『新东方在线托福听力』为托福考生准备了ETS托福TPO/Official 61 Passage2阅读练习，TPO/Official 61 Passage2原文答案解析，TPO/Official 61 Passage2阅读原文练习音频翻译。

托福TPO/Official 61 Passage2阅读原文题目答案及解析-新东方托福官方网校手机版

北美大平原干旱草原上自然生长着大约140种草类。

Around 140 species of grasses naturally occur in the arid Great Plains grasslands of North America.

草原草类成功的关键之一是它们在干旱环境中保存水的能力。

One key to the prairie grasses' success is their ability to conserve water in a dry environment.

像大多数植物一样，草类通过根部吸收水分，并通过叶片上微小的嘴形阀门（气孔）以水蒸气的形式失去水分。

Like most plants, grasses take in water through their roots and lose it as water vapor through tiny mouth-shaped valves, or stomata, in their leaves.

叶片表面越大，气孔越多，植物通过蒸发失去过多水分并枯萎的风险就越大。

The larger the surface of the leaf and the more stomata it bears, the greater the risk that the plant will lose too much moisture through evaporation and collapse.

草类通过减少气孔数量和叶片设计（呈狭窄叶片状）来避免这种伤害。

Grasses are protected from this trauma by having a reduced number of stomata and by the design of their leaves, which take the form of narrow blades.

更重要的是，这些缩小叶片的表面通常被改良——带有褶边或覆盖着毛状物——这样风无法吹过表面带走水分。

What's more, the surfaces of these reduced leaves are often modified—corrugated with ridges or covered in hairs—so that the wind can't sweep across the surface and draw out moisture.

粗糙的表面在叶片旁保留了一层薄薄的湿空气，从而有助于减少大气的“蒸发需求”或干燥能力。

The roughened surface holds a thin layer of humid air next to the leaf and thus helps to reduce the "evaporative demand," or drying power, of the atmosphere.

一些草类，包括西部麦草、六月草和蓝禾草，在干旱时期会卷起叶片边缘，以防止组织干燥。

Some grasses, including western wheatgrass, June grass, and blue grama, roll up the edges of their leaves during times of drought to help keep their tissues from drying out.

为什么不将气孔紧紧关闭以封住叶片内的水分？

Why aren't the stomata kept tightly closed to seal moisture inside the leaf?

原因是气孔还为植物的光合作用提供新鲜空气。

The reason is that the stomata also supply plants with fresh air for photosynthesis.

如果植物封住气孔，由于缺乏二氧化碳，这一维持生命的过程将停止。

If plants sealed their stomata, this life-sustaining process would come to a halt for lack of carbon dioxide.

但如果气孔完全打开，植物会因通过敞开的气孔失去水分而面临死亡风险。

But if the stomata are thrown wide open, the plants risk death due to the loss of moisture through their gaping valves.

草原草类通过战略性时间安排解决这一困境。

Prairie grasses resolve this dilemma by strategic scheduling.

在正午烈日下，气孔关闭，以保留水蒸气并阻止二氧化碳进入。

In the fierce blaze of the midday sun, the stomata close so that water vapor is held in and carbon dioxide is kept out.

在这种状态下，叶片可以捕获太阳能并将其储存在富含能量的分子中（这一过程需要阳光但不需要二氧化碳）。

In this state, the leaf can capture solar energy and store it in energy-rich molecules (a process that requires sunlight but not carbon dioxide).

然后，在凉爽的夜晚，当蒸发需求下降时，气孔迅速打开，让水蒸气缓慢流出，同时允许二氧化碳涌入叶片。

Then, in the cool of the evening, when the evaporative demand drops off, the stomata snap open, letting water vapor trickle out but also permitting carbon dioxide to flood into the leaf.

通过调动白天储存的能量，叶片利用二氧化碳制造生长所需的糖类和其他分子（这一过程可在完全黑暗中完成）。

By mobilizing the energy that was stockpiled earlier in the day, the leaf uses this carbon dioxide to manufacture the sugars and other molecules that it needs for growth (a process that can be accomplished in total darkness).

结果是草原草类部分为夜间活动；它们大部分生长发生在夜间或清晨。

The result is that prairie grasses are partially nocturnal; they do most of their growing at night or in the early hours of the morning.

草原草类还有另一种巧妙的方式来躲避太阳的需求。

Prairie grasses also have another ingenious way of evading the demands of the sun.

像许多草原生物（草原犬鼠、地松鼠、棉尾兔、獾等）一样，它们在地底下避难。

Like many grassland creatures (prairie dogs, ground squirrels, cottontails, badgers, and so on), they take refuge underground.

我们所认为的“草”——地上的叶片和茎——实际上只占植物体不到一半。

What we think of as "grass"—the aboveground leaves and stems—actually constitutes less than half of the organism.

按重量计，植物体的60%到80%通常生长在地下。

Between 60 and 80 percent of the plant, by weight, typically grows below ground.

一株3米高的大蓝茎草通过粗大、纤维状的根系固定在地下，根系可深入地下达3.6米。

A 3-meter stand of big bluestem is anchored underground by a mass of coarse, fibrous roots that reaches as much as 3.6 meters into the earth.

蓝禾草则很少将种子头抬高到地面以上，但其细小、分枝的根系网络有时可深入土壤近两米寻找水源。

Blue grama, for its part, seldom lifts its seed heads very far above the ground, but its network of fine, branching roots can sometimes probe the soil for water almost two meters down.

这些广泛的根系渴求地穿透土壤，意图吸取每一滴可用的水。

These extensive systems of roots push thirstily through the soil, intent on sucking up every available drop of water.

但如果土壤非常干燥，如干旱期间，根系无法吸取足够的水分来弥补气孔的损失。

But if the soil is very dry, as it is during periods of drought, the roots can't draw in enough moisture to keep pace with losses from the stomata.

草类通过将其最宝贵的资源（包括糖类和蛋白质）从叶片转移到根部，特别是根茎（地下茎）来应对。

Grasses respond by transferring their most valuable resources (including sugars and proteins) from their leaves into their roots and, especially, into their rhizomes (underground stems).

地上部分枯萎、干枯，对外界而言仿佛死去，植物在地下节俭地生存，依靠储存的物资等待天气好转。

Dead to the world above ground—withered and crisp—the plants live frugally below the surface, drawing on their cached supplies and biding their time until the weather improves.

当雨水最终回归（这是不可避免的），草类迅速活跃起来，发出新的根茎，进而长出新的叶片和根系，形成一个蓬勃生长的嫩芽网络。

When the rains eventually return, as they inevitably do, the grasses explode into action, sending out fresh rhizomes, which in turn put out fresh leaves and roots, to produce a burgeoning network of tender growth.

惊人韧性的蓝禾草可以从休眠中复苏，变绿并在仅五毫米的降雨量下继续生长。

The amazingly resilient blue grama can revive from dormancy, become green, and grow on as little as five millimeters of rainfall.

In paragraph 1, why does the author mention ridges and hairs?

What is the dilemma mentioned in the opening sentence of paragraph3?

In paragraph 3, why does the author include the phrase "can be accomplished in total darkness"?

According to paragraph 3, prairie grasses do each of the following during the night EXCEPT

According to paragraph 4, why do prairie dogs, ground squirrels, and other grassland creatures burrow underground?

Paragraph 5 suggests that when there is plenty of water available in the soil, grasses

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.

The word “resilient” in the passage is closest in meaning to

Look at the four squares  that indicate where the following sentence could be added to the passage. Leaf design, then, is valuable in moisture retention, but it is not the only effective strategy.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

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 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.

Grasses of the Prairie Around 140 species of grasses naturally occur in the arid Great Plains grasslands of North America. One key to the prairie grasses' success is their ability to conserve water in a dry environment. Like most plants, grasses take in water through their roots and lose it as water vapor through tiny mouth-shaped valves, or stomata, in their leaves. The larger the surface of the leaf and the more stomata it bears, the greater the risk that the plant will lose too much moisture through evaporation and collapse. Grasses are protected from this trauma by having a reduced number of stomata and by the design of their leaves, which take the form of narrow blades. What's more, the surfaces of these reduced leaves are often modified-corrugated with ridges or covered in hairs-so that the wind can't sweep across the surface and draw out moisture. The roughened surface holds a thin layer of humid air next to the leaf and thus helps to reduce the "evaporative demand," or drying power, of the atmosphere. Some grasses, including western wheatgrass, June grass, and blue grama, roll up the edges of their leaves during times of drought to help keep their tissues from drying out. Why aren't the stomata kept tightly closed to seal moisture inside the leaf? The reason is that the stomata also supply plants with fresh air for photosynthesis. If plants sealed their stomata, this life-sustaining process would come to a halt for lack of carbon dioxide. But if the stomata are thrown wide open, the plants risk death due to the loss of moisture through their gaping valves. Prairie grasses resolve this dilemma by strategic scheduling. In the fierce blaze of the midday sun, the stomata close so that water vapor is held in and carbon dioxide is kept out. In this state, the leaf can capture solar energy and store it in energy-rich molecules (a process that requires sunlight but not carbon dioxide). Then, in the cool of the evening, when the evaporative demand drops off, the stomata snap open, letting water vapor trickle out but also permitting carbon dioxide to flood into the leaf. By mobilizing the energy that was stockpiled earlier in the day, the leaf uses this carbon dioxide to manufacture the sugars and other molecules that it needs for growth (a process that can be accomplished in total darkness). The result is that prairie grasses are partially nocturnal; they do most of their growing at night or in the early hours of the morning. Prairie grasses also have another ingenious way of evading the demands of the sun. Like many grassland creatures (prairie dogs, ground squirrels, cottontails, badgers, and so on), they take refuge underground. What we think of as "grass"-the aboveground leaves and stems-actually constitutes less than half of the organism. Between 60 and 80 percent of the plant, by weight, typically grows below ground. A3-meter stand of big bluestem is anchored underground by a mass of coarse, fibrous roots that reaches as much as 3.6 meters into the earth. Blue grama, for its part, seldom lifts its seed heads very far above the ground, but its network of fine, branching roots can sometimes probe the soil for water almost two meters down. These extensive systems of roots push thirstily through the soil, intent on sucking up every available drop of water. But if the soil is very dry, as it is during periods of drought, the roots can't draw in enough moisture to keep pace with losses from the stomata. Grasses respond by transferring their most valuable resources (including sugars and proteins) from their leaves into their roots and, especially, into their rhizomes (underground stems). Dead to the world above ground-withered and crisp-the plants live frugally below the surface, drawing on their cached supplies and biding their time until the weather improves. When the rains eventually return, as they inevitably do, the grasses explode into action, sending out fresh rhizomes, which in turn put out fresh leaves and roots, to produce a burgeoning network of tender growth. The amazingly resilient blue grama can revive from dormancy, become green, and grow on as little as five millimeters of rainfall.