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Plants in the Deserts

　　Some cacti, like the saguaro, grow to tree size, but true trees need more moisture than most desert environments can supply, so they are scarce on deserts. Close to streambeds, cottonwoods can sometimes be found. Though these streams are dry most of the year, water flows there longest and is usually available fairly close to the surface. Elsewhere, trees must send taproots deep into the hard baked desert soil to draw on underground water. Perhaps the most widespread family of trees on the world's deserts is the acacia, whose taproots drill down as far as 25 feet(7. 5 meters). The mesquite common on North American deserts in both tree and shrub forms, does not begin to grow above ground until its root system is completely developed, ensuring the plant a supply of moisture. The roots of shrubs and trees help to hold the desert soil in place. Their stalks and branches also act as screens to keep the wind from sweeping great drifts of sand along the surface. These services are vital if a desert is to support life. Scientists estimate that a desert needs year round plant cover over 20 to 40 percent of its surface. If shrubs are too far apart - separated by a distance greater than five times their height - soil around them is likely to blow away. Without the shelter of established shrubs, new seedlings will have difficulty in getting a start. On the other hand, plants that are too close together may compete for underground moisture. To protect themselves from this competition some shrubs give off a substance that kills young plants that sprout too close to them. In addition to a few varieties of trees and tough shrubs, most deserts have grasses, herbs, and other annual plants. These do not compete for moisture with the longer lived growth. They spring up quickly after rains, when the surface is moist. Then, for a brief time, the desert can be literally carpeted with color. Almost as quickly as they appeared, these small plants die away. But they have developed special ways of ensuring the life of another generation when rains come again.
　　全文翻譯：沙漠中的植物一些仙人掌，如撒瓜羅，能長到象樹那么高。但真正的樹卻需要比大多數(shù)沙漠所能提供的更多的水份，所以樹在沙漠里是鮮見的。在小溪河床附近，有時能發(fā)現(xiàn)三角葉楊。盡管一年的大多數(shù)時間里這些小溪都是干涸的，那里卻是水流得時間最長的地方 而且水份相當(dāng)靠近地表。 其它地方樹木的主根必須深入受炙烤而堅硬的沙漠底部的土壤以 吸取地下水。  在沙漠里分布最廣的樹或許是刺魏，其主根能深達(dá) 25 英尺(合 7 5 米)。  牧豆樹屬植物不論是喬木和灌木，在北美沙漠中常�？梢姡�在它根部系統(tǒng)完全生長發(fā)達(dá)到能保 證提供充足的水分時才長出地面。 灌木和樹的根有助于固定沙漠中的土壤，它們的莖和樹 枝同時起屏障的作用，防止風(fēng)從沙漠表面吹起大堆的沙。  如果沙漠要支持生命，這種作用 必不可少。  科學(xué)家估計一年之中植物必須覆蓋沙漠表面的  20%到  40%。  如果灌木間隔太 遠(yuǎn)--大于它們高度的 5 倍--周圍的土壤就可能被吹走。 如果沒有這些灌木作為保護(hù)，新的種 子很難生長。 另一方面，植物相互靠得太近卻會引起競爭地下水分。  為避免競爭，一些灌木會釋放一種物質(zhì)來殺死那些距他們太近發(fā)芽的植物。  除了一些種類的樹木和頑強的灌木 外，大多數(shù)沙漠里還有青草、草本植物和其它年生植物。 它們并不與長期生長的樹木競爭 水份。當(dāng)雨后地表還潮濕時它們就迅速發(fā)芽，然后在一個短時間里，給沙漠鋪上綠色地毯。 這些小植物很快就消失了，幾乎就象它們長出來時那樣迅速，但它們已發(fā)展了特殊方式來保 證在下次降雨來時下一代的生命。