Day Length and Flowering Plants

Paragraph 1：Most trees change with the seasons. Deciduous trees lose their leaves as winter approaches (or, in the seasonal tropics, as the dry season approaches) and enter a state of dormancy. This is not a simple shutting down. Dormancy takes weeks of preparation. Before trees shed their leaves, they withdraw much of the nutrients within them, including the protein of chlorophyll, leaving some of the other pigments behind to provide at least some of the glorious autumn colors; and to conserve water, they stop up the vessel ends that service the leaves. 

 

 

 

1. According to paragraph 1, all of the following are ways in which trees prepare to enter a state of dormancy as winter approaches EXCEPT: 

 

A. They remove nutrients from their leaves. 

 

B. They remove chlorophyll from their leaves. 

 

C. They add autumn-colored pigments to their leaves.

 

D. They stop up the ends of the vessels that carry water to their leaves. 

 

 

 

 

 

Paragraph 2：How do trees sense seasonal change? There are many clues to season, including temperature and rainfall. But shifts in temperature and rain are capricious: they are not reliable signals. Sometimes a winter may be warm — but frost is never far away. Some autumns and springs are freezing, some balmy. The one invariable, at any particular latitude on any particular date, is the length of the day. So at least in high latitudes, where day length varies enormously from season to season, plants in general take this as their primary guide to action — while allowing themselves to be fine-tuned by other cues, including temperature. So temperate trees invariably produce their leaves and/or flowers in the spring, following the rigid pattern of solar astronomy, but they adjust their exact date of blossoming to the local weather. This phenomenon — judging time of year by length of day — is called photoperiodism. Most of the basic research on photoperiodism has been done on crop plants. But trees and crop plants work in the same way. 

 

 

 

2. Why does the author mention that “Some autumns and springs are freezing, some balmy”? 

 

A. To argue that shifts in temperature and rainfall can benefit trees depending on the season 

 

B. To argue that temperate trees vary the times of the year when they product their leaves and flowers depending on the weather 

 

C. To help explain why trees cannot depend on temperature and rainfall as primary indicators of the seasons

 

D. To show how much seasonal weather may vary at different  latitudes 

 

 

3. According to paragraph 2, each of the following is true of the phenomenon called photoperiodism EXCEPT: 

 

A. It applies equally to trees and crop plants. 

 

B. It explains why temperate trees always blossom in the spring. 

 

C. It alone determines the timing of blossoming for trees and crop plants regardless of weather conditions.

 

D. It has been more heavily researched in crop plants than in trees. 

 

 

 

 

 

Paragraph 3：Knowledge of photoperiodism dates from the 1920s, when agricultural scientists in America found that plants like tobacco, soybeans, spinach, and some wheat and potatoes would not flower if the days were shorter than a certain critical number of hours (often around twelve). However, other plants would not flower if the days were too long: strawberries and chrysanthemums were among those that remained resolutely sterile if the days were longer than sixteen hours. There were some, though, that didn’t seem to mind the length of day. The three groups became known as long-day, short-day, and day-neutral. Lone-day plants generally flower in high summer, and short-day plants in spring or autumn. 

 

 

 

4. What can be inferred from paragraph 3 about day-neutral plants? 

 

A. They will flower only if days are between 12 and 16 hours in length. 

 

B. They will flower only if days are shorter than 12 hours or longer than 16 hours. 

 

C. Their flowering depends on factors other than length of day.

 

D. Their flowering is not affected by weather conditions. 

 

 

 

 

 

Paragraph 4：As a further refinement, plants also seem aware that absolute day length has different significance at different latitudes. At very high latitudes, the longest days are 24 hours — the Sun never sets — and a fourteen-hour day is of modest duration. But in the subtropics, fourteen hours is a long day — as long as any day gets. Sometimes the same species may grow both at high and at low latitudes, including, for example, the aspens of North America. Then the northern ones will treat a fourteen-hour day as short, and the more equatorial ones will treat a fourteen-hour day as long. 

 

 

 

5. Which of the following can be inferred from paragraph 4 about the aspens of North America? 

 

A. They grow best in the subtropics. 

 

B. They are better at measuring absolute day length than many other types of trees are. 

 

C. They are more numerous in high latitudes than in low latitudes. 

 

D. They can tolerate fourteen or more hours of sunlight a day.

 

 

 

 

 

Paragraph 5：In the late 1930s it became clear that plants do not measure the length of the day but of the night. If the light is turned on even briefly during the night — a minute from a 25-watt bulb would do — short-day plants such as strawberry will not flower. In contrast, a long-day plant that flowers in sixteen hours of light and eight hours of dark will also flower with eight hours of light and sixteen hours of dark – if the darkness is interrupted by a brief light. In truth, long-day plants should be called short-night plants, and short-day plants are really long-night plants. 

 

 

 

6. Why does the author mention “a minute from a 25-watt bulb would do”? 

 

A. To illustrate the flowering plants’ sensitivity to light

 

B. To highlight the flowering plants’ need for light 

 

C. To provide a method used to encourage long-day plants to flower 

 

D. To support the idea that plants sometimes require an artificial light source in order to flower 

 

 

 

7. According to paragraph 5, why should long-day plants be called short-night plants and short-day plants be called long-night plants? 

 

A. Because for a day to be long the night must be short, and for a day to be short the night must be long 

 

B. Because flowering generally occurs at night rather than during the day 

 

C. Because plants generally will not flower if a brief period of light interrupts the darkness 

 

D. Because plants determine the time of flowering by measuring the length of the night rather than the length of the day

 

 

 

 

 

Paragraph 6：In the next few years the underlying mechanism became clear, and it is remarkably simple. Inevitably it depends on a pigment — for pigments by definition are chemical agents that absorb and reflect light, and so mediate a plant’s (or an animal’s) responses to it. In this case the pigment is phytochrome. Phytochrome exists in two forms, to either suppress or promote flowering, and light changes them from one form to the other. Again, these insights have been put to commercial use. Growers of chrysanthemums used to keep the lights in the greenhouse on at night to delay flowering until early winter — until, in the 1930s, they saw that a brief burst of light at night would produce the same effect, and much more cheaply. 

 

 

 

8. According to paragraph 6, a brief burst of light in the greenhouse at night has which of the following effects on chrysanthemums? 

 

A. It stops them from flowering.

 

B. It weakens their phytochrome. 

 

C. It makes them flower no matter what the season is. 

 

D. It makes them behave as if the night were longer than it actually is. 

 

 

 

 

 

Paragraph 3：Knowledge of photoperiodism dates from the 1920s, when agricultural scientists in America found that plants like tobacco, soybeans, spinach, and some wheat and potatoes would not flower if the days were shorter than a certain critical number of hours (often around twelve3). However, other plants would not flower if the days were too long: strawberries and chrysanthemums were among those that remained resolutely sterile if the days were longer than sixteen hours. ■There were some, though, that didn’t seem to mind the length of day. ■The three groups became known as long-day, short-day, and day-neutral. ■Lone-day plants generally flower in high summer, and short-day plants in spring or autumn.■ 

 

 

 

9. Look at the four squares ■ that indicate where the following sentence could be added to the passage. Where would the sentence best fit? 

 

On the other hand, the flowering of a day-neutral plant is not limited to a single season or even two: day-neutral varieties of roses, for example, can flower at any time from early spring to late autumn.

 

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 selected THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. 

 

Plants depend on various seasonal cues to prepare for dormancy and to begin flowering.

 

 

 

Answer Choices: 

 

A. While temperature and rainfall are not the main seasonal cues controlling when plants will flower, they play a major role in determining when plants begin preparations to enter dormancy. 

 

B. Length of day is the primary cue to season for many plants because it is the least variable and most reliable indicator of the time of year, while weather plays a secondary role. 

 

C. Plants are commonly grouped according to different day-length requirements for flowering — some require a long day, some require a short day, and others are indifferent to day length. 

 

D. Long-day plants are found at high latitudes because they require the most daylight to flower, whereas short-day plants appear at low latitudes because they require the least daylight to flower. 

 

E. The fact that a brief burst of light will stop short-day plants from flowering helps show that plants actually measure the length of the night rather than the length of the day. 

 

F. Short-day plants have one form of phytochrome while long-day plants have another, allowing short-day plants to flower earlier and long-day plants to flower later. 

 

 

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