PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 3. PLANT TISSUES

3.6. Anatomical, morphological, and functional features of vascular tissues

The Emergence of Vascular Tissues is a defining characteristic of higher plants. Among modern higher plants, only bryophytes lack true vascular tissues, which places them among primitive organisms. The formation of specialized vascular tissues significantly advanced plants not only in morphological complexity but also in structural Organization. Thanks to vascular tissues, all plant Organs became interconnected into a unified, continuous, and harmoniously functioning system.

Vascular tissues provide the upward and downward transport of nutrients and are represented by the xylem (from the Greek xylon, meaning wood) and the phloem (from the Greek phloios, meaning bark).

Water and dissolved mineral nutrients absorbed from the soil by the roots, as well as Organic compounds synthesized within the roots themselves, are transported upward from the roots to the aerial PARTS OF THE plant through the xylem.

Through the phloem, assimilation products—predominantly CARBOHYDRATES—are transported from photosynthetic organs (leaves, green stems) to the roots and other plant parts (buds, flowers, fruits).

Xylem and phloem are complex tissues comprising not only conducting elements but also elements of other tissues—mechanical, fundamental (parenchyma), and occasionally secretory. Vascular tissues originate from lateral vascular Meristems: the primary meristem (procambium) and the secondary meristem (cambium).

3.6.1. Xylem. Characteristics of Water-conducting elements of the xylem

The water-conducting (tracheal) elements of the xylem are tracheids and tracheae (vessels). Fully formed (mature) tracheal elements are dead Cells. Their protoplast disintegrates, and The Cell lumen gradually becomes filled with aqueous solutions. Functionally they are similar, but they differ significantly in morphological features.

The most ancient water-conducting elements are tracheids (Fig. 33). They have already been identified in rhyniophytes. Tracheids are long, narrow, prosenchymatous cells with tapered ends. The length of tracheids ranges from 1 to 4 mm, and in agave, it can reach up to 10 mm. At the same time, the diameter of tracheids is only a fraction of a millimeter. Tracheids are characterized by intrusive (sliding) growth, whereby newly forming tracheids wedge their tapered ends between already established cells to form strands. The oblique ends of the tracheid Cell wall lack pits. Pits are located exclusively on the vertical walls, which results in a relatively slow Movement of water through tracheids. In primitive tracheids, The cell wall has a Primary Structure, though patches of secondary wall develop within it in the form of rings or spirals. These secondary thickened areas of the cell wall become impregnated with Lignin and lignify. Due to these spiral and annular thickenings, the thin-walled Regions of the tracheid do not collapse under water stress, enabling tracheids to perform their water-conducting function. Tracheids with annular and spiral thickenings of the cell wall are called annular and spiral tracheids. These are the most ancient types of tracheids. During evolution, their structure became more refined. The cell wall grew progressively stronger as the primary wall, upon cessation of tracheid growth, was replaced by a secondary wall, which subsequently underwent lignification. The close arrangement of annular and spiral thickenings, along with the formation of interconnecting bridges between them, gave rise to reticulate tracheids. Only the areas bearing pits remained unlignified. The configuration of pits can vary. When pits are elongated transversely and arranged one above another, the tracheids are termed scalariform; when rounded, they are termed pitted. The presence of pits on the longitudinal walls facilitates the lateral Transport of substances. In addition to water-conducting Functions, scalariform and pitted tracheids also serve a mechanical (supporting) role.

Class="center">Fig. 33. Various types of tracheids: A—spiral tracheid with spiral thickening of the cell wall; B—pitted tracheid with rounded bordered pits; C—fiber tracheid with slit-like bordered pits

It is hypothesized that scalariform tracheids with bordered pits may have given rise to vessel elements, or tracheae. The transformation of scalariform tracheids into vessel elements can be envisioned as follows. Initially, the inclination of the terminal cell walls shifted, becoming progressively more horizontal. Scalariformly arranged elongated or rounded pits partially shifted onto this region of the cell wall. Under METABOLISM/18.html">The Influence of water flow, the closing membranes of the pits gradually broke down, forming through-openings—perforations—in the oblique cell wall. Thus, the cell walls of adjacent cells transformed into

scalariform perforation plates with numerous bars. In the course of evolution, the wall segments separating the perforations disappeared, the number of perforations in the cell wall decreased, and they became wider. Concurrently with the reduction in the number of perforations, the inclination of the cell wall changed until it became horizontal. Subsequently, all partitions within the horizontal wall disappeared, and the perforation became simple.

Evidence supporting the occurrence of such a transformation from scalariform tracheids with bordered pits into vessel elements is the presence of elements with various types of scalariform perforation plates in the xylem of Representatives of the families Degeneriaceae, Magnoliaceae, and several others. Primitive vessel elements differ from tracheids by their shorter length and greater width. The length of vessel elements varies (0.1 mm in seepweed (Suaeda fruticosa), 1.1 mm in magnolias, 1.6 mm in katsura tree (Cercidiphyllum japonicum)). As the length of a vessel element decreases, its width increases. In English oak, the diameter of a vessel element is approximately 0.6 mm. The most primitive types of vessels possess very long elements and long perforation plates with scalariform perforations, sometimes featuring over 50 bars on a very oblique wall. Highly advanced vessel types possess short elements and simple perforations on a transverse perforation plate (A. L. Takhtajan, 1964) (Fig. 34).

Fig. 34. Stages of evolution of scalariform perforation (from left to right): from primitive perforation with numerous bars to specialized perforation having

only a few primitive bars

A vessel, or Trachea, is a water-conducting element of the xylem consisting of numerous cells arranged end-to-end with perforated horizontal walls, known as vessel elements (Fig. 35). Much like tracheids, the cell wall of tracheas exhibits varied structure, configuration, and pit arrangement. Furthermore, like tracheids, tracheas are dead at maturity. Annular, spiral, scalariform, and pitted vessels are distinguished (Fig. 36). Both the types of tracheids and tracheas and The sequence of their appearance in ontogeny mirror their evolutionary sequence: annular and spiral elements form first, followed later—once the organ ceases longitudinal expansion—by reticulate, scalariform, and pitted elements.

Fig. 35. Stages of evolution of vessel elements with simple perforation

Fig. 36. Vessel types: A—annular; B—spiral; C—reticulate; D—scalariform; E—pitted: 1—primary wall of the vessel; 2—annular thickening (fragment of the secondary wall); 3—spiral thickening; 4—secondary wall; 5—pits of varying shape and arrangement within the secondary wall

The length of vessels varies among different plants. It is entirely possible that the length of the vessels equals the length of the plant itself.

Xylem development in ontogeny. Classification of xylem. During ontogeny, vessel elements develop from procambium or cambium cells, much like tracheids. As a vessel element forms, The structure of the lateral walls in the differentiating meristematic cells changes: they become secondary walls and acquire a distinctive appearance, which is reflected in the name of the vessels. If the vertical walls of the vessels retain a primary structure, while the secondary thickenings appear as rings or spirals, the vessels are termed annular and spiral. Typically, these vessels function for a short period and are frequently destroyed under the pressure of other, later-developing xylem elements. In the course of ontogeny, scalariform and pitted vessels develop alongside annular and spiral ones. They are named According to the arrangement and outline of the pits on their vertical walls. The horizontal walls retain their primary structure and gradually break down, resulting in the formation of perforations surrounded by a secondary cell wall rim that acts as a structural "strut" to increase vessel strength. During the formation of a vessel element, the living Contents of the meristematic cells degenerate (Fig. 37).

Fig. 37. Diagram of pitted vessel development during ontogeny: A—strand of vascular meristem (procambium); B—enlargement of procambial cells; C—Cytology/cytology/16.html">Early stages of vessel element formation: Development of the primary cell wall with primary pit fields on the vertical walls, and appearance of perforations on the horizontal walls; D—Formation of the secondary cell wall and pits on the vertical walls, breakdown of the horizontal cell wall, and formation of large perforations; E—fragment of a mature pitted vessel: 1—primary cell wall of the vessel element; 2—primary pit fields; 3—perforations in the horizontal walls; 4—remnants of the primary horizontal cell wall; 5—Nucleus; 6—cytoplasmic strands; 7—vacuole; 8—formation of the secondary cell wall and pits on the vertical walls, degeneration of the living cell contents; 9—pits; 10—lignification of the secondary cell wall; 11—simple perforation

Depending on which vascular meristem gives rise to the xylem, it is classified into primary (originating from the procambium) and secondary (from the cambium). Primary xylem consists of protoxylem and metaxylem. Protoxylem comprises exclusively annular and spiral tracheae or tracheids surrounded by parenchyma. Metaxylem develops from scalariform and pitted elements. In addition, vertical parenchymatous strands and mechanical tissue elements may occur among the tracheary elements of the metaxylem. Protoxylem forms during the early Stages of Ontogeny in plant parts exhibiting active cell growth. Because the cell walls of annular and spiral protoxylem elements are relatively fragile, they soon disintegrate. Metaxylem develops following the protoxylem and can persist within the plant body for a prolonged period. A characteristic feature of primary xylem is the absence of horizontal parenchyma, meaning primary medullary rays are lacking.

Secondary xylem is formed in gymnosperms and dicotyledonous angiosperms whose axial organs are capable of secondary thickening in girth. Compared to primary xylem, secondary xylem is a more complex conducting tissue. Its water-conducting elements are represented by thick-walled scalariform and pitted tracheids or tracheae. Elements with helical cell wall thickenings are also occasionally found, with the turns of the helix closely spaced. Mechanical tissue elements—specifically wood fibers, or libriform fibers—develop in great Abundance within the secondary xylem. In addition to vertical (axial) parenchyma, secondary xylem also produces horizontal (ray) parenchyma. Ray parenchyma consists of one or several rows of cells oriented perpendicularly to The surface of the axial organs (ROOT and stem), forming what are known as primary medullary rays within the stem.

Various water-conducting elements develop in the xylem across different groups of higher plants. In horsetails, clubmosses, ferns, and the majority of gymnosperms, tracheids serve as the water-conducting elements. However, in the bracken fern Pteridium aquilinum, scalariform vessels are present in the xylem alongside scalariform tracheids. Among the Ephedrales, Gnetales, and Welwitschiales of the gymnosperms, the xylem is composed of vessels. The vast majority of angiosperms are vessel-bearing plants, with only a few lacking vessels in their xylem. Vessels are absent in representatives of the Winteraceae, Nymphaeaceae, Nelumbonaceae, Hydrocharitaceae, and several other families. In members of the Alismataceae, Butomaceae, Agavaceae, and Amaryllidaceae, vessels develop exclusively in the roots. The metaxylem in the roots of certain monocots contains the most primitive vessel forms, characterized by scalariform perforation plates with more than 100 bars on the horizontal walls.

In woody angiosperms, all types of vessel elements can be found in the xylem—ranging from the most primitive ones featuring perforations with numerous bars, to the highly advanced ones with simple perforations. In herbaceous dicots, the perforations are simple.

The presence of vessels in the xylem of both highly evolved and more primitive higher plants suggests that vessels may have arisen polyphyletically across different taxa at various times during evolution. The absence of vessels in A number of angiosperms may be either primary (e.g., Nymphaeaceae) or secondary (e.g., Lemnaceae), associated with the transition to an aquatic lifestyle or parasitism.

3.6.2. Phloem. Characteristics of phloem elements

The conducting elements of the phloem in angiosperms are sieve tubes, which consist of individual elements (sieve tube members). In clubmosses, horsetails, ferns, and gymnosperms, the conducting elements are represented by sieve cells—more ancient and primitive structures compared to sieve tubes. Sieve cells are living, prosenchymatous, highly specialized phloem elements responsible for the basipetal Transport of Assimilates. The wall of a sieve cell retains a primary structure, with tapering ends and sieve areas formed on the longitudinal walls. A sieve area is a specialized region of the cell wall perforated by minute pores through which plasmodesmata pass, facilitating communication between adjacent cells. Sieve areas were first described by T. Hartig in 1837. In gymnosperms, sieve cells are functionally associated with albuminous cells (Strasburger cells), which are characterized by high enzymatic activity. During ontogenetic development, sieve cells and albuminous cells originate from different vascular meristem cells.

Over the course of evolution, sieve tube members are believed to have evolved from sieve cells. The transformation of sieve cells into sieve tube members is analogous to The conversion of tracheids into vessel elements. Gradually, the end walls of sieve cells became more oblique, and some sieve areas shifted from the lateral walls to the terminal ends. In The final stage of evolution, the terminal wall of the sieve cell became horizontal, and the sieve area transformed into a sieve plate. A sieve plate differs from a sieve area by having large, patent pores (perforations) that can reach several micrometers in diameter. Each perforation of the sieve plate is lined with callose, a specialized polysaccharide surrounding the plasmodesmata that traverse the perforated cell wall regions. Sieve plates can be compound (consisting of several sieve areas) or simple (a single plate formed on the horizontal wall). Compound sieve plates are more primitive and are found in plants with long, obliquely oriented sieve tube members.

Thus, sieve tubes are composed of superposed elements in contact via sieve plates. Sieve plates ensure a closer interaction between sieve tube members than sieve areas do between sieve cells; consequently, sieve tubes conduct assimilates more efficiently than sieve cells. The most highly specialized sieve elements possess simple sieve plates with large perforations on their horizontal walls and a minimal number of sieve areas on their longitudinal walls. Furthermore, each sieve tube member is associated with one or more companion cells (Fig. 38).

Fig. 38. Phloem elements of a pumpkin stem: A—cross section; B—longitudinal section: 1—sieve tube member; 2—sieve plate; 3—Cytoplasm in a sieve tube member; 4—companion cell; 5—phloem parenchyma cells; 6—perforations in the sieve plate; 7—sieve areas on the vertical walls of sieve tube members

Phloem development in ontogeny. Classification of phloem. During ontogeny, sieve tube members and companion cells arise from the mitotic division of a common meristematic cell, meaning they are genetically related (Fig. 39). From the moment of their formation, the presence of plasmodesmata establishes a close functional connection between them. However, the functions of sieve tubes and companion cells differ. Sieve tubes perform a transport function, whereas companion cells, acting as catalysts, perform a secretory one. Companion cells take part in the metabolism of dissolved substances and their transport into sieve elements.

Fig. 39. Ontogenetic development diagram of a sieve tube member and a companion cell: A—strand of vascular meristem (procambium); B—enlargement of the procambial cell; C—onset of procambial cell nuclear division; D—cytokinesis, formation of the cell plate; E—initial stage of sieve tube member and companion Cell Formation; F—degeneration of sieve tube member components, division of the companion cell; G—mature sieve tube member and companion cells: 1—primary cell wall; 2—cell plate; 3—onset of nacreous wall formation in the sieve tube member; 4—plasmodesmata on the horizontal walls of the sieve tube member; 5—nucleus; 6—cytoplasm; 7—Plastids; 8—tonoplast of the vacuole; 9—P-protein; 10—degenerating nucleus;

11—sieve plate with plasmodesmata

Due to their functional differences, sieve tube members and companion cells also vary in morphological features. The wall of the sieve element is primary; it can vary in thickness, but it never lignifies and contains a high amount of pectic substances, which give it a characteristic nacreous luster. Sieve elements are living, but the protoplast of the sieve tube member undergoes significant changes. Cyclosis is absent in the cell. As the sieve element matures, the Cell Nucleus degenerates as an integral structure. Ribosomes disappear, and The Endoplasmic reticulum becomes agranular (smooth). By the time the formation of the sieve tube member wall is complete, dictyosomes are no longer detectable within it. The tonoplast breaks down, and the boundary between the vacuole and the cytoplasm disappears. P-protein (phloem protein) appears in the sieve elements; its fibrils are initially dispersed throughout the cytoplasm and subsequently take up a parietal position in the cell. The exact function of P-protein has not been established. Perhaps its fibrils play

a mechanical role by holding the remaining cell Organelles in specific positions. In the fully formed sieve tube member, the Plasmalemma is preserved, which determines the selective permeability of the cytoplasm, along with Mitochondria, amyloplasts, and proteoplasts.

Sieve tubes function for a relatively short time. In annuals and most perennial plants, by the end of the growing season, the perforation Pores in the sieve plates become plugged with callose, forming a callus. Such sieve elements lose their ability to conduct plastic substances and die off. In some woody dicotyledonous plants, sieve tubes function for up to three years (e.g., linden), while in monocots (such as palms), they presumably function much longer.

The structure of companion cells indicates their high physiological activity. The cytoplasm of these cells is weakly vacuolated. The cells contain large nuclei with prominent nucleoli, numerous mitochondria, and an endoplasmic reticulum. However, the most characteristic feature emphasizing the high functional activity of companion cells is the presence of abundant ribosomes. It has been established that companion cells are capable of secreting sugars into the phloem conducting elements against a concentration gradient. It is hypothesized that continuous exchange between sieve tube members and companion cells maintains a sucrose concentration gradient that ensures a strictly defined speed and direction of its movement. The velocity of assimilate transport in sieve tubes has been found to range between 10–100 cm per hour. Thus, companion cells are responsible for regulating substance transport through the phloem. In addition to them, parenchymal cells present in the phloem also participate in The transfer of substances into sieve elements. Parenchymal cells located adjacent to sieve elements can participate in the "loading" and "unloading" of conducting elements and differentiate into transfer (transfusion) cells.

Like xylem, phloem is subdivided based on its time of origin within the plant body into primary (derived from the procambium) and secondary (derived from the cambium).

Primary phloem consists of protophloem and metaphloem. The protophloem forms only sieve tubes and a small amount of diffusely arranged vertical parenchyma. In the metaphloem, companion cells are formed in addition to the aforementioned elements. Horizontal parenchyma is absent in the primary phloem.

Secondary phloem is a more complex tissue. In addition to sieve tubes and companion cells, it features well-developed vertical and horizontal parenchymal systems, as well as mechanical tissues—bast fibers. In fiber crops (such as flax, hemp, jute, and kenaf), these fibers do not undergo lignification.

3.6.3. Vascular Bundles and Their Types

Since both organic nutrients (assimilates) and Mineral Substances can be transported exclusively in solution, it is vital that close contact is maintained between the xylem and the phloem. The combination of xylem, phloem, mechanical tissues, and parenchyma forms the vascular bundles, which are classified according to the mutual arrangement of the xylem and phloem, as well as the presence or absence of meristematic tissue within the bundle.

Depending on the relative positioning of the xylem and phloem, vascular bundles are subdivided into radial, concentric, collateral, and bicollateral types (Fig. 40).

Fig. 40. Diagrammatic structure of vascular bundles: A—open collateral; B—open bicollateral; C—closed collateral; D—concentric amphivasal; E—concentric amphicribral; F—radial: 1—primary phloem; 2—primary xylem; 3—cambium; 4—secondary phloem; 5—secondary xylem

Radial vascular bundles occur exclusively in young roots of monocotyledonous and dicotyledonous plants. In such bundles, the elements of the primary xylem and primary phloem do not Touch each other; instead, they lie along alternate radii separated by parenchyma.

In concentric bundles, either the xylem is surrounded by the phloem, or the phloem is surrounded by the xylem. When the xylem is encircled by the phloem, the concentric bundle is termed amphicribral. These bundles are typical of ferns. Conversely, when the phloem is enclosed by the xylem, the bundles are called amphivasal, and they are found in the rhizomes of certain monocots (Fig. 41).

Fig. 41. Concentric vascular bundles: A—amphivasal bundle in the rhizome of lily of the valley (Convallaria majalis); B—amphicribral bundle in the rhizome of bracken fern (Pteridium aquilinum); 1—primary phloem; 2—primary xylem; 3—fundamental parenchyma of the rhizome

The most widespread type of vascular bundle is the collateral bundle, in which the xylem and phloem lie adjacent to each other. Such bundles are formed in both dicots and monocots. Collateral bundles are typically enclosed by a bundle sheath composed of sclerenchyma or living parenchymatous cells. Sometimes, sclerenchyma develops only on the peripheral side of the bundle as a cap, or two distinct strands of sclerenchyma are formed—one above the phloem and another below the xylem (Figs. 42, 43).

Fig. 42. Cross-section through an open collateral bundle in the stem of Iberian birthwort (Aristolochia iberica): A—secondary phloem; B—secondary xylem: 1—sieve tube; 2—companion cell; 3—cambium; 4—vessels of secondary xylem; 5—wood (sclerenchymatous) parenchyma

Fig. 43. Cross-section through a closed collateral bundle in a corn stem (Zea mays): 1—primary phloem; 2—primary xylem; 3—sclerenchymatous bundle sheath; 4—fundamental stem parenchyma surrounding the bundle; 5—wood (sclerenchymatous) parenchyma

In representatives of the pumpkin family (Cucurbitaceae), two strands of phloem adjoin the xylem. Such bundles are termed bicollateral. The inner, poorly developed phloem strand is represented by the primary phloem, while the outer strand consists of the secondary phloem (Fig. 44).

Fig. 44. Cross-section of a bicollateral bundle in a pumpkin stem (Cucurbita pepo): A—phloem; B—xylem: 1—primary phloem; 2—secondary phloem; 3—cambium; 4—vessels of secondary xylem; 5—secondary xylem vessel occluded with tyloses; 6—wood parenchyma; 7—primary xylem

If a layer of procambium persists within collateral bundles and subsequently develops into cambium, these bundles are classified as open bundles (see Fig. 42). In such bundles, The activity of the cambium leads to an increase in various secondary tissues, causing the axial organs to grow in girth. Open collateral bundles are characteristic of dicotyledonous plants. Conversely, if bundles lack meristematic tissue, they are referred to as closed (see Fig. 43). In closed vascular bundles, all the procambium is consumed in the Formation of primary tissues, rendering the plants incapable of secondary thickening. Closed vascular bundles are typical of monocots, whose axial organs retain their primary structure throughout the plant's life. An exception is found in woody monocots (such as palms, yuccas, and dracaenas), which exhibit anomalous stem thickening. Vascular bundles form an intricate network extending far beyond the Vegetative organs of the plant. They are present in all parts of the flower—the specialized reproductive organ of angiosperms: in sepals and petals, stamens, the Ovary of the pistil, ovules, and so forth. Through this complex Vascular System, all plant organs are interconnected, allowing the plant to function as a unified, integrated Organism.