Courtesy G. Lumis
Although there is an incredibly very wide variety of size, shape, texture, they all have one important thing in common. The leaf is where a tree's food is manufactured. Water and minerals from the soil combine with carbon dioxide from the air to ensure growth and development. A tree will suffer if insects eat all the leaves, as the nourishment needed cannot be made without them.
The leaf is divided into three parts - blade, vein system and stem. The blade of each leaf is made up of hundreds of tiny cells. The cells contain a green substance called chlorophyll, which is an important part of the food-making machinery. The veins of the leaf are the conduits, bringing in the sap that has been drawn up from the roots. Leaves collect carbon dioxide from the air and the sun provides the power to run the machinery. A product is formed that is much like starch. This food is sent to every part of the tree where it is used in building food, bark and other tissues.
We might imagine leaves having an arrangement with breathing with other animals and human beings. Animals use oxygen from the air giving back carbon dioxide to the air. The leaves take in the carbon dioxide, keep the carbon to build up the wood, and release oxygen into the air for all animals to use.
Acting as an enormous "carbon sink", trees soak up carbon dioxide from the air, producing life-giving oxygen in return. In fact, a medium-sized tree generates the same amount of oxygen as each one of us needs to breathe.
In a tree, 'breathing' takes place in the leaf. Chlorophyll (the substance causing the green colour) absorbs the CO2 and uses it along with water to dissolve minerals taken up through the roots. After the chemical reaction is completed, the leaf releases oxygen and water vapor through its pores.
Photosynthesis is a process by which CO2 and water are combined with sunlight and a pigment called chlorophyll. The chemical reactions result in the production of sugars which provide energy to the tree. The leaves use some of this energy, but the majority is transported, in the form of sugar solutions, to other parts of the tree that require it.
Transpiration, or water loss, also takes place in the leaves. As this occurs, water is drawn up from the roots through the vascular system to replace lost moisture.
In autumn broad-leafed trees display a brilliant colouring. This occurs when the removal of the green pigments (chlorophylls) takes place, leaving the yellow pigments. These along with other materials are stored in the branches during the winter are used by the tree to start further growth in the spring. Even the fallen leaf performs a necessary function. They benefit the soil by keeping it from being washed away by heavy rains and they prevent the ground from becoming too hard so that melting rain and snow can sink into the ground rather than flooding the surface of the earth.
The evergreen tree is a conifer. The leaves are in the shape of needles. These trees actually do get a new set of needles every year, but as the needles stay on the tree for more than a year they remain green. Amongst conifers there are exceptions. The larch in autumn turns yellow and the leaves fall off. The arbutus, native to the Pacific Northwest is a broad-leafed tree that keeps its green leaves all year round.