BOTANY: Lecture Course for Bachelor Degree Students Specializing in "Agronomy" - 2016

LECTURE 4. Definition of "Tissues". Types of Tissues

True and false Tissues. Classification of tissues by function, Cell shape, consistency, origin, and Anatomical and physiological features. Meristematic tissues. Origin, Structure, and Functions of Meristems. Structural Features of meristematic Cells. Primary and secondary meristems. Classification. Protective tissues: origin, structure, and functions. Types of protective tissues. Fundamental tissues: types, structure, functions, and classification. Mechanical tissues: types, structural features, similarities, differences, and significance. Conducting tissues: origin, structure, types, and significance. Secretory Tissues: STRUCTURE AND FUNCTIONS. External and Internal Secretory Structures. Economic use of plant tissues.

Tissue is an aggregation of cells similar in origin, structure, and function.

Tissues are formed during organ development through mitotic Cell Division, growth, and differentiation. Both living and dead cells perform tissue functions. They differ in cell shape, function, lifespan, etc.

Types of tissues

- by complexity: simple and complex (compound) — periderm, phloem, xylem, rhytidome (bark).

- by cell division capacity: meristematic (meristems) and permanent. Exception: fundamental tissues and primary protective tissues, which can give rise to secondary meristems such as phellogen and cambium during growth.

- by degree of differentiation: undifferentiated (meristematic), semi-differentiated (fundamental), and differentiated (mechanical, protective, conducting).

- by cell shape: parenchymatous and prosenchymatous.

- by function: meristematic, protective, secretory, fundamental (parenchyma), conducting, and mechanical (supportive).

Meristematic tissue (meristem)

Cells retain The ability to divide throughout their life. Based on origin, they are classified into primary and secondary; based on Location in the plant body, into apical, lateral (lateral meristems), intercalary, and traumatic (wound) meristems.

Composition: small isodiametric cells. Based on function and cell shape, they are divided into initial and derivative cells.

First-row initial cells → give rise to ROOT cap cells and dermatogen, which forms the protoderm, and subsequently the epiblem.

Second-row initial cells → give rise to the periblem, and through it, the ground meristem, which differentiates into the Cells of the primary root cortex.

The third row of initial cells forms the plerome, which develops into the pericycle and procambium. These transform into the stele (central vascular cylinder of the root).

The SHOOT apical meristem of angiosperms contains two (or more) rows of initial cells that are part of the tunica, surrounding a group of initial cells that make up the corpus. The outer layer of tunica cells transforms into the protoderm, which is the precursor of the epidermis. The inner layer of tunica cells or the outer cells of the corpus gives rise to the ground meristem, which forms storage (assimilation) or primary mechanical Tissues of the primary cortex. The initial cells of the corpus give rise to the procambium, which forms the cells of the phloem, xylem, mechanical tissues, and parenchyma that constitute the central vascular cylinder, or stele.

Primary meristems. Located in the seed embryo, establishing its Primary Structure. They include the root and shoot apical meristems, pericycle, procambium, and intercalary meristem.

Procambium is a lateral meristem arising from the promeristem of the shoot and root apical meristems. It differentiates within the organ as isolated strands or a continuous ring (fascicular and non-fascicular types of organ structure). It gives rise to primary conducting tissues (vessels, tracheids, sieve tubes, companion cells) as well as the cambium (fascicular cambium in the fascicular structural type).

Pericycle is a lateral meristem surrounding the central cylinder → in primary structure, it forms lateral roots; in Secondary structure, it gives rise to the phellogen and interfascicular cambium, and in dicotyledons, to bast fibers and sclerenchyma, or bast fibers and fundamental tissue.

Secondary meristems appear during plant organ growth from procambial cells and/or living parenchymatous cells; they ensure Secondary Growth, the Formation of secondary permanent tissues, and the Introduction/11.html">Secondary structure of Organs (cambium, fascicular and interfascicular cambium, and cork cambium (phellogen)).

Phellogen (cork cambium) is a lateral meristem from which secondary protective tissues — cork (phellem) and rhytidome — develop.

Cambium is a lateral secondary meristem.

Traumatic (wound) meristem. Secondary in origin. It arises in response to mechanical damage from living cells, resulting in The formation of a parenchymatous callus. Over time, a periderm forms on the callus, and other permanent tissues differentiate in its interior.

Depending on their location within plant organs, meristematic tissues are classified into apical, lateral, intercalary, and traumatic (wound) meristems.

Meristematic tissue is rich in phytohormones (Cytokinins, Auxins), Nucleic Acids, purine and pyrimidine bases, Vitamins (B1, B2, PP, E, carotenoids), and Enzymes.

Dermal Tissues (Protective Tissues)

Functions: protective; regulates the secretion or absorption of substances, light, and heat by plant organs.

By origin: primary (epidermis, epiblema), secondary (cork or phellem), and tertiary (bark or rhytidome).

Primary Dermal Tissue.

Epidermis is the primary dermal tissue of leaves, herbaceous stems, floral parts, and fruits.

It consists of a single layer of living cells and their derivatives, which can be subdivided into fundamental (protective), guard, and subsidiary cells.

Epidermal derivatives include the cuticle, trichomes (hairs), and emergences or prickles.

The epiblema (rhizodermis) is the single-layered primary dermal tissue of the root, covering The surface of the root Hair zone. Function: absorption of Water and essential substances from the substrate; secretion of substances into the soil to facilitate the dissolution of components and create a favorable environment for microorganisms.

Secondary Dermal Tissue.

Cork (phellem) is formed from the secondary lateral meristem, known as the cork cambium (phellogen). Cork is the primary dermal tissue of woody stems as well as the secondary conductive zone of roots in dicots and gymnosperms.

The cork cambium is a layer of dividing cells that originates from the epidermis, subepidermal layer, or deeper cortical layers.

Gas exchange and Transpiration in cork are facilitated by lenticels—Pores in the cork containing spherical filling cells and well-developed intercellular spaces.

Tertiary Dermal Tissue.

The rhytidome (bark) is a tissue found in woody stems and roots, representing a complex of dead cortical cells formed As a result of the repeated Development of the phellogen and cork in deeper layers of the cortex. → Cortical cells located outside the newly formed cork die off. The bark grows thicker annually from the inside due to the phellogen, while sloughing off from the outside as it dies. The bark provides a much more robust protective barrier than cork.

Fundamental Tissues (Ground Tissues) consist of living, isodiametric cells of various shapes with intercellular spaces. Parenchyma is distributed across all organs, situated between dermal, mechanical, and Vascular Tissues.

Based on their functions, they are categorized into assimilation, storage, aerating, and water-storage parenchyma.

Assimilation or chlorenchyma parenchyma (chlorenchyma) is located in leaves and the bark of young stems close to the surface, where sunlight penetrates. Their cells contain METABOLISM/14.html">Chloroplasts and carry out Photosynthesis. In most plant leaves, it is differentiated into palisade and spongy parenchyma. The former is typically located beneath the upper epidermis, while the latter lies beneath the lower epidermis. Spongy parenchyma consists of cells of various shapes permeated by large intercellular spaces, and aside from photosynthesis, it performs gas exchange and transpiration.

Fundamental parenchyma is located deeper beneath the organ's surface; its cells lack chloroplasts and serve to fill the interior of the organ.

Storage parenchyma stores Proteins, starch, inulin, oils, and sugars; it is located in the stem pith and root cortex, as well as in reproductive organs such as seeds, fruits, tubers, and bulbs.

Water-storage tissue (hydroparenchyma) is found in leaves, occurring either subepidermally or internally (in the leaves of agave and aloe, and the stems of cacti and euphorbias).

Aerating parenchyma (aerenchyma) is found in organs submerged in water, where large air-filled intercellular spaces form between parenchyma cells (such as in the floral stalks of water lilies and yellow water lilies, and the stems of pondweed, cottongrass, and bulrush). It features well-developed storage aerating cell groups with very large intercellular spaces connected into a unified ventilation network.

Absorptive parenchyma is characteristic of the root absorption zone and is located beneath the epiblema. The cells are living, possess cellulosic walls, and contain intercellular spaces.

Mechanical (Supportive) Tissue

They consist of a group of specialized cells that provide strength to the organs and the plant itself. These cells can be living or dead, thick-walled, uniformly or unevenly thickened, parenchymatous or prosenchymatous.

Based on their origin, structure, function, and location within plant organs, mechanical tissues are subdivided into collenchyma, sclerenchyma, and sclereids.

Collenchyma consists of living prosenchymatous or parenchymatous cells containing all Organelles, featuring unevenly thickened cell walls. Located close to the surface, they contain chloroplasts and are capable of reverting to a meristematic state. Based on the pattern of wall thickening, the following types are distinguished: angular, lamellar, and lacunar. It is characteristic of dicotyledonous plants.

Bast fibers are located in the peripheral part of the plant and constitute a component of the bark (bast, phloem). The cells are elongated (5–300 mm) with thickened walls, most commonly composed of Cellulose and pectic substances, and are arranged in groups along the plant organs.

Primary bast fibers develop from the procambium.

Secondary bast fibers develop from the cambium; they are thinner and shorter than primary bast fibers.

Wood fibers (libriform fibers) are part of the xylem, or wood. The cells are short (up to 5 mm in length) with tapered ends. Cell walls are thickened through lignification.

Sclerids, or stone cells, are a group of dead parenchymatous cells with thickened, lignified cell walls that are mineralized with silica or calcium, featuring simple and branched pits. Cell morphologies include astrosclereids, brachysclereids, and osteosclereids.

Vascular (Conducting) Tissues

In higher vascular plants, part of the vegetative body is located in the soil (roots), while another part is in the air (shoots and their derivatives). Roots extract water and soil nutrients from the ground. Leaves absorb CO₂ from the air and perform photosynthesis using light energy. Substances absorbed and synthesized by various PARTS OF THE plant are redistributed throughout its body via conducting tissues. The upward flow of water and solutes originates in the roots and is transported through tracheids and vessels (tracheae). The downward movement of predominantly organic substances from the leaves to the roots and Generative organs occurs via sieve tubes or sieve cells. Water transport is carried out by dead conducting tissues, whereas The transport of organic substances is performed by living ones.

Vessels (tracheae) develop from ordinary parenchymatous cells beneath the shoot apical meristem as a result of the dissolution of transverse cell walls.

Tracheids are single-celled, evolutionarily older conducting elements of the xylem in the form of long, dead prosenchymatous cells with oblique end walls. Their length ranges from 0.5 to 1.0 mm in angiosperms and up to 4 mm in gymnosperms. The Cell wall features pits and secondary thickenings of various forms (spiral, annular, scalariform, pitted). They communicate with each other through pits located on their tapered ends.

From an evolutionary perspective, tracheids are more primitive than vessels; the ends of their segments are oblique and feature multiple perforations. In clubmosses, horsetails, ferns, and conifers, they represent the sole type of conducting elements.

In flowering plants, the conducting system is more advanced—the xylem is represented by vessels whose structure is adapted to the movement of aqueous solutions. Perforations form in the lateral walls of the vessels.

Sieve cells are elongated (1.5–4.8 mm) cells with tapered ends and sieve areas on their lateral walls.

Xylem (from Greek xylon — wood) is a complex conducting tissue comprising conducting elements (vessels and tracheids), parenchymatous, and mechanical histological elements. It transports water and dissolved mineral salts.

Phloem (from Greek phloios — bark, bast) is a complex of conducting, parenchymatous, and mechanical elements through which organic substances are transported from the leaves to the roots.

Depending on the composition of their elements and their mutual arrangement within the vascular bundle, they can be: open (possessing a meristematic tissue), closed (lacking meristematic tissue), complete (containing both xylem and phloem); incomplete (containing either xylem or phloem); collateral (phloem and xylem elements form two parallel layers); bicollateral (two phloem layers—outer and inner—with a layer of xylem positioned between them); concentric (a xylem strand surrounded by a ring of phloem, or vice versa); radial (xylem and phloem layers are located on separate radii relative to the axis, characteristic exclusively of primary root structure).

Secretory Tissues

Plant cells produce numerous substances that are metabolic by-products and are not utilized by the plant. They are eliminated by specialized structures—secretory tissues—which, In addition to excreting substances, also serve to store them within the Organism in the form of relatively harmless compounds or provide material for the synthesis of new substances.

Excretion of substances can be:

- active (specialized glandular cells—water, salts, sugars, mucilaginous substances, enzymes, Essential Oils);

- passive (along with shed cells and organs—leaves, root cap—Amino Acids, CARBOHYDRATES, vitamins, Alkaloids, Glycosides).

Mechanisms of secretion:

1. extrusion (Golgi vesicles approach the Plasmalemma, fuse with it, and release their contents through The Cell wall);

2. active Transport of substances across the Cytoplasm;

3. filtration along a concentration gradient.

External secretory structures are located On the surface (glandular hairs, glandular scales, glands, omophores, nectaries, hydathodes):

Internal excretory tissues: secretory idioblasts, secretory cells (cavities), excretory ducts, laticifers. Schizogenous and lysigenous cavities. Schizogenous cavities are formed from intercellular spaces created by the Separation of cells; living cells adjacent to the cavity become epithelial and begin to secrete excretory substances into the cavity. Schizogenous cavities and resin ducts are characteristic of members of the families Apiaceae, Araliaceae, Asteraceae, and the division Pinophyta.

Lysigenous cavities are formed as a result of the dissolution of a group of cells filled with excretory substances. They originate at the site of a group of cells that disintegrate after accumulating these substances (Citrus).

Laticifers are living cells (or rows of cells) containing cytoplasm, multiple nuclei, and a vacuole filled with milky juice (latex). The wall of the laticifers consists of cellulose. Latex contains resins, rubber, essential oils, protein compounds, and alkaloids. Laticifers can be non-articulated (simple), which develop from a single cell containing a branched vacuole and multiple nuclei (members of the families Euphorbiaceae, Urticaceae, Apocynaceae), or articulated (compound), consisting of individual cylindrical cells arranged in rows, formed as a result of The breakdown of transverse walls in a vertical row of cells. The transverse cell walls dissolve, forming tubular vessels (families Papaveraceae, Asteraceae, Campanulaceae, Euphorbiaceae, Araceae, Convolvulaceae, etc.).