Delphinidin

The outermost layer of cells of all young plant organs is called the epidermis. Since it is in direct contact with the environment, it is subject to modification by the environment and often includes several different kinds of cells. The epidermis is usually one cell thick, but a few plants produce aerial roots called velamen roots (e.g., orchids) in which the epidermis may be several cells thick, with the outer cells functioning something like a sponge. Such a multiple-layered epidermis also occurs in the leaves of some tropical figs and members of the Pepper Family (Piperaceae), where it protects a plant from desiccation.

Most epidermal cells secrete a fatty substance called cutin within and on the surface of the outer walls. Cutin forms a protective layer called the cuticle. The thickness of the cuticle (or, more importantly, wax secreted on top of the cuticle by the epidermis) to a large extent determines how much water is lost through the cell walls by evaporation. The cuticle is also exceptionally resistant to bacteria and other disease organisms and has been recovered from fossil plants millions of years old. The waxes deposited on the cuticle in a number of plants can reach the surface by diffusion, migrate between cells, or travel through microscopic channels in the cell walls. The susceptibility of a plant to herbicides may depend on the thickness of these wax layers.

Some wax deposits are extensive enough to have commercial value. Carnauba wax, for example, is deposited on the leaves of the wax palm. It and other waxes are harvested for use in polishes and, in the past, for phonograph records. In colonial times, a wax obtained from boiling leaves and fruits of the wax myrtle was used to make bayberry candles. In leaves, the epidermal cell walls perpendicular to the surface often assume bizarre shapes that, under the microscope, give them the appearance of pieces of a jigsaw puzzle.

Epidermal cells of roots produce tubular extensions called root hairs a short distance behind the growing tips. The root hairs greatly increase the absorptive area of the surface. Hairs of a different nature occur on the epidermis of above-ground parts of plants. These hairs form outgrowths consisting of one to several cells. Leaves also have numerous small pores, the stomata, bordered by pairs of specialized epidermal cells called guard cells.

Guard cells differ in shape from other epidermal cells; they also differ in that chloroplasts are present
within them. Some epidermal cells may be modified as glands that secrete protective or other substances, or modified as hairs that either reduce water loss or repel insects and animals that might otherwise consume them.