The bark cells are called phloem, meaning bark in Greek. These cells carry sugars and other materials produced by the plant. Annual Rings New layers of wood are added each year between the bark and the previous year's wood. These are called growth rings or annual rings and may be used to age a tree. Annual rings vary in size and thickness according to the season and environmental conditions when they are formed.
Cells produced in the spring are larger with thinner cell walls.
These are the light-colored rings, and the wood is called early or spring wood. Cells produced in the summer are smaller, and this late or summer wood has a higher density and darker color. Bark Trees have bark that protects the tree from pest attacks and environmental impacts such as fire and mechanical injury. Trees with thick bark are more resistant to injury than those with thin bark. As the tree grows in diameter, the bark has to give, forming ridges and cracks.