Unfortunately, in many parts of the world, man has not realized that the third of these services is the most important. (76) In his eagerness to make money from trees, he has cut them down in large numbers, only to find that without them he has lost the best friends he had. And besides, he is usually too careless to plant and look after new trees. So the forests slowly disappear. This does not only mean that man will have fewer trees. The results are even more serious: for where there are trees, their roots break up soil—allowing the rain to sink in and also bind the soil, thus preventing it from being washed away easily; but where there are no trees, the rain falls on hard ground and flows away, causing floods and carrying away the rich top-soil. When all the top—soil is gone, nothing remains but worthless desert. What is the most important service of trees to man according to the passage?