Why Plants have Leaves

The main job of a leaf is to make food (see left navigation for a separate page about this) for a plant. Leaves do this by using sunlight for energy to take apart water from the ground and carbon dioxide from the air. Leaves use parts of water and carbon dioxide to make sugar. This food making is called photosynthesis, a large word which means making something new (synthesis) using light (photo).

Water gets to the leaf through a set of pipes inside the tree which goes all the way down to the roots. Roots take water in from the soil so that it can be used by the tree. There are also chemicals in the water which help the plant stay healthy, a lot like vitamins help people stay healthy. In winter, the ground freezes. This makes it hard for water to get into the soil. Roots can't get as much water from the soil in winter as in the warm seasons of the year.

Carbon dioxide mixed in the air comes into the leaf through small openings on the bottom side of the leaf. These small openings also let air back out, air which has a little extra oxygen in it from the leaf's using carbon dioxide and water to make food. The small openings also let moisture get out of the leaf. This can make problems for the plant in winter when water is not as easy to get. Many trees in New York State get rid of their leaves before the ground freezes. This helps protect them from losing too much water during the winter.

When leaves are making food, they use the chemical chlorophyll to get energy from sunlight. Chlorophyll is green. When the leaf is ready to stop making food in the fall, it stops making chlorophyll. Without chlorophyll, the other chemicals that are in leaves can be seen. These chemicals show up as different colors. For a more complete explanation, visit the Web site Chemistry of Fall Colors.