Plant Structure and Functions

Pharmaceutical Botany with Taxonomy: Plant Structure and Functions

1. Structural Organization of Plants

  • Cells: At the lowest level are cells (the basic unit of structure and function).

  • Examples: Parenchyma, Sclerenchyma, Vessel elements

  • Tissues: Cells are organized together to form tissues (groups of similar cells performing a specific function).

  • Examples: Xylem, Phloem

  • Organs: Tissues are organized together to form organs (two or more tissues performing specific functions).

  • Examples: Leaves, Stamens, Roots, Stems

  • Organ Systems: Organs are organized together to form organ systems (groups of organs working together for complex functions).

  • Examples: Flowers, Shoots, Root System

2. Plant Cell Types

Parenchyma Cells

  • Characteristics:

  • Least specialized plant cells

  • Thin and flexible primary cell walls

  • Large central vacuole

  • Living at functional maturity

  • Functions:

  • Carry on most of the plant's metabolic functions (photosynthesis, storage, secretion)

  • Retain the ability to divide and differentiate into other cell types (unspecialized)

Collenchyma Cells

  • Characteristics:

  • Unevenly thickened primary cell walls (especially at the corners)

  • Living at functional maturity

  • Flexible and extensible (allow growth without breaking)

  • Functions:

  • Provide mechanical support to growing herbaceous stems, petioles, and young plant structures

Sclerenchyma Cells

  • Characteristics:

  • Very thick, lignified secondary cell walls with a narrow lumen

  • Dead at functional maturity

  • Cannot increase in length or elongate

  • Subtypes:

  • Sclerenchyma Fibers: Long, slender cells with a more or less regular secondary cell wall; found in bundles.

  • Sclereids: Shorter cells with an irregular shape and thick, pitted walls (e.g., stone cells in pears).

3. Tissue Systems in a Plant

A. Dermal Tissue System

  • Characteristics: The "skin" or protective outer covering of the plant; usually consists of a single layer of tightly packed epidermal parenchyma cells.

  • Key Structures:

  • Cuticle: Waxy layer covering the epidermis to prevent water loss.

  • Guard Cells & Stomatal Pores: Regulate gas exchange and transpiration.

  • Trichomes: Hair-like epidermal projections that reduce water loss, reflect excess light, or defend against herbivores.

B. Ground Tissue System

  • Characteristics: Makes up the bulk of the plant body; located between dermal and vascular tissues.

  • Composition: Dominantly made up of parenchyma cells, with collenchyma and sclerenchyma cells for support.

  • Functions: Photosynthesis, storage (starch, water), and mechanical support.

C. Vascular Tissue System

Continuous transport network composed of Xylem and Phloem:

  • Xylem:

  • Function: Conducts water and dissolved inorganic minerals upward from roots to shoots.

  • Cell Types:

  • Tracheids: Long, tapered, non-living conductive cells with pitted walls.

  • Vessel Elements: Shorter, wider non-living conductive cells with open end plates (perforation plates).

  • Xylem Parenchyma (Rays): Living cells involved in lateral transport and storage.

  • Xylem Fibers: Non-living sclerenchyma fibers providing structural support.

  • Phloem:
    Function: Transports sucrose, amino acids, and other organic compounds throughout the plant.

  • Cell Types:

  • Sieve-Tube Members: Living conductive cells lacking a nucleus at maturity, joined end-to-end via sieve plates.

  • Companion Cells: Living parenchyma cells connected to sieve-tube members via plasmodesmata; manage metabolic functions.

  • Phloem Parenchyma: Living cells for nutrient storage and lateral movement.

  • Phloem Fibers: Thick-walled sclerenchyma fibers providing mechanical support.

4. Plant Growth and Development

Types of Plant Growth

Feature

Determinate Growth

Indeterminate Growth

Cell Fate

Cells differentiate; developmental fate is sealed

Meristems remain active throughout life

Body Plan

Pre-programmed body plan

No fixed body plan

Duration

Growth stops after reaching a certain age/size

Growth continues throughout the plant's life

Examples

Leaves, flowers, fruits

Shoots, roots

Meristems

Specialized regions of undifferentiated cells that continuously divide to produce new tissue:

  • Apical Meristems:

  • Located at the tips of roots and shoots (shoot tips, root tips).

  • Responsible for primary growth (increasing the length/height of the plant).

  • Gives rise to primary meristems: Protoderm (dermal tissue), Ground Meristem (ground tissue), and Procambium (vascular tissue).

  • Lateral Meristems:

  • Located near the periphery along the length of roots and stems (cylindrical regions).

  • Responsible for secondary growth (increasing plant girth/thickness).

  • Composed of Vascular Cambium (produces secondary xylem and phloem) and Cork Cambium (produces protective cork layers).

5. Root Anatomy and Structure

Primary Growth in the Root

  • Root Cap: Protective thimble-like covering over the apical meristem; secretes a polysaccharide slime (mucigel) to lubricate movement through soil.

  • Apical Meristem: Active region of cell division.

  • Quiescent Center: A population of slowly dividing cells inside the apical meristem that acts as a reservoir; highly resistant to radiation and chemical damage.

  • Primary Growth Zones:

  1. Zone of Cell Division: Contains the apical meristem and primary meristems (protoderm, ground meristem, procambium).

  2. Zone of Elongation: Region where newly formed cells absorb water and rapidly lengthen.

  3. Zone of Maturation: Region where cells complete differentiation into functional dermal, ground, and vascular cells (root hairs form here).

Cross-Sectional Anatomy

  • Dicot Root:
    Epidermis: Outer dermal layer.

  • Cortex: Ground tissue layer used for food/starch storage.

  • Endodermis: Inner boundary of the cortex lined with a waterproof Casparian Strip to regulate mineral uptake.

  • Pericycle: Outer ring of the vascular cylinder; initiates the growth of lateral roots.

  • Vascular Center: Xylem forms an X-shaped or star-shaped central core, with phloem situated between the arms of the star.

  • Monocot Root:

  • Epidermis: Outer protective dermal layer.

  • Cortex: Broad ground tissue zone.

  • Endodermis & Pericycle: Ring layer surrounding the vascular bundle. (Note: Monocot roots rarely branch into extensive secondary growth).

  • Vascular Center: Alternating xylem and phloem strands arranged in a distinct ring around a central Pith (ground tissue core).

6. Shoot Anatomy and Secondary Growth

Primary Growth of Shoots

  • Apical Meristem: Located at the terminal shoot tip; continuously produces new stem tissues and leaf primordia.

  • Leaf Primordia: Young, developing leaves flanking the apical meristem.

  • Axillary Meristems (Axillary Buds): Dormant meristematic clusters located at the axils of leaves that have the capacity to activate and form lateral branches or flowers.

Secondary Growth

  • Secondary Plant Body: Composed of tissues produced by lateral meristems that increase plant thickness.

  • Vascular Cambium: Ring of meristematic tissue that produces Secondary Xylem (wood) toward the interior and Secondary Phloem (inner bark) toward the exterior.

  • Cork Cambium (Phellogen): Ring of meristematic tissue located in the outer bark that produces Cork cells (Phellem) to replace the epidermis as the stem expands.