托福备考-托福阅读背景知识 生物适应性

浏览次数：2612    发布时间：2012-11-07

 

托福阅读并不要求掌握相关学科的背景知识，但是文章中常会出现一些专有名词，影响了学生的答题情况。如果考生具备这方面的相关信息便能够有效提高答题速度和正确率。因此，我们为大家列出了一些出现频率较高的题材来 辅助学习，后面将持续更新。

 

Desert, term applied to regions of the earth that are characterized by less than 254 mm (10 in) of annual rainfall, an evaporation rate that exceeds precipitation, and, in most cases, a high average temperature. Because of a lack of moisture in the soil and low humidity in the atmosphere, most of the sunlight penetrates to the ground. Daytime temperatures can reach 55° C (131° F) in the shade. At night the desert floor radiates heat back to the atmosphere, and the temperature can drop to near freezing.

 

All but the most arid desert lands support life that is frequently abundant and well adapted to the scarcity of water and the daytime heat.

Desert plants have evolved ways of conserving and efficiently using the water available to them. Some flowering desert plants are ephemeral; they live for a few days at most. Their seeds lie dormant in the soil, sometimes for years, until a soaking rain enables them to germinate and quickly bloom. Woody desert plants either have long root systems that reach deep water sources or have spreading shallow roots that are able to take up surface moisture quickly from heavy dews and occasional rains. Desert plants usually have small leaves. This conserves water by reducing surface area from which transpiration can take place. Other plants drop their leaves during the dry period. The process of photosynthesis—by which sunlight is converted to energy and usually conducted primarily in leaves—is taken over in the desert by the stems. A number of desert plants are succulents, storing water in leaves, stems, and roots. Thorns, which are modified leaves, serve to guard the water from animal invaders. These plants may take in and store carbon dioxide only at night; during the day their stomata, or pores, are closed to prevent evaporation. Desert plants growing on saline soils may concentrate salt in their sap and then secrete the salt through their leaves.

 

Among the desert animals, the few amphibian species are capable of long-term dormancy during dry periods. When the rains come, they mature rapidly, mate, and lay eggs. Many birds and rodents reproduce only during or following periods of winter rain that stimulate the growth of vegetation. Some desert rodents, such as the North American kangaroo rat and the African gerbil, feed on dry seeds; their metabolic processes are extremely efficient at conserving and recycling water, and their urine is highly concentrated. A number of desert mammals, such as the camel, are able to withstand considerable dehydration. Most desert mammals and reptiles are nocturnal, remaining in cool underground burrows or in the shade by day. Some desert reptiles, such as the horned toad, can control their metabolic heat production by varying their rate of heartbeat and the rate of body metabolism. Some mammals, among them the desert oryx, vary their body temperatures, storing heat by day and releasing it at night.

 

本文主要介绍了沙漠中的动植物是如何适应极端环境的。首先说了沙漠最大的问题就是缺水，如何适应这种缺水环境就是各种动植物存活的关键。然后说了植物是如何适应缺水环境的：有些是周期性植物，只在湿度较高时才生长；常年生长的植物采取另一些办法，例如，叶子表面产生一层蜡质，减少水分蒸发；有些叶子成了刺；有些的根系特别发达；等等。然后，另起一段将动物是如何适应的：产生高盐度的尿液，调整呼吸，等等。随后，还对比了在沙漠和在极地生活的同一种动物的异同。

 

更多关于美国留学的信息请洽博纳寰美，电话0755-83587379或者直接点击网页右边的在线交流！

[返回] [关闭] [打印]