阅读下面短文，用适当的词填空。

　　Trees are useful to human beings in three important ways：they 　　1　　 them with food and other produces; they give them 　　2　　; and they help to 　　3　　 floods．　　4　　 in many parts of the world, people haven't realized that the 　　5　　 of these services is the most important．To get quick profits from the tree, they have cut them in 　　6　　 numbers, only to find that they have 　　7　　 their good friends．Two thousand years ago a rich and 　　8　　 country cut down its trees to build warships, with which it seized an empire．It gained the empire, 　　9　　, without its trees, its 　　10　　 became hard and poor．When the empire fell to pieces, the home country found itself faced with floods and starvation．

　　It is difficult for the government to 　　11　　 the villagers to see this．They are usually too lazy or too careless to plant and look after new trees．　　12　　 the government has a good system of control, the forest will slowly 　　13　　．This does not only mean that the villagers' sons and grandsons will have fewer trees．The results are even more 　　14　　．Where there are trees, their roots break the soil up－allowing rain to sink in．The roots also bind the soil, 　　15　　 preventing it from being washed away easily．If there are no trees, rain falls on hard ground and flows away from the surface, carrying away with it, the rich top-soil．When all the top-soil is gone, nothing 　　16　　 but a worthless desert．

1．________　2．________　3．________　4．________　5．________

6．________　7．________　8．________　9．________　10．________

11．________　12．________　13．________　14．________　15．________

16．________