292 LIVING THINGS IN THEIR ENVIRONMENT 



trees. Many insect larvae, as the forest tent caterpillar, 

 the caterpillars of the gypsy moth, the tussock moth, and 

 the brown-tail moth eat the leaves of forest trees. Beetles 

 bore in the wood or eat the roots, and scores of other in- 

 sects do their best to destroy our forests. Fortunately 

 the birds which live there feed on the insects and keep 

 the destruction down. So we may say the future of our 

 forests rests largely with our birds. If we kill the birds, 

 we also may kill our trees. In addition, man often does 

 harm by setting fires which destroy hundreds of thousands 

 of trees, or sheep are allowed to browse in forests and 

 destroy young seedling trees. 



.-bark 



Rh. 



- -Cambium 



layer- 



This tree grows both inward and outward from the cambium layer. How much 

 did it grow last year ? How old is the tree ? How can you tell this ? 



How Do Trees Grow ? If you break off a rapidly grow- 

 ing shoot from a forest tree, you will find it much softer 

 than an older branch. A cut trunk shows a series of well- 

 marked rings of growth. The branch or tree trunk grows 

 from an area just under the bark. This soft area, known 

 as the cambium, is a place where the cells of the tree are 

 rapidly multiplying in warm weather. They grow in 

 both directions, inward to form wood and outward to 

 form bark. Each winter growth slows up, and in spring it 

 becomes more rapid. This irregularity in growth causes 

 the rings of growth seen in trees. It is thus true that each 



