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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. A 3-meter stand of big bluestem is anchored underground by a mass of coarse, fibrous roots that reaches as much as 36 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.

OFFICIAL61 Look at the four squares[■]that indicate where the following sentence could be added to the passage. Where would the sentence best fit? Click on a square[■]to add the sentence to the passage.

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