Day 81
Passage 81
Photoperiodism in Plants
1
Flowering is crucially important for a plant to complete its life cycle. Although people have long known that plants such as tulips flower in the spring and chrysanthemums flower in the fall, it wasn't until the early 1900s that a little more became known about what actually caused flowering. Beginning in 1910, Wightman Garner and Henry Allard conducted experiments to test the effect of day length on flowering by growing plants in growth chambers, in which timers were used to control the length of light and dark periods. They discovered that plants such as barley flowered when the day's length was longer than a certain critical length. These plants, which they named long-day plants (LDPs), flowered mainly in the summer as the days were getting longer. Others, such as soybeans, flowered when the day length was shorter than a certain critical length. These short-day plants (SDPs) flowered in the fall as the days were getting shorter. Still others such as broad beans and buck wheat flowered almost at the same time regardless of day length and were called day-neutral plants. Based on these discoveries, they coined the term "photoperiodism" in 1920 to describe a plant's ability to flower in response to changes in the photoperiod, the relative lengths of day and night.
2
With respect to photoperiodism, all flowering plants have been placed in one of the three categories. However, scientists later discovered that, despite their names, it was the uninterrupted length of night rather than the length of day that was the most important factor in determining when and whether plants would bloom. For example, if SDPs were grown under short-day conditions but the dark period was interrupted by a flash of light, the SDPs would not flower. However, an interruption of the light period with dark had no effect. Thus, SDPs should more accurately be called long-night plants and LDPs short-night plants to emphasize the key role played by darkness in photoperiodism. [1] Most plants required several weeks of the appropriate long-night or short-night cycle before they would flower.
3
The late 1920s also saw the discovery of a biological clock in living organisms. It was shown that the movement of leaves on a bean plant, from horizontal at noon to vertical at midnight, continued uninterruptedly for several days, even when plants were placed in total darkness and at a constant temperature, and that the time between given points in the cycle, such as the most vertical leaf position, was almost but not exactly 24 hours—about 25.4 hours. In fact, many other cycles have now been found with similar characteristics in virtually all groups of plants and animals. There is strong evidence that the clocks are internal and not driven by the change in the environment.
4
Photoperiodism takes place in specialized pigment materials called phytochrome receptors. Phytochrome receptors respond most to the energy contained in red and far-infrared light waves. Two types of receptors, called P(red) and P(far-red), intercept these waves and coordinate plant metabolism processes accordingly. P(red) receptors absorb red light waves during the day, while P(far-red) receptors absorb far-red waves during dark periods. As each plant type requires only a certain amount of light wave, too much of one or not enough of the other prevents flowering from taking place. Not only can phytochrome receptors absorb available light energy, but they act as light sensors that integrate with a plant's biological clock to provide a means for a plant to adapt to its lighting conditions. It is suggested that under appropriate conditions, these interactions between phytochrome and the plant's biological clock are thought to activate the genes for flowering.
5
The photoperiodic requirements of plants depend on geographic origin and distribution: short-day plants grow in tropical and subtropical regions and long-day plants occur mainly in temperate and northern latitudes. This factor indicates the adaptability of the photoperiodic process not only to day length as an ecological factor but also to the entire complex of external factors. Photoperiodism is a unique clock that synchronizes the rhythm of ontogenesis with the seasonal rhythm. For example, short-day plants have adapted both to hot and dry summer in the subtropics and to periodic downpours; they do not flower or bear fruit during the longer days of these seasons.
——2011年01月15日北美考试机经
According to Paragraph 2, why does the author say "SDPs should more accurately be called long-night plants"?
A. Because most plants flower at night.
B. Because the flowing of plants is more affected by continued darkness.
C. Because plants only flower when the night is long enough.
D. Because plants mainly flower when the day is short.
核心词汇：
词汇练习：
阅读下列句子，用所给单词（或词组）的正确形式填空：
with respect to vertical temperate subtropical relative
regardless pigment internal horizontal geographic
constant adapt to accurately
1. These laws are universal in their application, _____ of cultural beliefs, geography, or climate. (OG: Applied Arts and Fine Arts)
2. Thus, city surfaces, as well as the air above them, tend to be drier between episodes of rain; with little water available for the cooling process of evaporation, _____ humidities are usually lower. (TPO-23: Urban Climates)
3. He found that under these circumstances, the birds oriented _____ the new "Sun". (TPO-11: Orientation and Navigation)
4. We now know the age of the glaciation _____ from radiometric dating of the carbon-14 in logs buried in the drift. (TPO-19: Discovering the Ice Ages)
5. As a method of spanning space, the arch can support greater weight than a _____ beam. (TPO-3: Architecture)
6. It works in compression to divert the weight above it out to the sides, where the weight is borne by the _____ elements on either side of the arch. (TPO-3: Architecture)
7. Maintenance for a given body design of an organism is relatively ____. (Online Test: Opportunists and Competitors)
8. In addition to the _____ variability of the global climate system itself, there is the added factor of external influences, such as volcanoes and solar activity. (TPO-10: Variations in the Climate)
9. It is possible that tubes made from animal bones were used for spraying because hollow bones, some stained with _____, have been found nearby. (Online Test: Lascaux Cave Paintings)
10. Animals also have to _____ desert conditions, and they may do it through two forms of behavioral adaptation: they either escape or retreat. (TPO-26: Survival of Plants and Animals in Desert Conditions)
11. Among the main factors are Teotihuacán's _____ location on a natural trade route to the south and east of the Valley of Mexico, the obsidian resources in the Teotihuacán Valley itself, and the valley's potential for extensive irrigation. (TPO-8: The Rise of Teotihuacán)
12. Many are found in tropical and _____ areas of the world, where accumulation of high concentrations of metals may afford some protection against plant-eating insects and microbial pathogens. (TPO-5: Minerals and Plants)
13. Glaciers are sometimes classified by temperature as faster-flowing ____ glaciers or as slower-flowing polar glaciers. (TPO-15: Glacier Formation)
参考答案：
1. regardless 2. relative 3. with respect to 4. accurately 5. horizontal
6. vertical 7. constant 8. internal 9. pigment 10. adapt to
11. geographic 12. subtropical 13. temperate
注释
[1]与今日百句译（Sentence 81）相同的句型。