A typical leaf is mainly composed of three parts: leaf, petiole and stipule. A leaf with these three parts at the same time is called a complete leaf, without any of them.

One or two leaves are called incomplete leaves. The leaves are usually flaky, the upper end of the petiole supports the leaves, the lower end is connected with the stem node, and the stipules are attached to the petiole.

Both sides of the base or axils of leaves have the function of protecting young leaves, which are generally much smaller than leaves.

When the blade is sliced horizontally, the following structures can be observed from the outside to the inside.

1. epidermis: a layer of primary protective tissue on the surface of leaves, usually divided into upper epidermis and lower epidermis. The upper epidermis is located on the ventral surface and the lower epidermis is located on the dorsal surface. Epidermal cells are flat,

Closely arranged, usually without chloroplasts, usually with cuticle outside. Some epidermal cells often differentiate to form stomata or protrude outward to form trichomes.

2. mesophyll: assimilated parenchyma in epidermis, which usually has the following two kinds.

(1) palisade tissue: directly below the upper epidermis, the cells are usually 1 to several layers, long cylindrical, perpendicular to the epidermal cells, closely arranged in a grid, containing more chloroplasts. On two-sided leaves or needle-shaped leaves, palisade tissue is also distributed above the lower epidermis or around the whole inner epidermis, but there are also some aquatic and shade plants with no palisade tissue at all.

(2) Sponge tissue: The cells are irregular in shape and contain less chloroplasts. They are located under the fence organization, with unclear levels and loose arrangement, and look like sponges.

(3) Veins: vascular bundles running through mesophyll. The vascular bundles of the main vein are thicker, while those of the lateral veins and venules are smaller, usually with limited external toughness with xylem at the top and less double toughness with xylem at the middle. Vascular bundle is mainly surrounded by parenchyma, and when it is close to epidermis, it often has thick horny tissue or thick-walled tissue.

These tissues usually have more prominent parts below the main vein and are particularly developed. Calcium oxalate crystals are very common in leaf tissues, with different shapes and different plant species.