The air enters the tree through the leaves and small openings in the bark, which are easily seen in such trees as the cherry and birch.

Trees breathe constantly, but they digest and assimilate food only during the day and in the presence of light.

In the process of digestion and assimilation they give off oxygen in abundance, but they retain most of the carbonic acid gas, which is a plant food, and whatever part of it is not used immediately is stored up by the tree and used for its growth and development.

Trees also give off their excess moisture through the leaves and bark.

Otherwise they would become waterlogged during periods when the water is rising rapidly from the roots.
After the first year, trees grow by increasing the thickness of the older buds.