Plant Tissues: Types and Functions

• Plant tissues consist of similar cells performing specific functions; tissue types create organs.
• Histology refers to the science of studying tissues.
• Plasmodesmata are channels connecting plant cells and facilitate substance exchange and communication.

Types of Plant Tissues

• Simple tissues comprise only one type of cell.
• Compound tissues consist of more than one type of cell.

Compound Tissues

• Compound tissue is divided into dermal, ground, and vascular tissues.
• Dermal tissues form the protective outer layer of the plant.
• Ground tissues provide support, storage, and perform photosynthesis.
• Vascular tissues transport water, nutrients, and sugars.

Dermal Tissue

• Dermal tissue protects the plant and regulates environmental interactions.

Epidermis in Herbaceous Plants

• Epidermis is a single layer of tightly packed cells that forms the "skin" of the plant.
• It covers leaves, stems, and roots, serving as the first line of defense.
• The epidermis protects against water loss by secreting a waxy coating called the cuticle.
• Stomata within the epidermis facilitate gas exchange.

Stomata

• Stomata are tiny openings found on the epidermis of above-ground plant parts like leaves, flower petals, petioles, and herbaceous stems.
• Guard cells surround and control the opening and closing of the stomatal pore.
• Stomata regulate the entry of carbon dioxide and exit of oxygen and water vapor, enabling gas exchange.
• Stomata facilitate photosynthesis, where sunlight converts carbon dioxide and water into sugars, releasing oxygen as a byproduct. Stomata regulate water loss via transpiration.

Periderm in Woody Plants

• Periderm is the dermal tissue in woody plants that undergo secondary growth, replacing the epidermis.
• Is a multilayered tissue made up of cork cells (phellem), phellogen (cork cambium), and phelloderm (secondary cortex).
• Cork cells (phellem) are non-living, protective cells that insulate the plant.
• Phellogen (cork cambium) produces cork cells and phelloderm.
• Phelloderm (secondary cortex) is a thin layer of living cells.

Trichomes (Plant Hairs)

• Trichomes are found on the epidermis of stems, leaves, and other above-ground parts.
• They create a barrier, deterring insects, act as a natural sunscreen by reflecting sunlight and protecting against harmful UV rays.
• Some trichomes secrete substances like oils or toxins to repel herbivores or pathogens.

Root Hairs

• Root hairs are epidermal extensions found on roots.
• Functions include increasing surface area for absorbing water and nutrients and anchoring the plant for efficient resource uptake.

Ground Tissue

• Ground tissue is inside the epidermis but not part of the vascular tissue.
• Includes parenchyma, collenchyma, and sclerenchyma cell types

Parenchyma

• Parenchyma cells have thin and flexible cell walls and are typically spherical or oval-shaped.
• Intercellular spaces are present between cells and make up the majority of the plant body. Photosynthesis occurs in chloroplasts within parenchyma cells, especially in leaves.
• Parenchyma cells store starch in roots, stems, and fruits and can regenerate and heal plant tissues.

Collenchyma

• Collenchyma cells have unevenly thickened cell walls and lack intercellular spaces.
• Collenchyma provides structural support for young stems and leaves.
• They often form strands, like the strings in celery stalks, containing cellulose, and lignin for additional strength.

Sclerenchyma:

• Sclerenchyma cells have significantly thickened and uniformly strong cell walls and lack intercellular spaces.
• Sclerenchyma cells usually die after the tissue forms, with two types: fibers and sclereids
• Fibers are long, thread-like cells that contribute to the plant's structural framework and are commercially used for textiles like hemp and flax.
• Sclereids are short, irregularly shaped cells that provide protection in hard structures like nutshells and seed coats.

Vascular Tissue

• Vascular tissues transport substances throughout the plant, divided into two types: xylem and phloem.

Xylem

• Conducts water and minerals from roots to other parts of the plant.
• Composed of dead cells when functional.
• Cell types include tracheids (overlapping cells) and vessel elements (end-to-end arrangement forming tubes called vessels).

Phloem

• Phloem distributes the products of photosynthesis and solutes throughout the plant.
• Composed of living cells when functional.
• Sieve elements in phloem have sieve plates (with small pores) that allow nutrient flow between cells.

Vascular Bundles

• Arrangement of xylem and phloem in different patterns.
• Radial arrangement involves xylem and phloem in separate groups and is found in roots.
• Conjoint arrangement puts xylem and phloem on the same radius, found in stems and leaves (collateral and bicollateral).
• Concentric arrangement has one tissue surrounding the other (amphivasal and amphicribal).
• Collateral has pholem on one side of xylem
• Bicollateral has pholem on both sides of xylem
• Amphivasal has xylem surrounding pholem
• Amphicribal has pholem surrounding xylem

Tissues Based on Ability to Divide

• Meristematic tissues have cells capable of division, responsible for plant growth.
• Permanent tissues have cells that have lost the ability to divide, specialized for specific functions.

Meristematic Tissues

• Meristematic tissues consist of undifferentiated cells capable of forming new ones with small, densely packed cells
• Characterized by thin cell walls, large nuclei, little to no vacuoles, and no intercellular spaces.

Types of Meristematic Tissues

• Primary meristems are found at the growth tips of plants and are responsible for the elongation of stems and roots.
• Secondary meristems appear later in the life cycle of a plant.
• It develops through dedifferentiation and contribute to secondary growth (thickening) in the stems and roots of perennial herbs and woody plants.