Conceptual Overview

The periderm is the secondary protective (dermal) tissue that replaces the epidermis during growth in thickness of stems and roots of gymnosperms and dicotyledons (i.e., secondary growth). Unlike the epidermis, the periderm is a multilayered tissue system, the bulk of which usually constitutes the cork, or phellem. There are, however, some exceptions to this inasmuch as some other structures (e.g. potato skin and apple peel) are also periderm. Phellem (the cork) consists of cells that are dead at maturity, and their primary walls become covered from the inside by a secondary wall which consists of parallel suberin lamellae alternating with wax layers. The lateral meristem, (cork cambium or phellogen), is one cell layer thick and encircles the stem. It produces periderm centrifugally. The layer of cork cells formed is impermeable for water and gases, but is interrupted at certain points by lenticels which function to some extent similar to stomata in the epidermis, and permit gas diffusion. In some cases parenchyma cells are produced centripetally (i.e. to the inside of the stem or root) by the phellogen as a part of the periderm. These persistent living cells are called phelloderm and structurally appear similar to cells of the cortex. The number of layers of cork and phelloderm varies greatly among different species; some plants produce no phelloderm. The most important function of the periderm is to reduce the loss of water and solutes from interior tissues and to protect a plant from unfavorable environmental conditions.

Cork cambium in most plant stems arises from an outer (subepidermal) layer of the cortex, but in some plants it originates by the periclinal division of the epidermal cells or, alternatively, fairly deeply inside the stem, such as in the primary phloem. In roots, it arises in the pericycle.

In woody plants, with the growth of the main stem in thickness, new layers of cork cambium, and accordingly sequential periderms, are formed in the secondary phloem, cutting off old non-functional phloem tissues. As a result, rhytidome û or outer bark, is formed consisting entirely of dead cells.

It should be emphasized that bark is not wood. Wood refers only to the secondary xylem. Bark is generally accepted to include all tissues of a plant exterior to the vascular cambium. It can be divided into inner bark and outer bark. The inner bark includes the region from the vascular cambium to the innermost cork cambium (or phellogen), that is going from one secondary meristem to a second secondary meristem. The outer bark is composed of all tissues outside of the cork cambium (which are dead) and includes phellem, old secondary phloem, crushed primary phloem, crushed cortex, crushed epidermis and any prior periderms.

It is worth considering the differences between the phellogen (cork cambium) and the vascular cambium. It can be said that the phellogen:

• is not derived from any previous meristem, such as procambium.
• is initially derived from permanent tissues, mostly from the subepidermal layer of the cortex.
• may repeatedly originate progressively in the deeper layers of the stem.
• typically owes its origin to dedifferentiation and redifferentiation into meristematic cells.
• produces its derivatives mainly (or only) to the outside (generally only cork cells are produced).
• lacks intercellular spaces except where lenticels are present.
• produces a single kind of initial that is rectangular in cross-section, and wider tangentially than radially. The cork cells produced have the same shape as the phellogen cells.

As indicated above, phellem cells are dead at maturity, and appear in brick-like rows having their primary walls impregnated with suberin. The plasmodesmata are blocked and usually the contents are filled with tannins or resins, or possibly just air filled as in the case of bottle cork. Some cells, called phelloids, have non-cork construction ù not suberized but being thin-walled, sclerified or lignified.