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 worlds deserts is the acacia, whose taproots drill down as far as 25 feet . 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.
【托福词根联想】Word List 2
托福阅读词汇选集(1):A
托福词汇记忆两大误区解决办法
新托福考试词汇该如何备考 分类别记忆最有效
名师指点托福备考两大难题:词汇与口语
托福听力核心词汇:校园词汇
历年托福考试高频词汇-readmit
【托福词根的联想】Word List 3
托福备考误区:学习托福就是背单词
高效记忆托福单词:词根"form"同源词汇解析
托福阅读词汇选集(4):CD
【托福词根联想】Word List 1
【托福词根的联想】Word List 4
历年托福考试高频词汇-readily
托福词汇记忆方法优缺点介绍
托福词汇:"And"特殊用法及汉译
【托福词根联想】Word List 1
【托福词根联想】Word List
【托福词根的联想】Word List 3
【托福词根的联想】Word List 3
托福词汇记忆方法优缺点介绍
【托福词根联想】Word List 1
【托福词根联想】Word List 2
托福写作高分词汇:交通
【托福词根联想】Word List 1
【托福词根联想】Word List 2
托福单词拆分有效帮助记忆
托福听力场景词汇:学生用语
【托福词根的联想】Word List 2
名师介绍托福词汇记忆方法优缺点
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |