Underneath the bark, there is a thin layer of cells called the cambium (pronounced "cam-bee-um").
The cambium cells divide and make new wood on the inside and new inner bark on the outside. In this way, a tree gets bigger around as it grows!
A tree needs to get wider around as it grows. As a tree gets bigger, it needs more and more support to keep it from falling. The large tree trunk is what keeps the tree from falling!
Below is a photograph of a tree trunk unwrapped. You can click on the picture to see the full-sized image. Can you identify the inner bark, the cambium, and the wood?