Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter II. Histology
2.3. Dermal Tissues

They cover vegetative and Generative Organs, protecting the plant from excessive moisture loss, Temperature fluctuations, mechanical damage, and the penetration of parasites and pathogens. Based on their origin, protective Tissues are classified into primary, secondary, and tertiary. The primary tissues include the epidermis and epiblem, the secondary includes the periderm (cork), and the tertiary includes the bark (rhytidome).

The epidermis is a living protective tissue formed from the outer layer of the SHOOT apex (tunica). Epidermal Cells are parenchymatous, living, transparent, often with folded walls, and contain a large vacuole. The latter is filled with Cell sap and is sometimes pigmented with anthocyanin. The outer walls of epidermal cells generally thicken and become impregnated with cutin, forming a continuous, unstructured film known as the cuticle. The Protective Functions of the epidermis are enhanced by various appendages such as hairs, scales, prickles, spines, etc. Hairs, or trichomes, in most angiosperms take the form of papillae, bristles, star-shaped structures, or tufts. Some of them are quite rigid and protect plants from being eaten by animals. Essential Oils accumulate in the glandular hairs, which possess a glandular HEAD. Stinging hairs, found in nettles, contain acids; upon contact, their tip breaks off and the contents penetrate the Skin, causing irritation.

The epidermis contains Stomata, which consist of a stomatal pore underlain by a so-called substomatal air chamber filled with air (Fig. 22). The pore is flanked by two guard cells containing METABOLISM/14.html">Chloroplasts. The walls of the guard cells are unevenly thickened: those facing the stomatal pore are significantly thicker than those facing the epidermis. The size of the pore can be regulated depending on the intensity of Photosynthesis and the plant's Water status. Under sunlight, photosynthesis proceeds intensively in the guard cells; the accumulation of photosynthetic products promotes water uptake, increases cell volume, and induces a turgid state in the guard cells. The guard cells acquire a characteristic Kidney-like shape, their walls stretch, and the stomatal pore opens. When light intensity decreases, The formation of sugars and starch in the guard cells slows down, water exits the guard cells via osmosis, turgor drops, and the stomatal pore closes. Gas exchange and Transpiration are facilitated by the functioning of the stomata. In most plants, stomata are located on the lower surface of the leaves, in aquatic plants mostly on the upper surface, and in pine needles across the entire surface.

The epidermis covers the leaves of all plants, the stems of monocots throughout their entire life cycle, and young plant organs. It functions for a single growing season.

The protective tissue of the ROOT absorption zone bears root hairs and is called the epiblem (or rhizodermis). It is formed through the differentiation of dermatogen cells at the root apex. Sometimes it is classified as a specialized type of absorptive parenchyma.

The secondary protective tissue is the cork (phellem), which develops from a secondary meristem—the phellogen (cork cambium). The latter is formed by the division of epidermal, cortical parenchyma, or collenchyma cells. The divisions of phellogen cells by tangential walls produce daughter cells. Those deposited outward from the phellogen differentiate into cork cells, while those deposited inward become phelloderm cells. Consequently, a complex of tissues arises—phellogen, cork, and phelloderm—which together form the periderm.

Cork consists of regular radial rows of tightly packed cells whose walls become suberized due to impregnation with suberin. The Cell contents die off. Thus, a layer of dead, air-filled cells is formed, which is impermeable to water, gases, etc. Phelloderm cells are living and sometimes contain chlorophyll. Gas exchange and transpiration in cork occur through the functioning of lenticels. Lenticels represent clusters of rounded complementary cells that form intercellular spaces through which air penetrates into the inner stem tissues. They typically develop from the phellogen of lenticels in locations where stomata were previously situated in the epidermis. This phellogen produces complementary parenchymal cells outward, whose pressure ruptures the cork, thereby forming the lenticel.

Cork forms on perennial shoots and roots of dicotyledonous plants. Over time, bark (rhytidome), which is classified as a tertiary protective tissue, develops on tree trunks and roots. Its formation involves a phellogen that initiates as a continuous ring in the deep layers of the cortex. Its activity produces the periderm. The cortex located outside the newly formed cork is cut off from nutrient supplies and begins to die. Subsequently, a phellogen develops in deeper layers of the cortex, and new patches of living tissue become isolated and likewise die off. The complex consisting of peridermal layers and dead cortical tissue trapped between them is called the bark (rhytidome).