Leaf

Epidermal cells are on the upper and lower surfaces of a leaf. They have two features which prevent evaporative water loss: they are packed densely together and they are covered by a cuticle, a waxy layer secreted by the cells. The epidermis usually consists of a single layer of cells, although the specialized leaves of some desert plants have epidermal layers which are several cells thick. Epidermal cells often have large vacuoles which contain flavonoid pigments. Flavonoids generally absorb ultraviolet radiation, and may act as a sort of natural sunscreen for the internal layers of the leaf, by filtering out harmful ultraviolet radiation from the sun.

The leaf epidermis has small pores, called stomata, which open up for photosynthetic gas exchange and transpiration. Stomata are scattered throughout the epidermis, but are typically more numerous on the lower leaf surface. Each individual stoma (pore) is surrounded by a pair of specialized epidermal cells, called guard cells. In most species, the guard cells close their stomata during the night to prevent transpirational water loss, and open their stomata during the day so they can take up carbon A scanning electron micrograph (SEM) of open stomata on the surface of a tobacco leaf (Nicotiana tabacum). Stomata are breathing pores scattered over the leaf surface, and sometimes stem, that regulate the exchange of gases between the leaf's interior and the atmosphere. Stomatal closure is a natural response to darkness or drought as a means of conserving water. Each pore is controlled by the turgor of two guard cells (sides seen beneath pore's opening) on either side of it. When they are full of water the pore is open; when they lose turgor, the pore closes. Photo Researchers, Inc. Reproduced by permission.

dioxide for photosynthesis, and give off oxygen as a waste product.