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.

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，有時能發現三 角葉楊。

 盡管一年的大多數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 25 英尺(合 7       5 米)。

 牧

 豆樹屬植物不論是喬木和灌木，在北美沙漠中常常可見，在它根部系統完全生長發達到能保 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--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，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

 木會釋放一種物質來殺死那些距他們太近發芽的植物。  除了一些種類的樹木和頑強的灌木 外，大多數沙漠里還有青草、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。當雨后地表還潮濕時它們就迅速發芽，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，但它們已發展了特殊方式來保 證在下次降雨來時下一代的生命。

譯路通武漢漢口翻譯公司整理

2012.7.9