Bark isn't stretchy like skin, though. If there is a big tree nearby, look at its bark. That's what happens as the tree grows bigger around. Luckily, the cracks on a healthy tree are never deep enough to break completely through the bark. The whole tree, including the bark, is made up of cells.
Cells are like tiny building blocks.
There are leaf cells, wood cells, root cells, and bark cells - special cells for every part of the tree. Through a microscope, it's easy to see that bark is made up of layers of cells. Look at the close-up picture of bark. A kind of fatty material has separated the outer bark cells from their supply of water and food. This causes the outer cells to die.