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Leaves are the primary site of photosynthesis in plants. The structure of a leaf, shapes and sizes of leaves, leaf textures, and the arrangement of leaves on a plant, have all evolved to adapt to the environment the plant lives in and enable the leaves to absorb the greatest amount of light.

Structure of a simple leaf:

Axillary bud: Clump of growth tissue from which the leaf sprouted; potential site of a new branch or leaf.
Lamina: Leaf blade, primary photosynthesis area. Contains chlorophyll; underside has stomata for gas transfer.
Petiole: Stalk-like structure connecting lamina and stem. Can position the leaf for maximal light absorption. Sessile leaves do not have petioles: their laminae are directly attached to the stem.
Veins: Nutrient transport system, containing xylem and phloem. Xylem moves water and minerals from roots to leaves, phloem moves photosynthesis products (sugar) from leaves to other parts of a plant. Usually, xylem is on the upper side of a leaf and phloem is on the underside.

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