And it is green. Its main functions are photosynthesis

A leaf is an above-ground plant organ and it is green. Its main functions are photosynthesis and gas exchange. A leaf is often flat, so it absorbs the most light, and thin, so that the sunlight can get to the chloroplasts in the cells. Most leaves have stomata, which open and close. They regulate carbon dioxide, oxygen, and water vapour exchange with the atmosphere.
Plants with leaves all year round are evergreens, and those that shed their leaves are deciduous. Deciduous trees and shrubs generally lose their leaves in autumn. Before this happens, the leaves change colour. The leaves will grow back in spring. Leaves are normally green in color, which comes from chlorophyll found in the chloroplasts. Plants that lack chlorophyll cannot photosynthesize.
Leaves come in many shapes and sizes. The biggest undivided leaf is that of a giant edible arum. This lives in marshy parts of the tropical rain forest of Borneo. One of its leaves can be ten feet across and have a surface area of over 30 square feet (~2.8 sq. metres).^[1]
However, leaves are always thin so carbon dioxide can diffuse quickly to all cells.

Anatomy
A leaf is a plant organ and is made up of a collection of tissues in a regular organisation. The major tissue systems present are:

1. 
   The epidermis that covers the upper and lower surfaces
   
2. 
   The mesophyll (also called chlorenchyma) inside the leaf that is rich in chloroplasts
   
3. 
   The arrangement of veins (the vascular tissue)
   

The epidermis is the outer layer of cells covering the leaf. It forms the boundary separating the plant's inner cells from the external environment.
The epidermis is covered with pores called stomata. Each pore is surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts. Opening and closing of the stoma complex regulates the exchange of gases and water vapor between the outside air and the interior of the leaf. This allows photosynthesis, without letting the leaf dry out.