BIOLOGY Volume 1 - A Guide to General Biology - 2004

6. HISTOLOGY

6.2. Plant Tissues Composed of Several Cell Types

Plants contain Two Types of vascular tissue — xylem and phloem — each of which is made up of several Cell types (Fig. 6.1). Together they form the vascular tissue system, the Functions of which, related to the Transport of substances throughout the plant, are discussed in Ch. 13. The xylem transports primarily Water and mineral salts in an upward direction—from the roots to other PARTS OF THE plant—whereas the phloem mainly transports Organic compounds originating in the leaves and moving both upwards and downwards. The amount of xylem and phloem can increase through Secondary Growth (Ch. 22). Secondary xylem, which sometimes develops very extensively, is referred to as wood.

6.2.1. Xylem

Xylem performs two main functions in the plant: it transports water and dissolved mineral nutrients, and it provides structural support. Thus, xylem plays a dual role—physiological and structural. Xylem tissue consists of four types of elements: tracheids, vessels, parenchyma Cells, and fibers. These histological elements are shown in both transverse and longitudinal sections in Fig. 6.9.

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Fig. 6.9. Structure of primary xylem. A. Transverse section. B. Longitudinal section. C. Transverse section of primary xylem from a Helianthus stem. D. Cytology/practical/54.html">Longitudinal section of primary xylem from a Helianthus stem.

Tracheids

Tracheids are single, lignified, spindle-shaped cells. The ends of adjacent tracheids overlap in much the same way as the tapered ends of sclerenchyma fibers. This gives tracheids mechanical strength and provides structural support to the plant Organs. Tracheids are dead cells; at maturity, their lumens are empty. Among the water-conducting cells of vascular plants, tracheids represent the primary, primitive form; in ancient vascular plants, they are the only water-conducting cells. The vessels and xylem fibers of higher plants, described below, evolved from them. Despite their primitive nature, tracheids undoubtedly function with high efficiency, as evidenced by the fact that in conifers—most of which are woody species—water Transport from the roots to the aerial parts is carried out exclusively by tracheids. Water moves through the empty lumens of the tracheids without encountering any obstruction from living cell contents. It passes from one tracheid to another either through pits and their closing membranes (shown in Fig. 6.8) or through unlignified portions of The Cell walls. The pattern of Cell wall lignification in tracheids is similar to that described below for vessels. The structure of tracheids is illustrated in Fig. 6.10. In angiosperms, the number of tracheids is relatively small compared to the number of vessels. Vessels are considered more efficient adaptations for water transport than tracheids; the appearance of vessels is believed to be associated with the higher rates of Transpiration in angiosperms, which possess large leaf surfaces.

Fig. 6.10. Structure of tracheids. A. Tracheid with bordered pits (like vessels, tracheids may have annular, spiral, scalariform, and reticulate thickenings; see Fig. 6.12, G). B. Tracheids from macerated Pinus wood, ×120.

Vessels

Vessels are characteristic conducting elements of the angiosperm xylem. They are very long tubes formed by the fusion of a series of cells joined end-to-end. Each cell making up a xylem vessel corresponds to a tracheid and is called a vessel member (or vessel element). However, vessel members are shorter and wider than tracheids. The first xylem to appear during plant development is called primary xylem; it originates at the ROOT tips and SHOOT apices. Differentiated xylem vessel members appear in rows at the ends of procambial strands. A vessel is formed when adjacent members in a given row fuse as the dividing walls between them break down. Remnants of the ruptured end walls persist inside the vessel as rim-like structures. The fusion of vessel members is illustrated in Fig. 6.11.

Fig. 6.11. Fusion of individual members during vessel formation.

Protoxylem and Metaxylem

The earliest formed vessels—the protoxylem—develop at the apices of axial organs, directly beneath the apical meristem, in regions where the surrounding cells are still elongating. Mature protoxylem vessels are capable of stretching as the surrounding cells elongate because their Cellulose walls are not yet fully lignified—Lignin is deposited only in rings or spirals (Fig. 6.12). These lignin deposits allow the tubes to maintain sufficient rigidity during the growth of the stem or root. As the organ grows, new xylem vessels appear that undergo more extensive lignification and complete their development in the mature Regions of the organ, thus forming the metaxylem. Meanwhile, the earliest protoxylem vessels are stretched and eventually destroyed. Mature metaxylem vessels are incapable of stretching and growing. They are dead, rigid, fully lignified tubes. If their development were completed before the elongation of the surrounding living cells had ceased, they would severely hinder this process.

Metaxylem vessels exhibit three MAIN TYPES OF wall thickenings: scalariform, reticulate, and pitted (Fig. 6.12).

Fig. 6.12. Structure of protoxylem and metaxylem vessels. A. Protoxylem vessels. B. Micrograph of annular and spiral protoxylem vessels. C. Micrograph of reticulate metaxylem vessels from macerated wood. D. Pitted and reticulate metaxylem vessels. E. Micrograph of a pitted metaxylem vessel from macerated wood; F. Scanning electron micrograph of metaxylem vessels, ×18,000. The appearance of these vessels in transverse section varies depending on the level at which the section passes through the vessel; this can be understood by referring to the schematic diagram in Fig. 6.12, A (see the leftmost vessel). G. Transverse section through a bordered pit.

The long, hollow xylem tubes provide an ideal system for long-distance water conduction with minimal resistance. Just as in tracheids, water can pass from vessel to vessel through pits or unlignified regions of The cell wall. Due to lignification, the cell walls of vessels possess high tensile strength, which is also crucial because it prevents the tubes from collapsing when water moves through them under tension (Sec. 13.4).

Xylem also performs its second function—mechanical support—by virtue of being composed of a series of rigid, lignified tubes. In the primary plant body, the xylem occupies a central position in the roots, helping them resist the pulling forces exerted by aerial parts bending in gusts of wind. In the stem, vascular bundles either form a peripheral ring, as in dicotyledons, or are scattered throughout, as in monocotyledons; in both cases, the stem is permeated by discrete strands of xylem that provide structural support. The supportive function of xylem becomes especially important where secondary growth takes place. During this process, the amount of secondary xylem increases rapidly; it takes over the primary mechanical role from collenchyma and sclerenchyma, serving as the main support in large trees and shrubs. The radial growth of trunks is determined to a certain extent by the mechanical loads experienced by the plant, so that additional growth sometimes occurs to reinforce the structure and ensure maximum support.

Xylem Parenchyma

Wood parenchyma is found in both Primary and secondary xylem, though it is more abundant and plays a more significant role in the latter. Like any other parenchymal cells, wood parenchyma cells have thin cellulosic walls and living contents.

There are two systems of parenchyma in secondary xylem. Both originate from meristematic cells referred to as ray initials in the one case and fusiform initials in the other (Ch. 22). Ray parenchyma is more abundant. It forms radial layers of tissue known as medullary rays, which permeate the pith and serve as a living link between the pith and the bark. Various nutrients are stored here, along with accumulations of Tannins, crystals, and the like; this is also the site of radial transport of nutrients and water, as well as gas exchange via intercellular spaces.

Xylem vessels and phloem sieve tubes, together with their companion cells, typically develop from fusiform initials, though they occasionally give rise to parenchymal cells as well. These parenchymal cells form vertical rows within the secondary xylem.

Wood fibers

Wood fibers, much like xylem vessels, are believed to have evolved from tracheids. They are shorter and narrower than tracheids, and their walls are considerably thicker. However, their pits resemble those found in tracheids, and in cross-sections, fibers are sometimes difficult to distinguish from tracheids due to the existence of a series of transitional forms between the two. Wood fibers closely resemble the previously described sclerenchyma fibers, with their end walls also overlapping. Unlike xylem vessels, wood fibers do not conduct water; consequently, they can feature much thicker walls and narrower lumina, rendering them mechanically stronger—that is, providing the xylem with added structural support.