Trees are useful to man in three very important ways: they provide him with wood and other products they give him shade and they help to prevent droughts (干旱) and floods. Unfortunately, man has not realized that the third of these services is the most important. Two thousand years ago a rich and powerful country cut down its trees to build warships, with which to gain itself an empire (帝国). It gained the empire, but, without its trees, its soil became hard and poor. When the empire fell to pieces, the home country found itself threatened by flood and starvation. Even though a government realizes the importance of a plentiful supply of trees, it is difficult for it to persuade villagers to see this. The villagers want wood to cook their food with and they can earn money by selling wood. They are usually too lazy to plant and look after the trees. So, unless the government has a good system of control, or can educate the people, the forests will slowly disappear. This does not only mean that the villagers' children and grandchildren will have fewer trees. The results are even more serious, for where there are trees their roots break the soil up, allowing the rain to sink in, and also bind the soil, thus preventing its being washed a-way easily. But where there are no trees, the rain falls on hard ground and flows away from the surface, causing flood. What is the most important function of trees?