BOTANY WITH THE BASICS OF HYDROBOTANY - 2010

2. PLANT Histology

2.1. General characteristics of Tissues

In lower plants (Algae), the body consists of either a single Cell or many similar Cells, each performing equally all the Functions inherent to a cell. In higher plants (mosses, lower vascular plants, seed plants), the Organism contains A large number of cells that differ in Structure and function—specialized cells.

Groups of cells that are similar in structure, function, and origin are called tissues. True tissues arise As a result of Cell Division in various planes. False tissues are formed through the intertwining of filaments of a branched heterotrichous thallus. They are characteristic of certain algae.

The modern Classification of plant tissues is based on a complex of Anatomical and physiological features—similarity in structure, function, and origin. The following types of plant tissues are distinguished: meristematic, dermal (protective), mechanical, assimilating, absorbing, storage, conducting (vascular), secretory, and aerating tissues. All tissues, except meristematic ones, belong to permanent (specialized) tissues. Parenchymal cells, which vary in shape, size, and function, are located among the specialized tissues. These cells are referred to as the fundamental parenchyma.

Some tissues are located adjacent to each other, interact mutually, and extend throughout the plant, forming a complex system—a tissue system. At least three tissue systems can be distinguished in plants: the dermal system, the vascular (conducting) system, and the fundamental (ground) system. The dermal system forms the outer protective layer of the plant, while The Vascular System occupies the central part of its body. The fundamental tissue system includes the fundamental parenchyma, mechanical, assimilating, and storage tissues, among others.

2.2. Meristematic Tissues. The plant is characterized by localized growth in specific areas—growth points containing meristematic tissues (Meristems), whose primary function is The formation of new cells through division.

Meristems consist of small, tightly packed cells with thin cellulosic walls. Meristematic cells are most frequently parenchymal, and less commonly prosenchymal with pointed ends. The Cell protoplasts contain large nuclei, while other Organelles are in the stage of development. Vacuoles are either completely absent or very small.

The embryo in a seed consists entirely of meristematic tissues. During Plant GROWTH AND DEVELOPMENT, meristems persist at the apices of shoots and the tips of roots. These growth points are called growth cones (apical meristems). In cereals, meristems persist at the base of SHOOT internodes, forming intercalary growth zones.

In appearance, all meristematic Cells of the growth cone are similar, but in reality, they are heterogeneous. Initial cells, or initials, are located at the very apex. Their primary function is division. Each initial cell divides into two daughter cells. Both grow, reaching the shape and size of the mother cell. One of them remains an initial, while the other, after several divisions, forms derivative cells that, at some distance from the growth cone, differentiate into cells of permanent tissues.

Meristems are classified according to their Location within the plant and their origin.

According to their location in the plant, apical, lateral, and intercalary meristems are distinguished. Apical meristems include the meristematic Tissues of the stem and ROOT growth cones; lateral meristems include procambium, cambium (which produces phloem and xylem), and cork cambium, which are arranged in individual strands or a continuous ring along the circumference of the stem or root; intercalary meristems are located at the base of shoot internodes and leaves (cereals, horsetails, some Apiaceae). Apical and intercalary meristems drive the elongation of the stem and root, whereas lateral meristems drive thickening (Secondary Growth).

According to their origin, meristems are subdivided into Primary and secondary. Primary meristems originate from the embryonic tissue (apical and intercalary meristems, procambium). Secondary meristems are formed from primary or permanent tissues that have regained The ability to divide (cork cambium, cambium producing phloem and xylem, traumatic meristems arising in response to injury).

Permanent tissues formed by primary meristems are called primary tissues. These include, for example, all tissues of the Introduction/19.html">Primary Structure of the stem and root, and the leaf blade. Monocots generally consist entirely of primary tissues. Permanent tissues formed from secondary meristems belong to secondary tissues (secondary phloem, secondary xylem, periderm).

2.3. Dermal Tissues. Dermal tissues are located On the surface of all plant Organs. Their primary function is protection. In addition, these tissues perform functions such as gas exchange, Transpiration, secretion,

absorption, etc.

Primary, secondary, and tertiary dermal tissues are distinguished.

The epidermis (Skin), a primary dermal tissue, covers leaves, flowers, fruits, and stems with primary structure. The epidermis is typically single-layered. In plants of arid zones (oleander, ficus), an epidermis consisting of two or three cell layers is found.

The epidermis consists of three components.

1. Fundamental tissue, the cells of which form the major part of its surface. They are tightly packed, sometimes with sinuous walls, which contributes to a stronger connection between them. The outer Cell wall is thickened and covered with a cuticle. The epidermal fundamental cells are living, possessing a protoplast and a large central vacuole. Among Plastids, leucoplasts are characteristic of them. METABOLISM/14.html">Chloroplasts are rare, occurring mainly in aquatic plants and plants growing in deeply shaded environments.

2. Stomata and their associated subsidiary cells. Gas exchange between the internal Plant Tissues and the external environment, as well as Water evaporation through the epidermis, occurs via specialized structures—stomata. Stomata are located on all above-ground plant organs, but they are particularly abundant in the epidermis of leaf blades—several hundred per 1 mm².

Plants are able to regulate The amount of evaporated water by increasing or decreasing the size of the stomatal pore—through its opening and closing, which is driven osmotically.

3. Various outgrowths (hairs), or trichomes of various shapes. Hairs create pubescence on plant organs. Generally, more hairs develop on the lower side of the leaf.

As the plant grows and develops, the epidermis is replaced by a secondary dermal tissue—the periderm.

Typically, at the end of summer, the epidermal cells of the stem or the living cells underlying the epidermis acquire the ability to divide, transforming into a secondary meristematic tissue known as the cork cambium, or phellogen. Phellogen cells divide parallel to the surface, laying down numerous layers of cork (phellem) toward the outside and one to two layers of living phelloderm tissue toward the inside. This gives rise to a complex of tissues—cork, phellogen, and phelloderm—collectively referred to as the periderm.

The actual protective function in this complex is performed by the cork, which is continuously produced by the cork cambium. Cork protects plants against Temperature fluctuations, pathogen invasion, and desiccation.

Cork cells may become filled with resins and acquire a brown or yellow coloration.

The periderm forms on damaged areas of the plant body (traumatic periderm), isolating deeper tissues from unfavorable environmental impacts, primarily pathogen penetration.

Gas exchange and transpiration in the cork occur through lenticels, which are crater-like depressions surrounded by a raised border. These lenticels provide aeration for the internal tissues of the plant.

In most trees, the periderm is eventually replaced by a tertiary protective tissue known as bark. In such plants, following the Formation of the initial periderm, a new cork cambium develops deeper within, giving rise to subsequent periderm layers. The living tissues trapped between the periderm layers, deprived of water and nutrients, eventually die off. This creates a complex of dead tissues containing periderms, which is called bark. Bark provides a much more robust defense for plants than the periderm, additionally shielding them from overheating and scorching during forest fires.

Special features of protective tissues in aquatic plants. Submerged aquatic plants, or hydatophytes, lack a cuticle and stomata. When removed from water, they rapidly dry out and perish.

Amphibious plants, or hydrophytes, possess an epidermis with stomata, but their transpiration rate is extremely high, allowing them to grow only under conditions of a constant and intensive water supply.

2.4. The fundamental parenchyma consists of parenchymal cells of various shapes: spherical, oval, prismatic, polyhedral, cylindrical, elongated, and others.

Through differentiation, cells of the fundamental parenchyma give rise to various specialized tissues. Furthermore, they possess a crucial characteristic: the ability to regain meristematic activity and form secondary meristems. This enables plants to heal wounds, produce adventitious roots and shoots, and regenerate lost organs. Groups of parenchymal cells, and even individual cells, are capable of regenerating entire plants in cell and tissue culture.

Functions of the parenchyma include: assimilation, storage, excretion, aeration, mechanical support, and others.

Parenchymal cells containing chloroplasts perform an assimilatory function. They can also store nutrients and water. Upon the thickening and lignification of their cell walls,

they acquire a supportive (mechanical) function. By forming numerous intercellular spaces, the parenchyma becomes a component of the aeration system. Frequently, its cells play a conductive role by transporting substances in a radial direction. By synthesizing and accumulating secondary metabolites, cells of the fundamental parenchyma become elements of the secretory tissue. Consequently, the fundamental tissue system often incorporates assimilatory, storage, Secretory Tissues, aerenchyma, and collenchyma. However, these aforementioned tissues are highly specialized and are more commonly treated as distinct groups, as was done in the preceding subsections.

The fundamental tissues include absorbing, assimilating, storage, and water-storing parenchyma, as well as aerenchyma.

2.5. Absorbing tissues. Plants absorb substances necessary for their vital activity from the environment. Algae and higher aquatic plants absorb them across their entire body surface. Higher land plants possess specialized absorbing tissues for this purpose, also known as absorptive tissues.

Mosses lack roots; they absorb water primarily through rhizoids, which are epidermal outgrowths. Typically, rhizoids consist of a single, thin-walled, elongated cell with a rounded apex. When the apex comes into contact with a substrate, it produces branching outgrowths. Occasionally, rhizoids branch from their inception.

Sphagnum mosses contain hyaline cells in their stems and leaves that absorb water. Hyaline cells are large and dead, with walls featuring spiral thickenings and small pores opening outward. Water enters these hyaline cells via capillary action through the pores, filling them. This explains the high water-holding capacity of mosses, as they retain many times their own dry weight in water.

The piliferous layer serves as the root's protective tissue in the absorption zone just above the root apex. It is referred to as the rhizodermis or epiblem. Cells of this layer form outgrowths known as root hairs.

Root hairs make intimate contact with soil particles, absorbing water from them via osmosis and mineral nutrients through active and passive transport mechanisms. Root hairs measure 0.1–1 mm in length, with a density of several hundred per 1 mm² of root surface. They function for only a few days: they are damaged upon contact with hard soil particles and die off, being continuously replaced by new ones forming above the growth point.

In plants of hot, arid habitats, the epidermis contains specialized hairs capable of absorbing moisture directly from the air. Absorbing tissues also include the velamen, the protective tissue found on the aerial roots of epiphytic plants that use tree trunks and branches for support.

In cereal grains and seeds of other monocots, the embryo is connected to the endosperm by a modified cotyledon known as the scutellum. The scutellum consists of a single layer of living, thin-walled cells elongated perpendicularly to the endosperm. During seed germination, scutellar cells elongate, and their lateral walls lose contact due to partial maceration. Much like root hairs, scutellar cells absorb sugars, Amino Acids, and other organic substances from the endosperm and transfer them to the embryo. These cells secrete hydrolytic Enzymes into the endosperm to break down storage reserves (starch, Proteins, oils). Toward the end of germination, as the seedling emerges from the soil and transitions to autotrophic Nutrition, the scutellum cells die and flatten out.

2.6. Assimilating, or photosynthetic, tissues. The photosynthetic function is carried out by specialized tissues called assimilating (photosynthetic) tissues, or chlorenchyma. Chlorenchyma is represented by living, thin-walled parenchymal cells whose protoplasts contain chloroplasts. Three MAIN TYPES OF assimilating tissues are distinguished: palisade, spongy, and folded. As a rule, all of them are located within the leaf blades.

Palisade parenchyma, or palisade tissue, is the primary photosynthetic tissue in plants. Its cells are cylindrical in shape, tightly packed, and arranged perpendicularly to the upper epidermis within the leaves.

Typically, they form a single layer, less frequently two or three. Palisade cells contain a large number of chloroplasts, and their somewhat elongated shape facilitates the outflow of photosynthetic products.

Spongy, or loose, tissue is also located within leaves, typically beneath the palisade layer. As its name implies, it contains numerous intercellular spaces. Its cells are rounded or lobed in shape and contain fewer chloroplasts than palisade cells. Alongside Photosynthesis, a vital function of the spongy chlorenchyma is gas exchange and transpiration.

Pallisade chlorenchyma is found mainly in the needles and leaves of certain grasses. Its cell walls form internal folds, which significantly increase the surface area of the wall and, consequently, the parietal layer of

Cytoplasm containing chloroplasts.

In addition to those listed, other tissues whose cells contain chloroplasts can also be classified as assimilatory, even though photosynthesis is not their primary function. Examples include phelloderm, collenchyma, cells of the primary stem cortex, and sometimes the epidermal cells of aquatic plants, as well as aerial roots.

2.7. Storage Tissues. The accumulation and retention of nutrients take place in storage tissues. Sometimes these can be seen with the naked eye, for instance, on a freshly cut apple or watermelon.

Within the protoplast—cytoplasm, plastids, and spherosomes—substances can be deposited in solid (starch, protein), liquid (oils), and dissolved states. Vacuoles contain dissolved sugars (in sugar beet root crops and the flesh of succulent fruits) and soluble Polysaccharides such as inulin (in the roots and tubers of Jerusalem artichoke).

In annual plants, storage tissues are located primarily in seeds and fruits.

In perennial plants, reserve substances are deposited not only in seeds and fruits but also in vegetative organs—the bark, wood, and pith of the stem, as well as the bark and wood of the root. These substances are utilized by the plant for bud break in the spring and the growth of young shoots and roots. Furthermore, many plants possess specialized storage organs such as rhizomes, tubers, and bulbs.

Storage tissues also include water-storing (hydrenchyma) tissues. Water-storing tissue is characteristic of succulents—plants with fleshy vegetative organs (such as aloe, stonecrop, and cactus). The hyaline cells of sphagnum mosses can likewise be classified as water-storing tissues.

2.8. Aeration System. The aeration system comprises stomata in the epidermis, lenticels in the periderm, and intercellular spaces. These intercellular spaces can vary in size; by interconnecting, they form a continuous intercellular system. The COMPOSITION OF THE air within these spaces typically differs from atmospheric air because local gas exchange occurs continuously—one gas is absorbed while another is released. Moreover, the intensity of these processes can fluctuate and vary.

Tissue containing very large intercellular spaces is known as aerenchyma. Often, the intercellular spaces in aerenchyma exceed the size of the cells themselves. Aerenchyma consists of living, thin-walled cells.

Aerenchyma is typically well-developed in aquatic and marsh plants,

particularly in hydatophytes (fully submerged plants) whose roots, rhizomes, and sometimes stems exist in water, mud, or waterlogged soil, as well as in submerged leaves. Atmospheric air penetrates the plant through stomata or lenticels on organs situated above the water and travels via intercellular spaces to reach the Cells and Tissues of oxygen-deprived organs, accumulating within the large intercellular spaces. Aerenchyma also helps maintain the plant in an upright position and provides buoyancy by reducing its specific gravity.

Large intercellular spaces are also found in the floral petals of certain plants. The air trapped within them reflects the full spectrum of incident sunlight; thus, if the cells lack pigments, the petals appear white (as in water lilies, roses, and lilies). When flowers wilt and their cells die off, water from the vacuoles spills into the intercellular spaces, displacing the air and rendering the petals translucent.

2.9. Vascular (Conducting) Tissues. The Transport of substances throughout the plant is carried out by specialized Vascular Tissues. Aqueous solutions of mineral and certain organic substances are transported from the roots to the leaves via the xylem (ascending stream), while organic photosynthetic products travel from the leaves via the phloem (descending stream). The solution transported through these vascular tissues is commonly referred to as plant sap.

Vascular tissues—xylem and phloem—typically lie adjacent to one another and extend throughout the entire plant, forming the vascular system.

Xylem consists of dead elements with lignified walls, known as tracheids and vessels, collectively referred to as tracheary elements.

Tracheids are prosenchymatous cells ranging from 1–4 to 10 mm in length and averaging 10–100 μm in width, with obliquely truncated, pointed, or rounded ends. Their oblique walls feature numerous pits through which the ascending water stream flows. Pits are also present on the lateral cell walls, facilitating the radial filtration of water from one tracheid to another, as well as between tracheids and parenchyma cells.

A crucial role in enabling tracheary elements to perform their functions is played by their cell walls, which must be rigid and strong.

Based on the pattern of cell wall thickening, tracheids are divided into three types: annular, spiral, and pitted.

Tracheids are the most ancient water-conducting elements in plants. In early land plants, such as psilophytes, the xylem consisted exclusively of spiral and annular tracheids. In lower spore-bearing and gymnosperm plants,

water transport occurs primarily through tracheids. This movement encounters considerable resistance because tracheids have a narrow diameter and connect in a vertical series via even narrower bordered pits, which are frequently (in conifers) half-closed by a torus. Despite this resistance and these obstacles, water ascends to great heights in woody conifers—exceeding 100 m in redwoods—traveling through a vertical series of tens and hundreds of thousands of tracheids. Tracheids are also found in angiosperms, which indicates the ancient and relatively primitive nature of this cell type.

Vessels, or Trachea, are tubes composed of a vertical series of dead member cells featuring perforations in their transverse end walls. They are more advanced water-conducting elements because water can flow unimpeded from one member to the next through large openings.

During plant evolution, vessel members evolved from tracheids that gradually became shorter and wider due to a reduction in procambium and cambium cells.

Vessels are significantly longer than tracheids, reaching several meters (up to 3.6 m in oak) or potentially even tens of meters. In woody angiosperms, vessels consist of many thousands of short, wide members.

In annual plants, the xylem functions for a single year. In perennials, various changes occur within the xylem with age. Tracheary elements can perform their function anywhere from 1–2 to 40–50 years, after which they become plugged with air and mineral or organic substances, such as resin in conifers.

Age-related Changes in the xylem lead to the death of living cells. Prior to this, the cell walls of parenchyma cells undergo thickening and lignification. This process is called sclerosis, as the living cells are transformed into mechanical tissue. Sclerosis enhances the mechanical strength of plant organs.

Phloem. The main Structural components of the phloem, its conducting elements, are sieve tubes and companion cells. The latter are also referred to as lateral or accessory cells.

Sieve tubes are a vertical series of living member cells whose transverse walls feature perforations (sieves). The walls of two adjacent cells, pierced by pores, are called sieve plates. The pores on them are usually arranged

in groups, forming sieve areas. Strands of cytoplasm pass through the perforations, connecting the protoplasts of neighboring cells.

Sieve tube members have thin cellulosic walls and a living content that differs from the protoplasts of ordinary living cells. They possess a Plasmalemma surrounding the cytoplasm and lining the perforations, along with a smooth ER. A tonoplast is absent, and the cell sap mixes with the cytoplasm. Most other organelles—nuclei, plastids, Mitochondria, the Golgi apparatus, and Ribosomes—are also missing. All these features, combined with broad cytoplasmic strands in the perforations, serve as an adaptation for transporting a fairly concentrated (20%) solution of assimilates. This solution includes sugars (80–90% of the total solute concentration), amino acids, organic acids, phytohormones, Vitamins, and Other Compounds.

Next to each sieve tube member lie one or more companion cells. These are typical living cells with a cellulosic wall and a protoplast containing all organelles. Mitochondria are particularly abundant in these cells. Companion cells facilitate the movement of photosynthesis products from the leaf mesophyll cells into the sieve tubes and their unloading at consumption sites within the plant (growth points, storage organs, etc.).

In the fine leaf Veins, where an intense transition of photosynthesis products into the phloem occurs (phloem loading), each sieve tube member is accompanied by several large companion cells. In axial organs (stem, root), where assimilate export (phloem unloading) predominantly takes place, there is usually a single companion cell associated with a sieve member, or several forming a single-row vertical strand along its longitudinal wall.

Sieve tubes are significantly shorter than xylem vessels. Their length averages from 100 to 300 µm, while their width ranges between 20 and 30 µm.

Like the xylem, the phloem is a complex tissue. In addition to sieve tubes and parenchyma cells, it contains mechanical fibers known as bast fibers. These are prosenchymatous cells with tapered ends and thick, frequently lignified walls featuring simple pits. The bast fibers of certain plants are utilized in the textile industry, with their length and lack of lignification being particularly prized. Long, unlignified fibers are found in flax, making them ideal for The production of high-quality linen fabrics.

Phloem is a short-lived tissue. Sieve tubes typically function for one season, more rarely for two, and exceptionally for several years. Furthermore, sieve tubes function longer in monocots than in dicots. The brief lifespan of phloem conducting elements is attributed to the absence of a nucleus and the accumulation of callose—a specialized polysaccharide that is deposited on the walls of perforations and sieve plates, progressively narrowing the pore lumen until it becomes clogged. Sieve tubes that cease to function eventually die off.

Vascular bundles. The plant conducting system comprises the conducting tissues: xylem and phloem. Typically, strands of these tissues run side by side through the plant, forming vascular bundles—a complex tissue complex with diverse functions. The conducting elements of the xylem and phloem transport substances throughout the plant, while the xylem, with its lignified walls, also provides structural support to soft tissues. Parenchyma is invariably present in the bundles, its cells carrying out radial transport and nutrient storage. Mechanical tissues, most commonly sclerenchymatous fibers, are also typically present in the bundles, giving rise to another of their names: fibrovascular bundles.

A vascular bundle containing a layer of cambium is termed open. Open fibrovascular bundles are capable of secondary thickening—growing in thickness—due to The activity of the cambium. Closed bundles lacking cambium are characteristic of monocots, whereas open bundles are typical of dicots.

The arrangement of vascular bundles in the root is considered the simplest: a single complex radial bundle runs through the entire root. In leaves, vascular bundles form veins that are distributed differently across plant species. In mosses, one or two unbranched veins run through the leaf. In ferns, the veins branch to form a dense network. Dicotyledonous plants possess a single main vein in the leaf, whereas monocots have several. Main veins in dicots branch extensively, and fine veins fuse to form anastomoses, ultimately producing a reticulate venation pattern. In monocots, the main veins run parallel along the leaf and do not always share the same thickness, sometimes alternating between thicker and thinner veins. Longitudinal main veins are interconnected by transverse cross-links composed of fine veins, which occur singly or form a complex network.

The arrangement of vascular bundles in stems is the most complex because their conducting system is intimately connected with the leaves.

2.10. Mechanical (supportive) tissues. Submerged higher aquatic plants (hydatophytes) and algae do not require structural support, as they are sustained by water, the density of which greatly exceeds that of air. Amphibious plants (hydrophytes) do possess mechanical tissues.

For small land plants living in humid environments, structural support is provided by tough, elastic cell walls and cellular turgor (mosses). Such support is insufficient for larger terrestrial plants. Following the transition of plants to land, evolution led to The Emergence of specialized mechanical tissues that support the plant body, maintain its shape, and preserve its spatial orientation. Indeed, mechanical tissues made it possible for plants to increase in size to the scale of tall trees (pine, oak, eucalyptus, sequoia).

There are two types of mechanical tissues: collenchyma and sclerenchyma.

Collenchyma consists of more or less elongated cells (up to 1–2 mm). These cells are living, containing all protoplast components, including chloroplasts, as well as a central vacuole. The cell walls are cellulosic, rich in hemicelluloses, Pectins, and water (up to 60–70%), and feature uneven thickening. Collenchyma is typically found in young, growing organs of dicotyledonous plants. In monocots, collenchyma is generally absent.

Sclerenchyma is the primary mechanical tissue in plants, characteristic of both monocots and dicots. Its cells are typically dead, featuring very thick, lignified walls with simple pits. Sclerenchyma is divided into two categories: fibers and sclereids.

Sclerenchymatous fibers are long prosenchymatous cells with tapered ends. The living Contents of the fibers degenerate early, rendering the cells dead. Sclerenchymatous fibers form an exceptionally strong tissue.

Bast fibers from various plants serve as raw Materials for the textile industry (flax, ramie, kenaf, hemp, etc.).

Sclereids, or stone cells, are usually parenchymatous, more rarely elongated and branched. Their walls are heavily thickened and lignified, featuring simple rounded pits whose channels may branch. The cells are dead, their living contents having disintegrated.

Based on cell shape, sclereids are subdivided into parenchymatous brachysclereids and elongated, branched astrosclereids. Brachysclereids occur singly or in groups within fruits (pear, sea buckthorn),

stems (broom), rhizomes (peony), and roots (horseradish). They can form dense stony tissue (nut shells, cherry and plum pits, etc.). Astrosclereids are large, branched, often star-shaped supportive cells. They appear as idioblasts in the leaf mesophyll (tea, camellia, olive), as well as in tissues with large intercellular spaces in aquatic plants (water lilies, yellow pond lilies).

2.11. Secretory tissues. If a plant releases certain substances into the environment, this is associated with reproduction (attracting pollinators and seed dispersers) or defense against unfavorable environmental factors (pathogenic Fungi and Bacteria, low and high temperatures, etc.). Some loss of substances is linked to leaf fall, the death of branch segments, the shedding of outer bark layers, root hairs, and so forth. For the most part, however, substances within the plant are reused and reutilized—this applies particularly to nitrogen.

From the aforementioned, it becomes clear why plants possess excretory tissues rather than a unified excretory system. Moreover, these tissues bear a closer resemblance to secretory tissues (and are frequently referred to as such) and, in part, to storage tissues.

Excretory tissues are conventionally divided into two groups: external (or exogenous) and internal (or endogenous). The former secrete substances into the external environment, whereas the latter accumulate them within the plant inside specialized cells and receptacles.

External excretory tissues include glandular hairs, external glands, nectaries, and hydathodes.

Glandular hairs exhibit considerable structural diversity; however, as a rule, they all feature a stalk and a HEAD, which may be either unicellular or multicellular. Typically, glandular hairs originate from epidermal cells.

Essential oil glandular hairs are frequently found in plants. Essential Oils represent a complex mixture of Aromatic Compounds that impart characteristic fragrances to flowers and other plant organs (such as roses, lavender, parsley, rosemary, caraway, mint, cloves, and linden).

Stinging hairs of nettles also belong to glandular structures. Each stinging Hair consists of a single large flask-shaped cell, the expanded lower portion of which is embedded in the leaf mesophyll. The upper part of the cell is elongated and gradually tapers. It possesses a thickened cell wall impregnated with silica, except for the narrow apical tip, which bears a tiny thin-walled head. When an animal or human brushes against the hair, the head easily breaks off, forming

sharp edges that pierce the skin, thereby injecting the stinging cell sap into the body.

External glands differ from hairs in that their formation involves not only epidermal cells but also deeper underlying tissue layers. Furthermore, these glands are generally more multicellular, and the endings of vascular bundles frequently extend toward them.

In insectivorous or carnivorous plants (such as sundew, butterwort, and pitcher plants), glands secrete a sticky mucilage to trap insects along with digestive enzymes for their assimilation.

Nectaries secrete a sugary fluid—nectar—outwardly. They are most commonly located within flowers—at the base of stamens, pistils, on petals, or sepals—though they may also occur on vegetative organs such as leaves, stipules, and stems.

Certain plants feature hydathodes, or water stomata, on their leaves, through which water is excreted in a liquid droplet state. This process is known as guttation. It typically occurs under conditions where soil moisture is abundant, the air is humid, and transpiration is suppressed. Guttation is particularly pronounced in tropical rainforests, where a steady drizzle can literally be observed beneath individual trees.

Guttation facilitates the Movement of water and mineral nutrients throughout the plant under conditions of reduced transpiration.

Internal secretory tissues are represented by individual cells, multicellular reservoirs, and laticifers.

Secretory cells are scattered individually or in groups among the cells of other tissues. Many of them differ from surrounding cells in size and shape, functioning as idioblasts. Depending on the predominant substances they contain, oil, mucilage, tannin, and crystal-bearing cells are distinguished.

Oil cells synthesize essential oils that gradually fill them, displacing the protoplast. Essential oil cells are found in representatives of families such as Lauraceae, Piperaceae, Magnoliaceae, and others.

Mucilage cells accumulate mucilage, which is predominantly carbohydrate in nature. They are characteristic of cacti. Sometimes bundles of calcium oxalate raphides are immersed in the mucilage.

Tannins frequently accumulate in secretory cells. The living contents of such cells die, and they become filled with tannins. Upon oxidation, tannins acquire a reddish-brown color, making tannin cells clearly visible under a Microscope.

Secretory cells also include crystal-bearing cells, which contain calcium oxalate crystals and their aggregates, known as druses.

Multicellular secretory structures are called reservoirs. In these, secretions are typically released into the intercellular spaces. Such reservoirs may take the form of long, often branching channels or rounded cavities. Depending on the mode of formation of the intercellular spaces, reservoirs are divided into schizogenous and lysigenous.

Schizogenous reservoirs appear as long, branching tubes, which is why they are specifically referred to as canals. They accumulate balsams (in conifers, St. John's wort, legumes) or mucilages (some ferns). Well-known is the oleoresin of the resin canals in many conifers. Balsam is a mixture of Essential oils and resins. The components of oleoresin have wide Applications in medicine, engineering, the chemical industry, and other fields.

Lysigenous reservoirs arise as a result of the breakdown (lysis) of a group of cells. A cavity is formed, filled with secretion and remnants of cell walls and protoplasts. The resulting reservoirs often have a regular, rounded shape (as seen in citrus peel).

In certain plants (such as spurges and dandelions), when organs are damaged, a milk-like liquid is released—latex, which is the cell sap of laticifers or laticiferous vessels.

At the center of a laticifer lies a vacuole containing latex—an emulsion of lipid-like substances and a solution of numerous compounds. Its composition includes proteins, amino acids, sugars, Glycosides, Alkaloids, tannins, mineral salts, and frequently rubber and gutta-percha. These latter substances primarily form the emulsion, imparting a white color to the latex. Latex may also have other colors: orange in greater celandine, yellowish-brown in hemp, while in mulberry, the sap is completely transparent and colorless.

It is hypothesized that laticifers perform several functions in the plant: conducting, storage, and protective. The conducting function is related to the fact that laticifers extend throughout the entire plant and can transport organic substances similarly to the phloem. The storage function is evidenced by the accumulation in latex of a range of typical reserve substances, such as sugars, inulin, and proteins. The protective role manifests, firstly, in the fact that latex contains certain toxic substances (such as glycosides and alkaloids) that protect plants from being consumed by animals. Furthermore, upon plant damage, rubber and gutta-percha flow out together with the latex, thicken in the air, and seal the wounds. Plants whose

latex is rich in rubber and gutta-percha are used for their extraction. Rubber is obtained on a large scale from the rubber tree Hevea. Vulcanized rubber, treated with sulfur, is used to manufacture rubber products. Pure rubber is used to make adhesive plasters and mustard plasters. Gutta-percha is not extracted in large quantities, but minor amounts are obtained from the shrub guayule. Gutta-percha is used to make insulation for submarine cables.

The milky sap from the leaves and unripe capsules of the opium poppy is known as opium. It contains A number of alkaloids that have a profound effect on The Human Body. Although some of these compounds are highly toxic, they are used in medicine as therapeutic drugs (such as morphine, codeine, and papaverine).

Selection/41.html">Review Questions and Tasks

1. WHAT IS A plant tissue? What are the key FEATURES OF PLANT tissues?

2. Characterize meristems. How are they classified?

3. Describe the functions and types of dermal (protective) tissues.

4. Characterize the epidermis. What are stomata and what is their structure?

5. How is the periderm formed? What is bark (rhytidome)?

6. What are the Characteristic Features of mechanical (supportive) tissues that enable their functions? What types of mechanical tissues do you know?

7. Characterize assimilating (photosynthetic) tissues.

8. Where in the plant are absorbing tissues located? Provide their characteristics.

9. Describe storage tissues.

10. Compare the Two Types of vascular tissue: xylem and phloem.

11. Characterize the tracheary elements of the xylem. How are they formed?

12. Describe the structure and formation of sieve tubes and companion cells in the phloem.

13. Discuss the features of secretory tissues. What groups of these tissues are you familiar with?

14. What do you know about the STRUCTURE AND FUNCTIONS of laticifers?

15. Describe the aeration system.

16. What is the fundamental parenchyma? What functions does it perform?

17. Describe the formation and composition of vascular bundles.

18. What types of vascular bundles do you know? In which plant organs are they found?

19. Describe the course and arrangement of vascular bundles throughout the plant body.