本篇文章共分 4 段，針對草原中草類的生存方式進行分析，內容包括其葉片的外型與氣孔結構與運作方式，以及這些草類發達的根系與極度乾旱時的生存機制。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Grasses of the Prairie 草原上的草

1. 草原草類的葉片結構

   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, roil up the edges of their leaves during times of drought to help keep their tissues from drying out.

   大約有 140 種草自然存在於北美乾旱的大平原草原中。草原中草成功的一個關鍵是它們在乾燥的環境中保存水分的能力。像大多數植物一樣，草通過它們的根部吸收水分，並通過它們的葉子上的小口形閥門（或氣孔）失去水分，成為水蒸氣。葉子的表面越大，氣孔越多，植物通過蒸發失去太多的水分和崩潰的風險就越大。草類通過減少氣孔的數量和設計它們的葉子來保護它們免受這種創傷，它們的葉子因而呈狹窄的葉片狀。更重要的是，這些縮小的葉子的表面經常被修正—比方有波紋的脊或覆蓋著毛—這樣風就不能掃過表面並抽出水分。粗糙的表面在葉子旁邊保留了一層薄薄的潮濕空氣，因此有助於減少大氣的「蒸發需求」，或乾燥能力。一些草，包括西部小麥草、六月草和藍草，在乾旱時期會把它們的葉子邊緣捲起來，以幫助保持它們的組織不乾枯。

2. 氣孔的調節方式

   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.

   為什麼氣孔不緊緊關閉，把水分封在葉子裡面？原因是氣孔也為植物提供光合作用的新鮮空氣。如果植物封閉氣孔，這一維持生命的過程就會因缺乏二氧化碳而停止。但如果氣孔大開，植物就有可能因水分從氣孔中流失而死亡。草原的草通過戰略性的安排來解決這一難題。在正午激烈的陽光下，氣孔關閉，使水蒸氣被擋在裡面，二氧化碳被擋在外面。在這種狀態下，葉片可以捕獲太陽能並將其儲存在富含能量的分子中（這一過程需要陽光，但不需要二氧化碳）。然後，在涼爽的夜晚，當蒸發需求下降時，氣孔突然打開，讓水蒸氣流出來，但也允許二氧化碳湧入葉片。通過調動白天早些時候儲存的能量，葉子利用這些二氧化碳製造它生長所需的糖和其他分子（這個過程可以在完全黑暗中完成）。其結果是，草原的草是半夜間活動的：它們在夜間或凌晨進行大部分的生長。

3. 根系深入土壤

   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.

   草原草還有一個巧妙的方法來逃避太陽的蒸發要求。像許多草原生物（草原犬、地松鼠、棉鼠、獾等）一樣，它們在地下避難。我們所認為的「草」—地上的葉子和莖—實際上只佔生物體的一半以下。按重量計算，60% 至 80% 的植物通常生長在地下。一株 3 米長的大青草被大量的粗纖維根系固定在地下，這些根系深入地下 36 米。藍草很少把它的種子穗抬到離地面很遠的地方，但它的細枝根系網絡有時可以在兩米以下的土壤中探尋水源。

4. 乾旱期的適應方式

   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.

   這些廣泛的根系乾渴地穿過土壤，打算吸食每一滴可用的水。但是，如果土壤非常乾燥，就像乾旱時期那樣，根系無法吸收足夠的水分，無法跟上氣孔的損失。草類的反應是將它們最寶貴的資源（包括糖和蛋白質）從葉片轉移到根部，特別是轉移到根莖（地下莖）。對地面上的世界來說，植物已經死了—枯萎且脆裂—但在地表下儉約地活著，利用它們儲存的物資，等待時機，直到天氣好轉。當雨水最終回來時，正如其不可避免的那樣，這些草就會爆發行動，發出新鮮的根莖，這些根莖又發出新鮮的葉子和根，以產生一個蓬勃發展的嫩芽網絡。彈性驚人的藍草可以從休眠中恢復過來，變得綠油油的，只需五毫米的降雨量就能生長。