Plant Organization: Organs, Tissues, and Cells

Plant Body

Composed of millions of tiny cells with different shapes and functions, forming tissues and organs.

Shoot System

The aboveground part of the plant body; consists of stems, leaves, buds, flowers, and fruits.

Root System

The belowground part of the plant body; consists of main roots and their branches.

Tissue

An aggregate of cells that function together.

Simple Tissues

Tissues composed of a single cell type.

Complex Tissues

Tissues composed of aggregates of different cell types.

Meristems

Specialized groups of dividing cells from which tissues are derived.

Protoderm

Meristem that gives rise to the future epidermis.

Ground Meristem

Meristem that becomes the ground tissues; surrounds the procambium.

Procambium

Meristem that develops into vascular tissues (xylem and phloem).

Dermal Tissue System

The outer protective covering of the plant body.

Ground Tissue System

The bulk of the plant body, involved in metabolic functions, storage, and support.

Vascular Tissue System

A transport system for water, minerals, and food.

Parenchyma Tissue

Living cells when functional; variable shape (commonly polyhedral); may have primary or both primary and secondary cell walls; may contain lignin, suberin, or cutin.

Functions of Parenchyma

Respiration, secretion, photosynthesis, storage, conduction, wound healing, and regeneration.

Parenchyma Locations

Found throughout the plant body: in cortex, pith, pith rays, xylem, and phloem.

Aerenchyma

Parenchyma with extensive connected air spaces; found in aquatic plants like water lilies.

Chlorenchyma

Parenchyma with many chloroplasts; functions in photosynthesis; found in leaves.

Storage Parenchyma

Parenchyma without chloroplasts; functions mostly in food or water storage.

Transfer Cells

Parenchyma with irregular inner wall extensions to increase plasma membrane surface area; found in nectaries of flowers and carnivorous plants; assist in dissolved substance transport.

Collenchyma Tissue

Living, elongated cells with unevenly thickened, non-lignified cell walls.

Collenchyma Location

Beneath the epidermis in young stems; may form a cylinder or patches; found along leaf veins.

Function of Collenchyma

Provides support in the primary plant body.

Sclerenchyma Tissue

May be living or dead when functional; thick, often lignified secondary walls.

Sclereids (Stone Cells)

Variable shape, generally shorter than fibers; found throughout the plant; gives pears a gritty texture.

Function of Sclereids

Mechanical protection.

Fibers

Generally very long cells; often dead when functional; found in stem cortex and associated with xylem and phloem; also found in monocot leaves.

Function of Fibers

Provides support and storage.

Epidermis

Outermost cell layer; often one cell thick; covers leaves, flowers, fruits, seeds, stems, and roots until secondary growth occurs.

Functions of Epidermis

Mechanical protection, water loss prevention (cuticle), and gas exchange (via stomata).

Cuticle

Fatty substance (cutin) secreted by epidermis; forms a protective layer resistant to bacteria and pathogens.

Multiple-Layered Epidermis

Found in leaves of tropical figs and Piperaceae; helps prevent desiccation.

Guard Cells

Specialized epidermal cells that regulate stomata and control gas and water vapor movement.

Subsidiary Cells

Support guard cells; surround them.

Trichomes

Outgrowths of epidermis.

Root Hairs

Type of trichome; increases absorption of water and minerals from soil.

Leaf Trichomes

Reflect sunlight, reduce leaf temperature and water loss; may serve for protection and chemical defense.

Functions of Trichomes

Absorption, temperature regulation, defense against insects and herbivores, and secretion of substances (glandular hairs).

Vascular Tissue System

Interconnected network of cells that transports water, minerals, and food throughout the plant.

Xylem

Principal water-conducting tissue; transports water, minerals, provides support and food storage.

Primary Xylem

Found in vascular bundles of leaves and young stems, and in the center of young roots (vascular cylinder); formed early in development.

Secondary Xylem

Forms later in mature stems and roots; arranged in cylindrical patterns.

Tracheary Elements

Specialized xylem cells; include tracheids and vessel elements.

Phloem

Principal food-conducting tissue; transports sugars, amino acids, lipids, micronutrients, hormones, floral stimulus, proteins, and RNAs.

Sieve Elements

Phloem cells for conduction.

Sieve Cells

Found in gymnosperms; associated with albuminous cells.

Sieve-Tube Elements

Found in angiosperms; associated with companion cells.

Secretory Cells

Derived from parenchyma; produce specialized substances.

Secretory Products

Nectar (from flowers); Oils (from citrus, mint, etc.); Mucilage (from glandular hairs of sundews); Latex (in Euphorbiaceae); Resins (in pine trees)