Leaves can be likened to solar panels. In a similar manner to solar panels, the broad side of every leaf is faced toward the sun so that the optimum amount of energy can be captured and photosynthesized.
Tiny cells in the leaves called chloroplasts actually handle photosynthesis. They are helped by molecules called chlorophyll, which give the leaves their green color. Chlorophyll changes the sun's energy into sugars that the tree can use as food.
The tree leaves also supply other materials to the tree or channel them to different parts of the tree. Tiny pores on plants' leaves are called stomata. These stomata breathe in carbon dioxide, which is essential for trees to survive, and breathe out oxygen, which is essential for people and animals to survive.
In addition, water is channeled up from the roots and around the tree through the vascular system that is partially located in the tree leaves.
Water is absorbed through roots and reaches the leaves through the tree's vascular tissue system. Excess water is removed from the leaves through transpiration, a process that is very similar to the way humans sweat. The leaves have minute openings in them through which the water evaporates.
Similar to humans, tree leaves also transpire more heavily during warm periods. The size of the tree and the amount of water it can absorb are factors that result in more or less transpiration for the tree.
There are other factors that influence the rate at which transpiration takes place. These include the size and shape of the leaf as well as the size of the stomata or pores within the leaf. Some leaves are also more waxy, which also allows them to conserve more water during periods of drought.
Conifer needles (which are a type of leaf) are stiff and waxy and are better at retaining water that leaves with broad leaves.
Leaves cool down the areas beneath the tree during hot seasons and lessen the drying action of heavy winds. They provide the raw material for homes for small animals like squirrels and birds, and tree leaves are also eaten by certain animals such as deer.
On the ground, they act as a mulch to keep the soil under the tree warm or cool and moist. When they decompose they return their nutrients back to the earth to be recycled and reused by the tree.