Conceptual Overview

The outer protective layer of tissue in the primary growth state is derived from the protoderm and is called the epidermis. This development occurs from the outer layer of the shoot apical meristem and therefore is primary in origin. The outer tissue layer in roots has almost no protective function, being adapted almost entirely for absorption and somewhat for anchorage, and therefore may not be considered a true epidermis. Rather, the term rhizodermis is logically employed in root systems, and is more completely discussed in the unit on "The Root: Primary State of Growth."

Epidermal cells are tightly packed and have no intercellular spaces, thus providing a means to minimize loss of water and the invasion of biotic or abiotic agents from outside of the plant. The two primary functions of epidermis are to control water and gaseous exchange. The epidermis is a complex tissue and consists of ground epidermal cells (or pavement cells), trichomes, and stomata. Trichomes are unicellular or multicellular extensions of the epidermal cell(s) and may be glandular or non-glandular. Glandular trichomes are discussed in the unit on "Secretory Structures." In accordance with the protective function of the tissue, the cell walls (especially the outer one) of epidermal cells may be very thick and occupy a large volume equal to about 20-30% of the total cell volume. Additional descriptions of epidermis are included in the units on leaves, stems and roots.

The epidermis is the outer layer of cells of all young above-ground organs. It (or rhizodermis in the case of primary roots) is usually destroyed by secondary growth. It lasts for varying periods of time on stems. It is destroyed by secondary growth in woody stems, although it usually persists on herbaceous stems even after secondary growth. It persists as the outer layer of leaves throughout the life of the leaf. The epidermis is formed from tunica cells in the shoot apex and is first differentiated as the protoderm from apical meristematic growth and, in most plants, continues its growth with the developing plant parts by anticlinal divisions that maintain it as a single layer of cells. In certain plants, however, the epidermal cells divide periclinally to form two or more layers of epidermis (e.g. Bigoniaceae, Moraceae, and Piperaceae).

Division in periclinal planes from the outer cortex also forms phellogen, but other periclinal divisions may give rise to buds that grow into new plants, and various types of epidermal appendages. The outer walls of epidermis in leaves and stems are covered with a cuticle of cutin and/or wax that varies in thickness with the kind of plant, and with its habitat. The cuticle restricts water loss from above-ground organs. The epidermis of submerged plants is covered with a mucilaginous layer of pectic compounds and, in some instances, a thin layer of cuticle. Epidermal cells of water plants and plants growing in deep shade have thinner walls and cuticles. Alternatively, in plants growing in dry habitats, the epidermal cell walls and cuticles are typically very thick. In pines (Pinus sp.) the epidermal cells of the needles have layers of secondary wall material that almost fill the cell cavity.

One of the most conspicuous aspects of the epidermis of leaves is its surface appearance. The surface may be smooth and shining because of a cuticle, dull and gray because of textured wax deposits, or variously textured because of a covering of trichomes. Trichomes are normally initiated by a periclinal division of a single protodermal cell. Some trichomes are soft and pliable, and others are stiff and rigid, even silicified. The cells of the mature trichome may lack protoplasm, but living cells may also be easily seen in which there is a streaming movement of living protoplasm. Many plants produce glandular trichomes having cells secreting an essential oil, nectar, salts, slimes, or other substances. The odor of tomato and potato leaves is from the essential oils stored in such glandular trichomes. The lower epidermis of white poplar (Populus alba) is white because of a thick felt of unbranched, tangled, non-glandular trichomes. The multicellular trichomes of the stinging nettle (Urtica urens) have a long cell terminated by a small bulbous tip that easily breaks off, releasing the accumulated muscle-stimulating substances that cause the sting.

Plants growing in bright sunlight usually have more trichomes per unit area of epidermis than plants of the same species growing in the shade. The difference is due to the relative smaller size of their epidermal cells of leaves.

The epidermis of leaves and stems bears stomata, which are openings surrounded by two guard cells, that are specialized epidermal cells. Stomata regulate gas and water exchange of the leaf by opening and closing. Stomatal frequency varies greatly,depending on location, age of development, and environmental conditions. The guard cells have prominent nuclei and chloroplasts which are capable of starch accumulation. The anticlinal wall away from the stomatal opening is thinner and more flexible than the other walls. Stomatal opening and closing take place as a result of changes in turgor pressure, potassium ion concentration, and to the radial arrangement of cellulose microfibrils in the guard cell wall facing the stomatal pore.

Several pathways of stomatal development can be recognized. If guard cells and the neighboring epidermal cells (including subsidiary cells) have a common origin, then the development is referred to as mesogenous. If there is no common development between the neighboring and subsidiary cells with the guard cells, the development is referred to as perigenous. Finally, if one or more, but not all of the neighboring cells are of common origin to the guard cells, the relationship is designated as mesoperigenous. The developmental patterns result in different configurations of stomatal complexes (i.e. guard cells and subsidiary cells). Briefly, they may be designated as follows:

Actinocytic - radiate-celled with several subsidiary cells radiating

Anisocytic - three unequal-sized subsidiary cells surrounding the stomatal complex

Anomocytic - no subsidiary cells to be found

Diacytic - two subsidiary cells with common walls at right angles to the long axes of the guard cells

Paracytic - two or more subsidiary cells border the stomatal complex with their long axes parallel with the long axes of the guard cells.