MEDICAL BOTANY - A.G. Serbin - 2003

SECTION 1. ANATOMY

PLANT TISSUES

MECHANICAL TISSUES

Mechanical, skeletal, or supportive Tissues provide plants with structural strength, maintain their spatial posture, and protect Organs from tearing, breaking, stretching, and damage. Mechanical tissues consist of parenchymatous or prosenchymatous Cells with thickened cellulosic or lignified Cell walls that can sometimes rival steel in durability. Mechanical tissues include collenchyma and sclerenchyma.

Collenchyma is a living tissue characterized by unevenly thickened cellulosic cell walls. Its cells range from parenchymatous to prosenchymatous, featuring slanted or tapered ends. The protoplast often contains METABOLISM/14.html">Chloroplasts, allowing the tissue to perform Photosynthesis. Collenchyma, typical of dicotyledonous plants, is located along the periphery of the stem in isolated patches, forming a continuous or interrupted ring. In ridged stems and petioles, it fills the ridges; in leaves, it reinforces the Veins and leaf margins. Depending on the pattern of wall thickening and cell packing density, collenchyma is classified into angular, lamellar, and lacunar types (Fig. 1.20, 1 – 3). Angular collenchyma is composed of tightly packed rectangular cells with walls thickened at the corners, and is particularly well-developed in petioles and leaf veins. Lamellar collenchyma features the thickening of tangential walls that run parallel to the organ's surface; it typically occurs in stems beneath the dermal tissue in several or many layers. Lacunar collenchyma consists of cells with more or less uniformly thickened walls, accompanied by intercellular spaces that vary in development depending on the organ and Water availability.

Class="center">Fig. 1.20. Mechanical tissues: 1 – 3 — collenchyma: 1 — lamellar; 2 — angular, 3 — lacunar; 4 – 7 — sclereids: 4 — brachysclereids; 5 — macrosclereids; 6 — osteosclereids; 7 — fiber-like sclereids; 8 – 9 — fibers: 8 — xylary (wood) fibers; 9 — phloem (bast) fibers in cross-section; 10 — parts of a bast fiber in longitudinal section: tip, narrow section with oblique slit-like pits in the layered wall, expanded section with a reticulate Cell wall; 11 — bundle of sclerenchymatous fibers (technical fiber)

Sclerenchyma is a dead tissue featuring thickened, lignified cell walls. It is subdivided into sclereids and fibers.

Sclereids (Fig. 1.20, 4 – 7) vary considerably in shape, size, and cell wall Structure, which holds taxonomic significance. Their walls are typically heavily thickened and pierced with slit-like and branched pits. Several types of sclereids are distinguished: brachysclereids, or stone cells — short, isodiametric sclereids developing in fruit pulps, bark, phloem, and the pith of axial organs; macrosclereids — elongated, rod-shaped cells that form the seed coat (in legumes); osteosclereids — shaped like dumbbells or long bones, found in leaf mesophyll, seed coats, and pericarp; astrosclereids, or star-shaped sclereids, along with branched, thin-walled trichosclereids — found in the leaves of certain dicots; and thread-like or fiber-like sclereids — located in axial organs.

Sclerenchymatous fibers are prosenchymatous cells arranged closely together in groups forming strands, cylinders, sheaths, or, less commonly, scattered individually. Fibers are classified according to their origin (primary, secondary), structure, and anatomical Location.

• Xylary (wood) fibers, or libriform fibers (Fig. 1.20, 8) — located in the xylem, providing it with strength and hardness. True libriform cells possess lignified walls ranging from 1 to 2 mm in length. Depending on their developmental stage and structure, varieties of libriform include substitute fibers and septate libriform fibers — partially lignified tissues containing living contents that perform supportive and storage Functions.

• Phloem (bast) fibers (Fig. 1.20, 9 – 11) are located in the bast (phloem). They are long (from 4 to 350 mm), narrow cells with thickened, striped-layered cell walls—sometimes cellulosic, but more often partially or fully lignified—penetrated by oblique pits. The stratification of the walls depends on the alternation of layers with varying contents of Cellulose, hemicellulose, and pectic substances, while their striped appearance results from the reticulate arrangement of fibrils. Fiber length, the shape of fiber tips, and The Nature of intercellular connections during The formation of fibrous strands are plant species-specific traits. For instance, flax fibers have tapered ends, hemp fibers have club-shaped ends, and kenaf fibers feature serrated, interlocking ends, ensuring a strong connection and the continuity of the fibrous strand.

• Cortical fibers are located in the bark of axial organs in bundles or individually. In monocotyledonous plants, they are frequently situated beneath the epidermis.

• Pericyclic, or perivascular, fibers originate from the pericycle and are situated along the periphery of the central cylinder.

• Sheathing fibers are formed from the procambium or ground tissue around vascular bundles, serving as a structural framework and support for them.