MEDICAL BOTANY - A.G. Serbin - 2003

SECTION 1. ANATOMY

PLANT TISSUES

DERMAL TISSUES

Depending on THE ORIGIN OF Dermal Tissues, their Structure AND Functions, the following types are distinguished: epidermis, periderm, bark (rhytidome), and epiblema.

The epidermis, or Skin, is a primary dermal tissue covering all PARTS OF THE primary plant body. It is formed from the protoderm, which is the outer layer of Cells of the apical Meristems. The epidermis is typically a single-layered, or less frequently, a multi-layered complex tissue that performs protective and regulatory-secretory functions: it protects the plant against Temperature fluctuations, mechanical and other injuries, regulates Transpiration, gas exchange, and external secretion. The epidermis covering the aerial parts comprises: fundamental epidermal cells with a cuticle, Stomata, and often trichomes. The epidermis covering underground Organs lacks stomata and trichomes and has no thick cuticle.

Fundamental epidermal cells (Fig. 1.12) are living, straight- or sinuate-walled, tightly packed cells elongated along the axis of the leaf (in monocots) or parenchymatous (in dicots). Epidermal cells generally lack actively functioning colored Plastids, but light-sensitive leucoplasts are typically arranged around The Nucleus. The coloration of cells occasionally observed depends on the presence of pigments in the vacuoles, such as anthocyanidins, flavones, flavonols, etc. Sometimes, calcium oxalate crystals or cystoliths are formed in the epidermis, serving as a diagnostic feature of plants. The walls of epidermal cells are unevenly thickened: lateral walls are thin, bottom walls are thicker, and upper walls bordering the external environment are thickened, cutinized, or mineralized, and covered with a protective layer of wax or cutin—the cuticle. The thickness and pattern of cuticle deposition vary among species and depend on environmental factors.

Class="center">Fig. 1.12. Epidermis from the leaf surface: A — monocots; B — dicots: 1 — epidermal cells; 2 — substomatal (accessory) cells; 3 — stomatal guard cells; 4 — cuticular folds

The stomatal apparatus, or stoma (Figs. 1.12 — 1.14), ensures gas exchange and transpiration. It consists of two guard cells, an intercellular space or stomatal pore situated between them, and subsidiary, or accessory, cells. Guard cells are characterized by a crescent shape in surface view, the presence of photosynthesizing METABOLISM/14.html">Chloroplasts, and unevenly thickened walls: only the walls adjacent to the subsidiary cells remain thin and elastic. This allows the guard cells to alter their shape and volume. These changes and the size of the stomatal pore are influenced both by the physiological state of the plant Organism (Water balance, intensity of Photosynthesis, transpiration, etc.) and by external factors such as light intensity, temperature, atmospheric pressure, and water regime. The Mechanism of stomatal operation is complex. It is based on changes in turgor pressure depending on the concentration of photosynthetic products: during the day, sugars enter the vacuoles, the concentration of Cell sap increases, leading to intensive water uptake from the subsidiary cells and an increase in turgor pressure. Consequently, the guard cells stretch, move apart, and the stomatal pore enlarges (Fig. 1.13, A). At night, the concentration of sugars decreases, turgor pressure drops, and the stomata "close" আত্মীয়.

Fig. 1.13. Structure of stomata and their arrangement relative to epidermal cells: A — stoma located in the same plane as epidermal cells; B, C — raised stoma; D — sunken stoma; 1 — guard cells; 2 — epidermal cells; 3 — subsidiary cells; 4 — outer (anterior) vestibule; 5 — inner (posterior) vestibule; 6 — substomatal cavity; 7 — stomatal pore

Depending on the mutual arrangement of subsidiary cells, their number, size, and shape, several types of stomatal apparatus are distinguished (Fig. 1.14), which is of taxonomic significance. Anomocytic type — the stoma is surrounded by cells that do not differ from the remaining epidermal cells (class Dicotyledones, families Asteraceae, Ranunculaceae, Solanaceae, Papaveraceae, Urticaceae, Geraniaceae, etc.). Anisocytic type — there are three subsidiary cells, one of which differs in size from the others (class Dicotyledones, families Brassicaceae, Crassulaceae, Malvaceae). Paracytic type — There are two or several subsidiary cells whose longitudinal axes are parallel to the stomatal pore (class Dicotyledones, families Fabaceae, Myrtaceae, Rosaceae, Apocynaceae). Diacytic type — there are two subsidiary cells whose adjacent sides are perpendicular to the stomatal pore (class Dicotyledones, families Lamiaceae, Caryophyllaceae). Tetracytic type — a stoma with four subsidiary cells, of which two are lateral and two are polar (class Monocotyledones, rarely Dicotyledones).

Fig. 1.14. Some types of the stomatal apparatus in angiosperms: A — anomocytic; B — anisocytic; C — paracytic; D — diacytic; E — tetracytic

The type of stomatal apparatus, the arrangement of stomata relative to the leaf surface (Fig. 1.13), the orientation of the stomatal pore relative to the longitudinal axis of the leaf, stomatal frequency per unit surface area, their relative dimensions, shape, and certain other features are specific to particular taxonomic and ecological groups of plants. For instance, in monocots, stomata are arranged in regular rows, and the stomatal pores are oriented along the axis of the organ, whereas in dicots, the arrangement of stomata is usually irregular, without a definite orientation of the stomatal pores (Fig. 1.12).

Epidermal cells can form specific outgrowths—trichomes (Fig. 1.15)—creating pubescence. Around trichomes, epidermal cells are often radially oriented, forming a rosette (Fig. 1.15, 3). The presence or absence of trichomes, their type, shape, structure, arrangement, and functioning pattern serve as diagnostic features. Trichomes are subdivided into simple and glandular hairs, scales, glands, and emergences.

Fig. 1.15. Epidermal trichomes: 1 — 11 — simple hairs: 1 — two-horned; 2 — retort-shaped; 3 — T-shaped; 4 — setaceous with a warted cuticle; 5 — whip-like with an elongated apical cell; 6 — multicellular, uniseriate, conical with a striate cuticle; 7 — hooked grabber; 8 — branched; 9 — stellate (top and side views); 10 — vesicular; 11 — papillose; 12 — 15 — glandular hairs: 12 — with a unicellular stalk and a multicellular HEAD; 13 — with a multicellular uniseriate long stalk and a unicellular small head; 14, 15 — with a multicellular head and a multicellular uni- or multiseriate stalk; 16 — 21 — emergences: 16 — stinging Hair (a — multicellular pedestal; b — ampulla; c — head); 17 — bristle; 18 — peltate gland; 19, 20 — thorn and spine (diagrams)

Simple, or covering, hairs (Fig. 1.15, 1 — 11) perform a protective function; they can be living or dead, unicellular or multicellular, uniseriate or multiseriate, unbranched, branched, and diverse in shape and size. Glandular, or capitate, hairs (Fig. 1.15, 12 — 15) and scales perform a protective-secretory function, consisting of a stalk, or stipe, and a secreting uni- or multicellular head of various shapes. Glands are trichomes with a short stalk and a well-developed multicellular secretory head covered by a cuticle. Emergences (Fig. 1.15, 16 — 20) are formed from both epidermal and underlying cells.

The periderm is a secondary complex dermal tissue. It forms on the stems of woody plants by the end of the first year of life, covers many underground organs, and occasionally fruits and other plant parts. It includes the meristematic tissue phellogen, or cork cambium, and derivatives of the phellogen—cork (phellem) and phelloderm (Fig. 1.16, A). Cork, or phellem, is a multi-layered, dead, dense, suberized, water- and gas-impermeable protective tissue. Phelloderm is a living, single- or multi-layered parenchymatous tissue. Differences in The structure of the periderm and primary cortex in various axial organs and among different plants are related to which tissue and at what depth within the cortex the phellogen originates. For example, it can develop from subepidermal cells or deeper layers of the cortical parenchyma (Fig. 1.16, A), in which case the Tissues of the primary cortex are fully or partially preserved. It can also arise from the pericycle, beneath the endodermis (Fig. 1.16, B), leading to the shedding of the entire primary cortex.

Fig. 1.16. Secondary dermal tissues: A, B — periderm with different locations of phellogen initiation; C, D — periderm with lenticels: On the surface of a twig, in cross-section; E — bark (rhytidome); 1 — epidermis; 2 — cork; 3 — phellogen; 4 — phelloderm; 5 — complementary tissue of the lenticel; 6 — cortical parenchyma; 7 — sclereids; 8 — cells with druses; 9 — endodermis

For water and gas exchange within the periderm, lenticels (Fig. 1.16, C, D) are formed from the phellogen beneath the epidermal stomata, appearing as loose areas, cracks, or swellings. They are round, oblong, lenticular, or of other shapes and specific coloration, which serves as a diagnostic feature for plants and Medicinal plant raw Materials. Lenticels function throughout the vegetation period and are closed for the winter by a layer of cork produced by the phellogen.

Bark (rhytidome) forms on tree trunks As a result of repeated initiation and activity of the phellogen. It consists of several periderms and layers of cortical tissue located between them (Fig. 1.16, E). Depending on the pattern of phellogen formation, scaly bark is distinguished—when phellogen layers are laid down at an angle to one another—and ring bark, when phellogen layers form parallel rings. Water and gas exchange through the bark is facilitated by fissures.

Epiblema — the protective and absorptive tissue of the ROOT (see p. 47).