Trees and Plants have 5 organs: Leaves, Stems, Roots, Flowers, and Fruit.
Trees have what is called a cambium layer, just underneath the bark. This layer is the vascular system of the tree. It sends nutrients throughout the tree.
Leaves are the food producers of the tree. They have tiny cells in them called chloroplast that contain chlorophyll. The chloroplasts collect the sunlight where a reaction called photosynthesis occurs. This process creates carbohydrates called photosynthate for the tree to stay alive and grow.
Trees need plenty of sunlight to grow. They often grow towards the sun to accumulate the most sun rays as possible. This is could cause the tree to lean further to one side towards the sun and grow thicker.
To feed itself or to give off oxygen to survive, the tree sends signals whenever environmental conditions change to the stomal located on the inside of the leaf. This allows the leaves to release nutrients into the tree where they are needed.
We actually depend on trees and trees depend on us. Trees take in carbon dioxide and release oxygen; people take in oxygen and release carbon dioxide. This creates a cycle and allows plants and beings to prosper.