During its growth, a large tree takes in by means of its roots many tons of water. Yet the tree retains only a very little of this water, perhaps 1%, in the growth process of the cells. The rest is given off through its leavessintosthe atmosphere.

　　These quantities are significant. Even a small tree, such as a fruit-tree, gives off four gallons of watersintosthe air every hour. An oak tree, with perhaps a quarter of a million leaves, gives off more than a thousand gallons a day in summertime.

　　When we think of a whole forest, the total must clearly be vast, even though the amounts vary according to the kind of tree.

　　A tree can be seen as a kind of pump with two purposes. It is always sucking up water from the ground, which would otherwise become far more loosely composed. At the same time, it increases the humidity (湿度) of the surrounding land, which would otherwise be much drier.

　　Trees and frosts also act a breakwater (防水堤)against rainstorms, which, especially on hillsides, would otherwise wash away much of the topsoil, both around the trees and lower down. The rich humus (腐殖土壤)that forms the floor of the forests acts as a sponge (海绵). It holds far more water than the bare earth would be able to hold.

　　The water that is not required by the tree passes through to the subsoil in an orderly manner, feeding the springs, the streams and rivers of the area.

　　As well as protecting the soil from the full effect of rain, the trees protect it also from erosion (腐蚀，侵蚀) caused by wind; for wind can blow away bear soil as surely as water can wash it away.

　　习题：1.“When we think of a whole forest, the total must clearly be vast, even though the amounts vary according to the kind of tree.”In this sentence, he total refers to__________.

　　A: all the trees in a whole forest

　　B: all the water taken in by the trees in a whole forest

　　C: all the water given off by the trees in a whole forest

　　D: both B and C

　　2. The water _____ passes through to the subsoil in an orderly manner.

　　A: requiring not by the trees

　　B: required not by the trees

　　C: not requiring by the trees

　　D: not required by the trees

　　3. The best title for this passage is _____.

　　A: Trees and Water

　　B: Tees and Forests

　　C: Trees and Soil

　　D: Tees and Surroundings.

　　注释：1 give off (散)发出(蒸气),发散(光线)

　　2 significant adj.有意义的,重大的,重要的

　　3 pump n.抽水机；打气机；唧筒

　　抽气；打气；(用抽水机)取水

　　A bicycle pump puts airsintosthe tires.

　　自行车打气筒把空气打进轮胎里。

　　4 otherwise adv.另外,否则,不同地,别的方式

　　adj.另外的,其他方面的

　　5 surrounding adj.周围的，附近的

　　n.[pl. ]周围的事物,环境

　　social surroundings

　　社会环境

　　railway surroundings

　　铁路周围环境

　　6 require需要

　　The floor requires washing.

　　地板该洗了。

　　I require two children to help me.

　　我需要两个孩子来帮我的忙。

　　要求

　　All passengers are required to show their tickets.

　　所有乘客都必须出示车票。

　　答案：1 C 2 D 3 D