Trees should only be pruned   when there is a good and clear reason for doing so and , he number of such reasons is small . Pruning involves the cutting away of overgrown and unwanted branches , and the inexperienced gardener can be encouraged by the thought that more damage results from doing it unnecessarily than from leaving the tree to grow in its own way . First , pruning may be done to make sure that trees have  desired shape or size . The object may be to get a tree of the right height , and at the same time to help the growth of small side branches , which will thicken its appearance or give it a special shape . Secondly , pruning may be done to make the tree healthier . You may cut out diseased or dead wood , or branches that are rubbing against each other and thus cause wounds . The health of a tree may be encouraged by removing branches that are blocking up the center and so preventing the free movement of air . One result of pruning is that an open wound is left on the tree and this provides an easy entry for disease , but it is a wound that will heal   . Often there is a race between the healing and the disease as to whether the tree will live or die , so that there is a period when the tree is at risk . It should be the aim of every gardener to reduce that risk of death as far as possible . It is important to make the area , which has been pruned , smooth and clean , for healing will be slowed down by roughness . You should allow the cut surface to dry for a few hours and then paint it with one of the materials available from garden shops produced especially for this purpose . Pruning is usually done in winter , for then you can see the shape of the tree clearly without interference   from the leaves and also it is very unlikely that the cuts you make will bleed . If this does happen , it is , of course , impossible to paint them properly . What was the author's purpose when writing this passage ?
To discuss different methods of pruning . To introduce some common knowledge of pruning . To explain how trees develop disease . To give practical instruction for pruning a tree .