Plant Tissue Types: Meristematic and Permanent

• Unit III covers plant tissue

Types of Plant Tissue

• Meristematic tissue exists in apical, lateral, and intercalary forms
• Permanent tissue can be simple or complex
• Simple permanent tissue includes parenchyma, collenchyma, and sclerenchyma
• Complex permanent tissue includes xylem and phloem
• Protective tissue comprises epidermis and cork (phellem)

Meristematic Tissue

• Nature: Continuously dividing cells help the plant increase in length and girth
• Cells are similar in structure with thin cellulose cell walls
• Cells may be spherical, oval, polygonal, or rectangular
• Compactly arranged cells contain no intercellular space
• Each cell has dense/abundant cytoplasm and a single, large nucleus
• Cells contain few or no vacuoles
• Occurrence: Growth tissues found in growing regions of the plant
• Classified as apical, lateral, or intercalary based on position
• Function: Continuously forms new cells

Apical, Lateral & Intercalary Meristems

• Apical meristems are at the growing tips of stems and roots and apexes of leaves
• Apical meristems cause root & stem elongation and increase plant height, known as primary growth
• Lateral meristems are beneath the bark (cork cambium) and in vascular bundles of dicot roots and stems (cambium)
• Lateral meristems occur in thin layers and are responsible for growth in thickness, leading to increased diameter and girth, known as secondary growth
• Intercalary meristems are located at the base of leaves or internode, i.e., stems of grasses and monocots, increasing organ length

Permanent Tissue

• Consists of cells derived from meristematic tissues
• Cells have lost the power of division and attained definite forms
• Classified into simple and complex types

Simple Permanent Tissue

• Composed of structurally and functionally similar cells of one type
• Types include parenchyma, collenchyma, and sclerenchyma

Complex Permanent Tissue

• Consist of more than one cell type that coordinate to perform a common function
• Transport water, mineral salts (nutrients), and food material to various parts of the plant body
• Types include xylem (or wood) and phloem (or bast)
• Xylem and phloem are conducting tissues, also known as vascular tissues, together constitute vascular bundles

Parenchyma

• Forms the bulk of the plant body
• Living cells that can divide
• Cells are rounded, isodiametric, oval, round, polygonal, or elongated
• Thin cell wall encloses dense cytoplasm with a small nucleus and large central vacuole
• Cells have living protoplasm and abundant intercellular spaces
• Occurrence: Widely distributed in plant body in stems, roots, leaves, flowers, and fruits
• Found in soft parts like the cortex of roots, ground tissues in stems, mesophyll of leaves, and pith, medullary rays, and packing tissue in xylem and phloem
• Functions as a packing tissue to fill spaces, maintaining shape and firmness due to turgidity
• Supports herbaceous plants due to turgidity
• Stores and assimilates food, acting as a food storage tissue
• Transports materials through cells or cell walls
• Metabolically active cells with intercellular air spaces allow gaseous exchange

Collenchyma

• Consists of living cells with features of parenchyma
• Characterized by extra cellulose deposition at cell corners and generally absent intercellular spaces
• Collenchyma cells are elongated and contain only a few chloroplasts
• Occurrence: Located below the epidermis of dicotyledon stems and petioles and in midribs of dicot leaves
• Absent in monocot stems, roots, and leaves
• Functions as a mechanical tissue providing support and elasticity
• Provides tensile strength with flexibility, allowing easy bending
• May manufacture sugar and starch if chloroplasts are present

Sclerenchyma

• Composed of dead cells devoid of protoplasm
• Walls are greatly thickened with lignin, called lignified
• Excessive thickening of the cell wall results in a nearly absent cell cavity or lumen
• Cells are closely packed without intercellular spaces
• Occurrence: Occurs in abundance in patches or definite layers
• Found in stems, roots, leaf veins, hard coverings of seeds and nuts
• Sclereids form the gritty part of ripe fruits and contribute to the hardness of seed coats and nutshells
• Functions mainly for mechanical support and protection, providing strength, rigidity, flexibility, and elasticity

Complex Permanent Tissue: Xylem

• Nature: A vascular and mechanical/conducting tissue
• Composed of four cell types: tracheids, vessels/tracheae, xylem parenchyma, and xylem sclerenchyma
• Except for xylem parenchyma, the xylem elements are dead with thick lignified walls
• Vessels are the most important cells: very long, tube-like structures formed by a row of cells placed end to end
• Tracheids are elongated cells with tapering ends and that conduct water
• Functions to carry water and mineral salts upward from the root to different parts of shoots
• Gives mechanical strength to the plant body

Pholem

• Contains tubes, but has no mechanical function
• Composed of four elements/cells: sieve tubes, companion cells, phloem parenchyma, and phloem fibers
• Except for the phloem fibers, pholem cells are living
• It's function is to transport photosynthetically prepared food materials from leaves to storage organs and from storage organs to growing regions

Protective Tissue

• Includes Epidermis and Cork (or phellem)

Epidermis

• Usually present in the outermost layer of leaves, flowers, stems, and roots
• One cell thick and covered with a cuticle, a waterproof layer of waxy cutin secreted by epidermal cells
• Cuticle thickness varies, thicker in xerophytic plants
• Epidermis cells are elongated, flattened, and lack intercellular space
• The inner contents are similar to parenchyma cells
• Functions to protect the plant from desiccation and infection
• The cuticle helps reduce water loss by evaporation and prevents entry of pathogens

Cork (Phellem)

• Outer protective tissue that undergoes changes as plants age
• A strip of secondary meristem, called phellogen or cork cambium replaces the epidermis of stems
• Cork cambium is a simple tissue of one cell type
• Cells of cork cambium are rectangular, vacuolated, and contain tannins and chloroplasts
• Cork cambium gives off new cells on both sides to form cork on the outer side and secondary cortex/phelloderm on the inner side
• Cells cut by cork cambium on the outer side ultimately form the thick bark
• Cork cells are dead, compactly arranged, and lack intercellular spaces
• Walls are heavily thickened by suberin
• Suberin: An organic substance, making the cells impermeable to water and gases
• Cork cells does not contain protoplasm, are filled with resin or tannins
• The cell walls become thick and waterproof due to addition of Suberin
• Functions protectively against desiccation, infection, and mechanical injury