leaf, in botany, any usually flattened green outgrowth from the stem of a vascular plant. As the primary sites of photosynthesis, leaves manufacture food for plants, which in turn ultimately nourish and sustain all land animals. Botanically, leaves are an integral part of the stem system. A leaf is a part of a plant attached to a stem resembling a flat structure. Leaves help plants collect sunlight, which they can then turn into energy (food) through a process called photosynthesis. Their flatness helps them in this task and they are thin to allow the sunlight easy entry into their cells. A simple leaf is singular and never divided into smaller leaflet units. It is always attached to a twig by its stem or the petiole. The margins, or edges, of the simple leaf can be smooth, jagged, lobed, or parted. Lobed leaves will have gaps between lobes but will never reach the midrib. All leaves change sunlight into energy through photosynthesis. The leaves are the primary food-making part of the plant. Leaves absorb carbon dioxide from the air, combine it with water that comes through the roots of the plants to make food (a sugar molecule known as glucose), and release oxygen into the air. Leaves can be either simple or compound in shape. Simple leaves have a single leaf blade, while compound leaves have many leaflets. Remember, you can tell the difference between a leaf and a leaflet by looking for the bud at the base of the petiole.Leaf cells are filled with chloroplasts, structures that contain specialized pigments called chlorophylls. Chlorophylls absorb light, allowing plants to collect energy from the sun. Leaves are green because chlorophylls absorb every color of UV light except for green. The first things that form are a tiny root (to get water and nutrients), and the stem and leaf cells. These cells then rapidly divide and divide, letting the stem push up and out of the soil towards the light. Once cells in the stem sense light, the tiny curled up leaves get the signal to open.