PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 3. PLANT TISSUES

3.4. Anatomical, morphological, and functional features of the fundamental (parenchymal) tissues

Parenchyma is a permanent tissue that constitutes the bulk of the plant body. It occupies the spaces between all other permanent, more specialized Tissues and is therefore found in all plant Organs. For this reason, parenchyma is sometimes referred to as the fundamental tissue. When turgescent, parenchyma Cells are tightly packed, providing additional structural support to plant organs, especially herbaceous ones. When Water is lost, the cells collapse and the plants wilt. Parenchyma is the only permanent tissue capable of dedifferentiation and regaining meristematic activity. This allows for wound healing following plant damage, as well as organ regeneration during vegetative propagation. Parenchymal tissue is most often primary in origin, developing from the ground meristem of ROOT and SHOOT apices. Its cells are thin-walled, typically isodiametric, and occasionally elongated, with simple pits and primary walls. Sometimes, The Cell walls of parenchyma thicken and become lignified; such parenchyma is termed sclerenchymatous. Parenchyma is well developed in the primary cortex of stems and roots, in the pith, medullary rays, leaf mesophyll, and succulent fruits.

Parenchyma is a multifunctional tissue. Its cells vary in Structure and perform different Functions depending on their Location in the plant body. Fundamental (parenchymal) tissues function primarily as nutritive tissues, although functionally they can fulfill many roles, ranging from assimilation to excretion. Regardless of its position in the plant body, parenchyma facilitates the Transport of substances horizontally via cell walls (the apoplast) and cytoplasmic pathways (the symplast).

3.4.1. Types and functions of parenchymal tissues

Depending on their function and cellular structure, parenchyma is classified into assimilatory (chlorophyll-bearing tissue or chlorenchyma), aeriferous (aerenchyma), storage, transfusion, wood (sclerenchymatous), and bast parenchyma.

Assimilatory parenchyma (chlorenchyma) performs Photosynthesis (Fig. 26). Despite its relatively simple structure, it is precisely this tissue where solar energy is fixed and converted, and where the bulk of organic matter on Earth is synthesized. It consists of thin-lived, living cells containing METABOLISM/14.html">Chloroplasts, which is why it is frequently called chlorenchyma. Chlorenchyma is present in all green PARTS OF THE plant.

Class="center">Fig. 26. Diagram of Cell Structure in Different types of chlorenchyma: A—palisade; B—spongy (loose); C—folded: 1—chloroplasts; 2—intercellular spaces; 3—Cell wall invagination

Most often, it lies directly beneath the epidermis. This surface-proximal location of assimilatory tissue gives plant organs their green color. The overwhelming majority of assimilatory tissue is located in leaves, where it forms the mesophyll (from the Greek mesos, middle, and phyllon, leaf). The shape of chlorenchyma cells varies, giving rise to three distinct types: palisade, spongy (loose), and folded.

Cells of the palisade chlorenchyma are tabular in shape, fit tightly together, and contain A large number of active chloroplasts. Photosynthesis proceeds intensively in this chlorenchyma, accompanied by active cyclosis—the circular or streaming movement of Cytoplasm. Spongy chlorenchyma consists of more or less rounded cells with relatively large intercellular spaces. It is located deeper than the palisade chlorenchyma; photosynthetic processes occur less actively here, but the well-developed system of intercellular spaces ensures efficient gas exchange and water vapor movement. Differentiation into palisade and spongy chlorenchyma is observed only in leaves with a dorsiventral structure exposed to bright light. In pine needles, which typically persist for several years and experience seasonal fluctuations in Temperature and humidity, folded chlorenchyma develops. This evolved as an adaptive trait because the needle-like leaf shape drastically reduces the evaporating surface while still requiring efficient photosynthesis. Consequently, the cell walls in the mesophyll parenchyma of pine needles grow inward to form folds. This significantly increases the surface area of the cytoplasm layer—complete with chloroplasts—adjoining The cell wall, as well as the overall photosynthetic surface.

Storage parenchyma can develop in various organs and parts of the plant. It accumulates metabolic surplus products for a given developmental stage: Proteins, CARBOHYDRATES (sugars, inulin, starch), and Lipids (Fig. 27). These may be stored in liquid (sugars, oils) or solid (starch, proteins, etc.) form. Reserve substances typically accumulate inside the cell, but may also be deposited in the cell wall. Storage tissues are well developed in fruits, metamorphosed underground organs—root crops, root tubers, etc.—as well as in the pith and cortex of stems. The parenchyma of Vascular Tissues can also serve as a storage site for reserves. In seeds, reserve nutrients are concentrated in the endosperm (Apiaceae, Poaceae), embryo cotyledons (Fabaceae), or the perisperm surrounding the embryo (Caryophyllaceae, Chenopodiaceae). In the date palm, hemicellulose serves as a reserve product and is deposited in the endosperm cell walls.

Fig. 27. Storage parenchyma cells containing various substances: A—inulin crystals in root tuber cells of the garden dahlia (Dahlia variabilis); B—starch grains in petiole cells of the zonal geranium (Pelargonium zonale); C—aleurone grains in castor bean seeds (Ricinus communis): 1—inulin crystals; 2—compound starch grains; 3—aleurone grain; 4—aleurone grain wall; 5—protein crystalloid; 6—protein globoid

In plants of arid habitats (xerophytes), water-storage parenchyma develops (cacti, Crassulaceae family). A well-developed water-storage tissue is present in the tuberous stem swellings of many epiphytic orchids, as well as in the metamorphosed roots of certain Liliaceae (Chlorophytum, asparagus). The thin-walled cells of the water-storage parenchyma contain abundant mucilage, which retains moisture. When the plant dries out, water-storage cells transfer water primarily to the assimilatory tissues.

The vast majority of reserve nutrients function as reserves and are re-engaged in metabolic processes at specific stages of plant development.

Air-storage parenchyma, or aerenchyma, is parenchyma featuring extensively developed intercellular spaces. The primary function of this tissue is the aeration of vegetative organs subjected to low oxygen conditions. It is characteristic of aquatic and marsh plants, but also occurs in terrestrial species (e.g., rushes growing in poorly aerated soils). The intercellular spaces of this tissue contain a significant amount of gas differing in composition from atmospheric air, which supplements the supply of oxygen or carbon dioxide to the leaf. Additionally, the aerenchyma of aquatic plants provides buoyancy. Aerenchyma develops in roots, stems, and leaves, and occasionally forms in fruits (snowberry) (Fig. 28).

Fig. 28. Aerenchyma: A—in the leaf petiole of the white water lily (Nymphaea alba); B—in the stem of the soft rush (Juncus effusus); C—in the stem of shining pondweed (Potamogeton lucens); D—in the root of creeping primrose-willow (Jussiaea repens): 1—cuticle; 2—epidermis; 3—cells of air-storage parenchyma (aerenchyma); 4—air cavities; 5—central cylinder

Transfusion (transfer) parenchyma forms in the leaves of conifers between the vascular bundles and the endodermis. It consists of living cells with primary walls and participates in translocation. Functionally, transfusion parenchyma cells are similar to albuminous cells (Strasburger cells), which are characteristic of gymnosperm phloem. This type of parenchyma, together with transfusion tracheids—dead water-conducting cells—comprises the transfusion tissue. It facilitates The transfer of both photosynthetic products from the leaf chlorenchyma to the vascular bundles, and water from the vascular bundles to the chlorophyll-bearing parenchyma.

Wood and bast parenchyma represent specialized types of parenchymal tissues.

Wood (sclerenchymatous) parenchyma is a living tissue with thickened cell walls that forms vertical rows (vertical or strand parenchyma) in the secondary xylem. The walls of wood parenchyma are slightly lignified with numerous simple pits. Strand parenchyma functions to store nutrients, with starch serving as the primary reserve substance. Strand parenchyma is particularly well expressed in woody plants.

Depending on their arrangement relative to xylem vessels, wood parenchyma is classified into apotracheal and paratracheal parenchyma.

Apotracheal parenchyma is not associated with vessels. It may be diffuse, terminal, or metatracheal (Fig. 29). Diffuse parenchyma cells are scattered randomly among the elements of the growth ring (annual ring). Apotracheal parenchyma formed at the end of the annual growth period (in autumn) is termed terminal. Apotracheal parenchyma arranged in short tangential bands relative to the surface is called metatracheal.

Fig. 29. Apotracheal parenchyma. Schematic arrangement of strand wood parenchyma cells in the secondary xylem of dicotyledonous trees: A — diffuse; B — metatracheal; C — terminal: 1 — vessels; 2 — strand (vertical) parenchyma; 3 — outer layer of late wood (annual growth ring boundary); 4 — wood of the new annual ring (early wood)

Paratracheal parenchyma surrounds the vessel, forming a complete or partial sheath. It can be vasicentric, aliform, or confluent (Fig. 30). Vasicentric parenchyma forms a complete sheath around the vessel. Aliform parenchyma adjoins only certain vessels, forming wing-like extensions. When the aliform sheaths of adjacent vessels merge with one another, the paratracheal parenchyma is referred to as confluent.

Fig. 30. Paratracheal parenchyma. Schematic arrangement of strand wood parenchyma cells in the secondary xylem of dicotyledonous trees: A, B — vasicentric parenchyma; C — aliform parenchyma; D — confluent parenchyma: 1 — vessels; 2 — strand (vertical) parenchyma

Bast (phloem) parenchyma is a component of the secondary phloem. Much like wood parenchyma, it is strand-like (vertical), directly adjacent to sieve elements, and interacts closely with them. It is believed that living cells of the phloem parenchyma produce and supply Enzymes to the sieve tubes that are necessary to accelerate The transport of photosynthetic products. In addition, phloem parenchyma can store nutrient reserves as well as Metabolic waste products (excretes). The neoformation of phloem parenchyma cells leads to phloem dilatation (expansion), while the thickening of the parenchymal cell walls culminates in The formation of sclereids, which helps increase the mechanical strength of the stem and root. Phloem parenchyma cells can dedifferentiate and form phellogen cells, which subsequently give rise to internal periderms.

3.5. Anatomical, Morphological, and Functional Features of Mechanical (Skeletal) Tissues

Mechanical tissues in the plant body perform a supporting function, providing a specific configuration to individual organs. Most commonly, they act as a skeletal framework, interspersed among other tissues to impart additional strength. These tissues enable plants to resist static (gravity) and dynamic (gusts of wind, heavy rain, etc.) loads. Their general and principal feature is the thickening of cell walls and the tight packing of structural elements. In the course of plant evolution, mechanical tissues emerged in connection with the refinement of branching and The Development of the vegetative mass.

Depending on the timing of their formation during plant ontogeny, Primary and secondary mechanical tissues are distinguished: primary tissues originate from the ground meristem, whereas secondary tissues develop from the cambium.

There are two MAIN TYPES OF mechanical tissues: collenchyma and sclerenchyma.

3.5.1. Collenchyma as a Type of Mechanical Tissue

Collenchyma (from Greek colla, glue, and enchyme, infusion) is a tissue composed of living cells, 1–2 mm in length, with unevenly thickened primary cell walls. These thickenings develop on the primary wall predominantly through the deposition of small amounts of Cellulose and abundant pectic substances. They are never impregnated with Lignin and do not undergo lignification; they remain elastic and stay alive, as a rule, throughout the entire life of the plant. Because the cell walls retain a Primary Structure, they are capable of stretching and do not impede the growth of adjacent cells. For this reason, collenchyma serves as the primary supporting tissue in young organs undergoing elongation. Collenchymal cells elongate parallel to the long axis of the organ in which the tissue develops. An important feature of collenchyma is that it performs its supportive function only in a turgid state. Otherwise, when the plant loses water, the thin Regions of the cell walls collapse, shoots lose their rigidity, and the plant wilts.

Collenchymatous tissues are of primary origin, often developing from the outer layers of the shoot apical meristem (collenchyma is absent in roots), and are therefore located (in dicots) beneath the epidermis in the primary cortex. Collenchyma typically forms a continuous cylinder or is localized in ridges (such as in the stems of deadnettle or the petioles of plantain and celery), which unquestionably enhances the mechanical strength of the organ. The presence of collenchyma in the leaf petioles of so-called compass plants or beneath the inflorescence (in sunflowers) enables these plants to reorient their leaves or inflorescences in space following THE POSITION OF the sun. In the leaves of certain dicots, collenchyma may be situated above and below the vascular bundles, providing structural support. In monocots, collenchyma is found in the nodes of grasses, protecting the plants against lodging.

Depending on the pattern of cell wall thickening, Three types of collenchyma are distinguished: angular, lamellar, and lacunar (Fig. 31).

Fig. 31. Types of collenchyma: A — lamellar collenchyma in the stem of a one-year-old shoot of English oak (Quercus robur); B — angular collenchyma in the leaf petiole of royal begonia (Begonia rex); C — lacunar collenchyma in the stem of deadly nightshade (Atropa belladonna): 1 — epidermis; 2 — cell wall thickening; 3 — cell lumen; 4 — intercellular spaces; 5 — developing periderm

In angular collenchyma, cell wall thickenings are localized at the cell corners where adjacent cells meet. As a result, the cells appear polygonal (5–6-sided), while the aggregated thickened walls of neighboring cells form 3–5-sided configurations. The thickened regions of the cellulosic walls have a glossy appearance. Angular collenchyma is well developed in the petioles of begonias, rhubarb, beets, pumpkin, sorrel, buckwheat, and other plants.

In lamellar collenchyma, the tangential cell walls (parallel to the organ surface) are significantly thickened. During Cytology/cytology/16.html">Early stages of tissue development, the radial walls in such cells remain thinner. Lamellar collenchyma represents a sturdier tissue than angular collenchyma. It develops in the stems of sunflowers, elderberry, eggplant, and masterwort.

Lacunar collenchyma initially begins to form as angular collenchyma, but subsequently large intercellular spaces arise due to cell Separation. These spaces act as schizogenous reservoirs that accumulate mucilaginous secretions or function in aeration. The cell lumen is correspondingly reduced and becomes barely visible. This type of collenchyma can be observed in the petioles of burdock, coltsfoot, and in the floral scapes of members of the Liliaceae family (such as Vallota, lilies, and garlic).

Collenchyma cells, located beneath the epidermis in the peripheral region of the primary cortex, may contain chloroplasts and perform not only a supportive function but also photosynthesis. Over time, the walls of collenchyma cells occasionally undergo further thickening and lignification, transforming the collenchyma into sclerenchyma. In some cases, collenchymal cells may lose their thickenings and revert to storage parenchyma.

3.5.2. Sclernechyma as a Type of Mechanical Tissue

Sclerenchyma (from Greek scleros, hard, and enchyme, infusion) is a much stronger mechanical tissue. It occurs in all plant organs and is located not only beneath the epidermis but also in other parts of the plant. Unlike collenchyma, sclerenchyma cells possess uniformly thickened cell walls interspersed with numerous simple pits (Fig. 32). Typically, sclerenchyma cells develop secondary cell walls. The walls of mature sclerenchyma cells become impregnated with lignin and lignify, while the living cell contents gradually degenerate and die. Thus, unlike collenchyma, mature sclerenchyma cells are most commonly dead. Such cells are incapable of stretching; therefore, their final maturation occurs only after the growth of surrounding cells has ceased. The walls of sclerenchymatous cells exhibit exceptionally high tensile strength, approaching that of steel. They withstand dynamic loads effectively without undergoing permanent deformation, thereby providing resistance to compression, tension, and bending. High tensile strength allows for significant stretching without damaging plant organs, while compressive strength ensures adequate resistance to bending.

Fig. 32. Sclerenchyma: A, B — longitudinal and transverse sections of fibers in the stem of zonal pelargonium (Pelargonium zonale); C — astrosclereid in the leaf blade of white water lily (Nymphaea alba); D — sclereid from a coconut shell (Cocos nucifera); E — stone cells from the fruit pulp of pear (Pyrus communis): 1 — secondary cell wall; 2 — pit canals; 3 — cell lumen; 4 — Nucleus; 5 — central region of the cell

In some instances, sclerenchyma cells do not undergo lignification, which is characteristic of fiber crops (fibers of flax, kenaf, and hemp dogbane). In textile plants, the fibers remain cellulosic, retaining flexibility and elasticity. In rare cases, owing to numerous branched simple pits, sclerenchyma cells retain their living contents (such as stone cells in fruit flesh).

Depending on their origin, sclerenchyma is divided into primary and secondary. Primary sclerenchyma most commonly originates from the shoot apical Meristems and is located in the stem beneath the epidermis (in monocotyledons). When formed from procambial strands, it is situated within the bundle sheath. Primary sclerenchyma can also develop from the pericycle. Pericyclic sclerenchyma is typical for the stems of birthwort, Solomon's seal, and flax.

Secondary sclerenchyma develops from the cambium and either forms part of the vascular bundle sheath or is diffusely scattered among the parenchymatous tissues of various plant parts.

Sclerenchyma is subdivided into fibers and sclereids (see Fig. 32).

Fibers are represented by strongly elongated prosenchymatous cells with tapered ends (slanted transverse walls). These cells grow via intrusive (gliding) growth and overlap one another, which increases the mechanical strength of the tissue. Fibers possess thick cell walls and very narrow cell lumens. Their length averages 1–4 mm, but in fiber crops it is significantly greater: up to 60 mm in flax and up to 350 mm in ramie. The longer the fibers, the higher their technical performance indicators. The fiber grade in flax reflects its length: the higher the grade, the longer the fiber. Fibers located in the phloem (often referred to as bast) are called bast fibers. Sclerenchymatous fibers found in the xylem (wood) are termed wood fibers, or libriform fibers. They are shorter than bast fibers (no more than 2 mm) and always become lignified. It is hypothesized that during evolution, libriform fibers originated from scalariform tracheids—the water-conducting elements of the xylem.

Fibers may also occur as part of other tissues, arranged either in groups or singly. In many plants, especially monocotyledons, fibers form the mechanical sheath of vascular bundles. The mechanical tissue elements that make up fibrovascular bundles are characterized by a relatively short length and thinner cell walls compared to mechanical elements located near the organ surface.

Sclereids are cells close in shape to parenchyma, featuring thick, secondary layered walls and simple branched pits. Their contents typically degenerate and die, though they sometimes persist, as in the fruits of pear and quince. In these plants, as the fruits ripen, the cells un-lignify and become thin-walled parenchymatous storage cells. Sclereids are usually most abundant in the bark (cranberry), pith, phloem of axial organs (cinchona tree), as well as in fruits and seeds (legumes), and leaves. Sclereids in various plant organs exhibit diverse shapes. Roundish-elliptical sclereids are termed brachysclereids (from Greek brachys — short) or stone cells. They typically occur in groups and are abundant in unripe pear and quince fruits. Astrosclereids (from Greek astron — star) are characterized by elongated cell branches. They are most commonly found in thick, leathery leaves (ficus, camellia, fig), in the petioles of certain plants (water lily, yellow pond-lily), and provide resistance against tearing. Sclereids resembling a leg bone are called osteosclereids (from Greek osteon — bone).

Sclereids may also occur individually, in which case they are referred to as idioblasts. Idioblasts form in the leaves of fig and camellia. In the endocarp of fleshy fruits such as cherry, plum, and walnut, as well as in the exocarp of dry fruits like hazel and filbert, idioblasts form a durable mechanical structure known as the stone (pit).

G. Haberlandt proposed referring to the entire assemblage of thick-walled, lignified plant cells as the stereome (from Greek stereos — solid, firm). The degree of mechanical tissue development largely depends on the plants' habitat. It is minimal in hydrophytes—plants of aquatic environments, and substantial in xerophytes—plants of arid habitats.

Mechanical tissues have a specific localization within the plant body: in roots, they are situated in the central region, whereas in stems, they form a peripheral ring. The exposure of shoots to diverse, rapidly fluctuating, and multidirectional loads (gusts of wind, raindrops, trampling by animals) requires a more efficient arrangement of supporting tissues. According to S. Schwendener's concepts, this arrangement in above-ground organs corresponds to the I-beam principle in structural engineering, providing maximum resistance of the organ to bending, tension, and fracture. As for the root as an underground organ, its spatial position is additionally supported by the soil; consequently, the mechanical Tissues of the root primarily provide compressive strength. It should be noted that the Structure of Plant organs adheres to THE PRINCIPLE OF achieving strength with economical material expenditure. The renowned Russian botanist V. F. Razdorsky, who studied structural and mechanical principles in plant design, compared them to reinforced concrete structures, in which mechanical tissues serve as the reinforcement (rebar), while all other living tissues act as the filler. The Structural Features of plants are taken into account in bionics—a scientific field dedicated to creating advanced technical systems.