MEDICAL BOTANY - A.G. Serbin - 2003

SECTION 1. ANATOMY

PLANT TISSUES

VASCULAR (CONDUCTING) TISSUES

These Tissues ensure the Transport of substances throughout the plant: the ascending current, which carries Water and dissolved mineral nutrients from the roots to the above-ground parts, and the descending current, which transports Photosynthesis products from the leaves to all other Organs. The ascending current is carried out through the tracheal elements of the xylem — vessels and tracheids — while the descending current moves through the sieve elements of the phloem (sieve Cells and sieve tubes with companion cells).

Vessels (or tracheae) are the most advanced and functionally efficient elements of the xylem. They develop from vertically arranged meristematic cells. During differentiation, they elongate, and their transverse walls become perforated (pored) or dissolve completely, while their longitudinal walls undergo localized thickening and lignification, resulting in the death of the protoplast. Mature vessels appear as segmented capillaries with porous walls or internal thickenings in the form of rings, spirals, or ladders (Fig. 1.21). The shape, dimensions of the segments, type of perforation and pitting, and The Nature of internal wall thickenings serve as important taxonomic features. Primary, procambial vessels are narrow, featuring annular and spiral thickenings. Secondary, cambial vessels are wider, with scalariform, pitted, or reticulate thickenings. Intermediate forms between all vessel types and individual vessel segments are frequently observed, such as annular-spiral or scalariform-pitted transitions. Vessels function for a relatively short time because they are gradually blocked by tyloses—parenchyma outgrowths protruding into the vessel lumen (Fig. 1.21, 7).

Class="center">Fig. 1.21. Vessels: 1 — annular; 2 — spiral; 3 — scalariform; 4 — pitted; 5 — reticulate; 6 — diagram of vessel segment articulation; 7 — spiral vessel with tyloses (longitudinal and transverse sections)

Tracheids are dead prosenchymatous cells with tapered ends and lignified Cell walls. They communicate with each other and transport substances via bordered pits. In the absence of pits, a tracheid is referred to as fiber-like and performs a mechanical function similar to libriform fibers. Like vessels, tracheids may feature internal scalariform and spiral thickenings (Fig. 1.23).

Sieve tubes (Fig. 1.22) develop from a vertical series of procambial or cambial cells. These cells elongate, and their transverse walls become perforated to form sieve plates. The tubular segments communicate through apertures within these plates known as sieve areas. The walls of sieve tubes are cellulosic, and although the protoplast persists, the nuclei and tonoplast disintegrate, causing the Cytoplasm to lose its selective permeability, irritability, and other living properties. Nevertheless, sieve tubes do not die because they are accompanied by subsidiary or companion cells, which arise from the longitudinal division of the sieve tube segment. These are living cells containing a Nucleus, dense cytoplasm, and a thin cellulosic Cell wall. They synthesize Enzymes that pass into the sieve tubes and sustain their metabolic activity. In autumn, the pores of the sieve plates become blocked with callose, which can redissolve in spring, thus restoring sieve tube function. Consequently, they may remain functional for several years.

Fig. 1.22. Phloem elements: 1 — 5 — development of a sieve tube with companion cells (in longitudinal and transverse sections): 1 — lateral meristem cells; 2 — 4 — Differentiation of the sieve tube segment and companion cells; 5 — mature sieve tube; 6 — Aging sieve tube containing callose; 7 — phloem elements in transverse section; a — sieve tube segment; b — sieve plate; c — companion cells; d — bast fibers; e — bast parenchyma

In plant organs, conducting tissues integrate with other elements to form complex tissues known as the xylem and phloem.

Xylem (wood) consists of primary (procambial) and secondary (cambial) elements that fulfill specific Functions: conducting tissues include vessels and tracheids; mechanical tissues comprise wood fibers (libriform); and storage tissues consist of wood parenchyma and substitute fibers (Fig. 1.23).

Fig. 1.23. Xylem elements: A — isolated elements in surface view; B — transverse section; 1 — vessel segment; 2, 3 — pitted and spiral vessels; 4 — 6 — tracheids; 4 — spiral; 5 — bordered-pitted; 6 — fiber-like; 7 — wood fiber; 8 — septate libriform fiber; 9 — substitute fiber; 10 — wood parenchyma; 11 — ray parenchyma

Phloem (bast) (Fig. 1.22) likewise comprises primary (procambial) and secondary (cambial) elements of various functions: conducting tissues (sieve cells or sieve tubes with companion cells), mechanical tissue (bast fibers), and storage tissue (bast parenchyma). Mechanical fibers are sometimes absent. Laticifers or other secretory structures frequently develop within the phloem.

The xylem and phloem generally accompany one another, forming conducting or vascular bundles (Fig. 1.24).

Fig. 1.24. Types of vascular bundles: A — collateral closed; B — collateral open; C — bicollateral; D — radial; E — concentric amphivasal (centro-phloem); F — concentric amphicribral (centro-xylem); 1 — phloem; 2 — xylem; 3 — cambium; 4 — sclerenchyma

Vascular bundles formed by the procambium that lack cambium are termed closed, whereas bundles containing cambium are designated as open due to their capacity for prolonged Secondary Growth. Depending on the mutual arrangement of the xylem and phloem, bundles are classified into collateral, bicollateral, concentric, and radial types.

Collateral bundles are characterized by the phloem and xylem lying side by side along the same radius. In axial organs, the phloem occupies the outer part of the bundle and the xylem the inner part, whereas in leaves this orientation is reversed. Collateral bundles can be closed (in monocotyledons) or open (in dicotyledons).

Bicollateral bundles are invariably open and possess two strands of phloem—inner and outer—with the xylem positioned between them. The cambium is located between the outer phloem and the xylem. Bicollateral vascular bundles are typical of representatives of families such as Cucurbitaceae, Solanaceae, Apocynaceae, and several others.

Concentric bundles are closed. They are amphicribral (centro-phloem) if the xylem surrounds the phloem, and amphivasal (centro-xylem) if the phloem surrounds the xylem. Amphicribral bundles are most commonly found in monocotyledonous plants, while amphivasal bundles occur typically in ferns (Pteridophytes).

Radial bundles are closed. In these bundles, the xylem and phloem alternate along different radii. Radial bundles are characteristic of the ROOT absorption zone as well as the root conduction zone in monocotyledonous plants.