The outermost layer of the leaf is the epidermis. It consists of the upper and lower epidermis, which are present on either side of the leaf. The epidermis aids in the regulation of gas exchange. It contains stomata, which are openings through which the exchange of gases takes place.

Two guard cells surround each stoma, regulating its opening and closing. Guard cells are the only epidermal cells to contain chloroplasts. The epidermis is usually one cell layer thick. However, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy layer known as the cuticle covers the leaves of all plant species.

The cuticle reduces the rate of water loss from the leaf surface. In the middle of the leaf are two layers of photosynthetic cells called the mesophyll. Just below the upper epidermis is the palisade layer. This tissue is made of elongated cells,each containing hundreds of chloroplasts, and is the main site of photosynthesis.

The palisade cells are close to the source of light, and the upper epidermis is relatively transparent, allowing light to pass through to the enormous numbers of chloroplasts which lie below. Below the palisade layer is spongy layar. They form the main gas exchange surface of the leaf. Water and mineral ions are supplied to the leaf by xylem. Phloem supplies the product of photosynthesis, sucrose to all other parts of the plant.