The tissues of a leaf

In common with stems and roots, leaves are made up of three main types of tissue: epidermal tissue, vascular tissue, and ground tissue. Each tissue forms a continuous system throughout the plant.

The epidermis covers and protects the leaves. It is the first line of defence against physical damage, infection, and being eaten. The upper epidermis consists of one or more layers of rectangular cells. In terrestrial plants, these epidermal cells secrete a waxy coating called the cuticle. The waxy cuticle is waterproof, minimising water loss from the surface of the leaf. It is often thicker on the upper surface, making this surface appear more shiny than the lower surface.

The epidermis is perforated by microscopic pores called stomata. Stomata allow carbon dioxide and oxygen to gain easy access into the plant, but also allow water to escape. Each stomata is flanked by a pair of guard cells that regulate the size of the pore, closing it in times of water stress. Water is more likely to be lost from the upper surface of a leaf because it is more exposed to sunlight. The upper surface usually has fewer stomata than the lower surface; this minimises water loss.

The vascular tissue consists of veins adapted to transpsort liquid substances around the plant, and it is made up of vascular bundles, groups of vessels running from the root up the stem and to the leaves. Xylem forms the upper part of a vascular bundle in the leaf, bringing water and mineral salts to the leaf. Phloem forms the lower part of a bundle, transporting sucrose and other products of photosynthesis away from the leaf.

Ground tissueis all the tissue in a plant other than the epidermis, reproductive tissue, and vascular tissue. It makes the bulk of a leaf and consists mainly of parenchyma cells reinforced by collenchyma and sclerenchyma.