Plant Tissue Types: Meristematic and Permanent

• Microscopic properties are important for plant identification.
• Microscopic properties assist in detection of adulterants.
• Tissues are groups of cells that form a structural and functional unit.

Plant Tissue Types

• Plant tissues are categorized into meristematic and permanent tissues.

Meristematic Tissues

• Meristematic tissues involve regions of cell division.
• Meristems are not yet committed to developing into a specialized cell type.
• Terminal (Apical) meristems are at the ends of stems, branches, and roots.
• Lateral meristems are parallel to the sides of plant parts, increasing width.
• Intercalary (Axillary) meristems appear at the base of leaves and branches.

Permanent Tissues

• Permanent Tissues are simple or complex.

• Simple tissues have one cell type and include ground tissues.

• Complex tissues have two or more cell types.

• Complex tissues include vascular tissues (xylem, pholem).

• Plant tissues fit into three main categories, dermal, ground and vascular.

• Dermal tissue includes the epidermis, trichomes (hairs), stomata, (guard cells) and root hairs.

• Ground tissue includes parenchyma, collenchyma and sclerenchyma.

• Vascular Tissue includes xylem and pholem.

Dermal Tissue

• Dermal tissue is generally a continuous single layer of tightly packed cells.
• It covers the outer surface of plant organs, stems, and leaves.
• Epidermal cells secrete a waxy cuticle.
• The waxy cuticle protects against ater loss.
• Epidermis is formed of a single layer of cells without intercellular spaces except for the stomata
• Stomata are modified epidemal cells responsible for gas exchange via the osteole (stoma).
• Stomata includes guard and subsidiary cells.
• Anticlinal walls are perpendicular to the surface.
• The cuticle is a protective film, it may be thin or thick and smooth or striated.
• Trichomes (hairs) are epidermal cells extended outward forming a small projection.

Cell Walls

• Cellulosic walls have a chief polysaccharide of plant cell wall.
• They also occur in all cell walls.
• Cellulose + iodine yields no blue color.
• Hemicellulose + iodine yields a blue color.
• Lignified walls have a complex polymer of phenolic compounds.
• They are present in the cell walls of tracheids, vessels, fibers, and sclereids.
• Lignified wall + phloroglucinol and HCl gives a red color.
• Suberized and cutinized walls consist of suberin and cutin are mixtures of polymerized fatty acids esters.
• Suberin thickenings are present in cork cells.
• Leaves are covered with cutin.
• Cutin and suberin plus Sudan III yields a red color.
• Chitinious walls form the insects cells.
• Heated with 50% potash for 1 hour they convert into chitosan with ammonia and acids.
• Chitin + 0.5% solution of iodine in potassium iodide then sulphuric acid yields a violet color.

Trichomes

• A trichome has two parts, a foot embedded in the epidemis and a free part body.
• When the trichome falls it leaves a cicatrix.
• Types of trichomes include non-glandular and glandular.
• Non-glandular trichomes can be unicellular or multicellular.
• Unicellular hairs can be unbranched e.g. Senna or branched e.g. Lavender.
• Multicellular can be unbranched or shaggy e.g. Cumin or branched e.g. Tobacco.
• Glandular hairs can be unbranched or branched.
• Unbranched hairs include unicellular stalk e.g. Digitalis.
• Branched hairs are a branched stalk with unicellular head e.g. Hyoscyamus.

Ground Tissue

• Ground tissue includes the parenchyma, collenchyma and schelernchyma.
• Parenchyma: Cells are living with thin cellulosic walls and wide intercellar spaces.
• Parenchyma is involved in storage, respiration, and photosynthesis.
• Parenchyma enables the plant to differentiate to other cell types.
• Collenchyma: Cells are living with primary cellulosic walls.
• They have no intercellar spaces.
• Offers hard elastic secondary wslls for flexible support.
• Schelernchyma: Cells are non-living at maturity with thick walled lignified cells.
• Includes Fibers and sclerides.
• Enables tolerance of pressure, stretching, and bending (Mechanical support).
• Sclereides are circular in shape.
• The wall is thick and lignified.
• Fibers are elongated cells with tapering ends and lignified secondary walls.

Mesophyll

• Mesophyll differentiates into palisade and spongy tissue.
• Palisade layer: One or more layers of columnar cells perpendicular to the epidermis with narrow intercellular spaces.
• The upper palisade forms a continuous (Senna) or discontinuous (Datura) layer over the midrib.
• The mesopyll is isobilateral or dorsiventral
• Spongy Tissue has intercellular spaces.

Vascular tissues

• Vascular tissue is a complex tissue.
• There are the two types of vascular tissue: xylem and pholem.
• Vascular tissue can be scattered or regularly arranged in ground tissue forming a ring.
• Scattered vascular tissue (Monocots).
• Regularly arranged vascular tissue (Dicots).

Vascular bundles

• Xylem conducts water.
• Xylem has vessels made of dead cells.
• Xylem demonstrates one way flow toward the leaves.
• Xylem has lignified walls.
• Xylem has end walls that disappear completely.
• Xylem is composed of xylem parenchyma, vessels, and tracheids.
• Phloem conducts food.
• Phloem elements are made of living cells.
• Phloem has bidirectional flow.
• Phloem vessels have no lignified walls.
• Phloem has end walls that do not disappear completely.
• Phloem has pit connections called plasmodesmata.
• Phloem is composed of pholem parenchyma, sieve tubes and companion cells.
• Xyem composed of traechids i.e. fiber and vessels i.e. major conducting elements.
• Vessels are derived from vertical series of cells dissolution of the wall makes a continuous tube.
• Phloem composed of sieve tubes formed from elongated cells.
• Sieve tubes intersect in the walls as sieve plates
• There are companion cells or plasmodesmata associated with the sieve tube structure, which is characterized by its dense protoplast and well developed nucleus and thin cellulose wall.