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，如撒瓜罗，能长到象树那么高。 但真正的树却需要比大多数沙漠所能提供的更多的水份，所以树在沙漠里是鲜见的。在小溪河床附近，有时能发现三角叶杨。 尽管一年的大多数时间里这些小溪都是干涸的，那里却是水流得时间最长的地方而且水份相当靠近地表。 其它地方树木的主根必须深入受炙烤而坚硬的沙漠底部的土壤以吸取地下水。 在沙漠里分布最广的树或许是刺魏，其主根能深达25英尺(合7 5米)。 牧豆树属植物不论是乔木和灌木，在北美沙漠中常常可见，在它根部系统完全生长发达到能保证提供充足的水分时才长出地面。 灌木和树的根有助于固定沙漠中的土壤，它们的茎和树枝同时起屏障的作用，防止风从沙漠表面吹起大堆的沙。 如果沙漠要支持生命，这种作用必不可少。 科学家估计一年之中植物必须覆盖沙漠表面的20%到40%。 如果灌木间隔太远--大于它们高度的5倍--周围的土壤就可能被吹走。 如果没有这些灌木作为保护，新的种子很难生长。 另一方面，植物相互靠得太近却会引起竞争地下水分。 为避免竞争，一些灌木会释放一种物质来杀死那些距他们太近发芽的植物。 除了一些种类的树木和顽强的灌木外，大多数沙漠里还有青草、草本植物和其它年生植物。 它们并不与长期生长的树木竞争水份。当雨后地表还潮湿时它们就迅速发芽，然后在一个短时间里，给沙漠铺上绿色地毯。 这些小植物很快就消失了，几乎就象它们长出来时那样迅速，但它们已发展了特殊方式来保证在下次降雨来时下一代的生命。
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