survive temperate climate. 

 

 

 

 

 

Paragraph 2: What environmental cues do plants use to determine the seasons? Most cues such as temperature or water availability are quite variable: autumn 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 autumn and winter. 

 

 

Paragraph 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, iris, clover and petunias) flower when the length of darkness is shorter than a species’ specific critical period. Long night plants (including asters, potatoes, soy beans, goldenrod and cockleburs) flower when the length of uninterrupted darkness is longer than the species’ specific critical period. Thus spinach is classified as a short night plant because it flowers only if the night is shorter than eleven hours (its critical 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 ten-hour nights. 

 

 

 

2. The author mentions “tomatoes, corn, snapdragons and roses” in order to

 

O name some of the most well-developed and commonly grown domestic plants 

 

O provide examples of plants whose flowering is not determined by length of day

 

O demonstrate that the neutral plants are more common than long-day or short-day plants 

 

O support the claim that some plants flower at night as well as during the day 

 

 

 

3. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

 

O Traditionally, long day and short day plants are better known as short night and long night plants because they actually flower during periods of continuous darkness. 

 

O The naming of long day and short day plants is traditional and depends on whether they flower during long days or long nights. 

 

O Whether plant is better described in terms of day length or night depends on whether it flowers during the day or during continuous darkness. 

 

O Despite their traditional names, the flowering of what are called long day and short day plants actually depends on the length of continuous darkness rather than on day length.

 

 

 

4. According to paragraph 3, why is spinach considered a short night plant?

 

O It will not flower if it experiences too many hours of uninterrupted darkness. 

 

O It flowers after certain number of short nights. 

 

O It flowers as soon as the nights become shorter than the days. 

 

O It flowers for only a short period of time usually at night. 

 

 

 

 

 

Paragraph 4: Plant scientists can induce flowering in the cocklebur by exposing leaves to long nights (longer than its 8.5 hour critical 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 molecule often called florigen (literally, flowering maker). Some researchers believe they are close to demonstrating a flower’s stimulating substance for a 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 control flowering. 

 

 

5. According to paragraph 4, what had experiments with cockleburs revealed about the flowering process?

 

O Flowering is triggered not by multiple chemicals but by a single hormone in the bud. 

 

O The entire plant must experience the crucial period of darkness in order to flower. 

 

O The signal to begin flowering can originate in the leaves.

 

O Flowering is not always related to length of darkness. 

 

 

 

6. According to paragraph 4, which of the following is true of the signal for the cocklebur to flower?

 

O Only certain leaves produced signal. 

 

O The signal must arise from the flower bud. 

 

O The signaling molecule has not been chemically identified.

 

O The length of uninterrupted darkness cockleburs need in order to signal varies a lot. 

 

 

Paragraph 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). 

 

 

 

7. According to paragraph 5, what is the function of plants’ phytochrome?

 

O Measuring time durations in the absence of environmental cues 

 

O Helping reset the plant’s biological clock through light detection

 

O Stimulating the growth for plants’ leaves 

 

O Determining the color of the plants’ leaves 

 

 

 

 

 

Paragraph 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 sixteen hours of darkness and eight hours of light. However, interrupting the middle of the dark period with just a minute or two of lights prevents flowering. Thus their flowering is controlled by the length of continuous darkness. It is evident that even brief exposure to sunlight or white light will reset their biological clocks. The color of the light used for the light exposure is also important. A nighttime flash of pure red light inhibits flowering, while flash of light at the far-red end of the spectrum has no effect on flowering, as if no light were detected. 

 

 

 

8. According to paragraph 6, which of the following types of light will not reset a cocklebur’s biological clock?

 

O White light 

 

O sunlight 

 

O pure red light 

 

O far-red light 

 

 

 

 

 

Paragraph 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 sixteen hours of darkness and eight hours of light. However, interrupting the middle of the dark period with just a minute or two of lights prevents flowering. ■Thus their flowering is controlled by the length of continuous darkness. ■It is evident that even brief exposure to sunlight or white light will reset their biological clocks. ■The color of the light used for the light exposure is also important. A nighttime flash of pure red light inhibits flowering, while flash of light at the far-red end of the spectrum has no effect on flowering, as if no light were detected.■ 

 

 

 

9. Look at the four squares [■] that indicate where the following sentence can be added to the passage.

 

But not all types of light will have this effect.

 

Where would the sentence best fit? 

 

 

 

 

 

10. Directions: 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 answer choices 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. 

 

 

 

Answer Choices

 

O 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. 

 

O 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.

 

O Plants measure darkness through an internal biological clock which is reset when the plant’s phytochrome detects certain types of light.

 

O 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. 

 

O Day neutral plants use the phytochrome system to detect environmental conditions other than light, which may affect the viability of their flowers and seeds. 

 

O 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. 

 

 

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