Plant Structure; Cell Types & Primary Growth I Study Notes

Lecture Overview

  • Lecture 4: Plant Structure; Cell Types & Primary Growth I

    • Plant structure

    • Cell types—continued

  • Primary growth

    • Meristems

    • Shoot growth

    • Root growth

  • Materials: Plant Lecture Slides were created by Dr. Mark Johnston

Tissue Systems in Vascular Plants

  • Three Tissue Systems:

    1. Dermal Tissue

    • Characteristics:

      • Single layer of cells.

      • Secretes the cuticle (waxy layer).

    1. Vascular Tissue

    • Components:

      • Xylem & Phloem

    • Functions: Support & supply nutrients.

    1. Ground Tissue

    • Definition: Tissue other than dermal or vascular.

    • Characteristics:

      • Makes up the bulk of the young plant.

      • Fills the space between dermal and vascular tissues.

      • Mostly composed of parenchyma.

    • Functions: Storage, photosynthesis, support.

Plant Cell Types

  • Dermal Tissue

    1. Epidermis

  • Ground Tissue

    1. Parenchyma

    2. Collenchyma

    3. Sclerenchyma

  • Vascular Tissue

    1. Tracheids & vessel elements

    2. Sieve elements

Epidermal Cells

  • Characteristics:

    • Outermost cells forming a single layer covering leaves, stems & roots of non-woody parts.

    • Functions include:

    • Waterproofing: Prevents water loss.

    • Protection from pathogens:

      • Viruses

      • Bacteria

      • Fungi

    • Specializations include:

      • Hairs (trichomes)

      • Nectary

      • Guard cells: enable gas exchange.

Cuticle

  • Composition: Formed by epidermal cells; outermost part is wax.

  • Functions: Provides protection against:

    • Water loss

    • Pathogens

    • Ultraviolet (UV) radiation

Trichomes

  • Secretory Trichomes:

    • Example: Cannabis trichomes (stalked gland of female plant)

  • Functions:

    • Secrete oils for defense.

    • Hinder crawling insects (defense).

  • Types of Trichomes:

    • Found on marjoram (mint family):

    • Secrete oils.

Parenchyma Cells

  • Functions:

    • Many and often specialized.

    • Responsible for most metabolic processes including photosynthesis, storage, and secretion.

  • Features:

    • Ground tissue type

    • Usually lack secondary wall

    • Can divide & differentiate at maturity

  • Examples:

    • Fruit flesh

    • Endosperm

    • Pith & cortex of stems & roots

    • Chloroplast-bearing cells in leaves.

Collenchyma Cells

  • Functions:

    • Provide support, especially for young & growing organs.

  • Characteristics:

    • Type of ground tissue.

    • Thick, uneven primary wall and no secondary wall.

    • Located just below epidermis; often occurs in strands.

    • Remains alive at maturity, can elongate.

  • Examples:

    • Celery leaf midrib & petiole.

Sclerenchyma Cells

  • Functions:

    • Provide support and protection to non-growing tissues.

  • Features:

    • Type of ground tissue.

    • Thick secondary wall containing lignin (a rigid polymer).

    • Cannot elongate at maturity and may be dead.

    • Two forms:

    • Fibers

    • Sclereids

  • Examples:

    • Sclerenchyma fibers found in burlap (jute fibers) and hemp (Cannabis sativa).

    • Sclereids observed in pear and nutshell.

Vascular Tissue

  • Water-conducting cells of xylem: Tracheids & vessel elements

    • Functions:

    • Conduct water and minerals.

    • Features:

    • Dead at maturity.

    • Secondary wall is lignified.

    • Two types exist: Tracheids and vessel elements.

Lignin

  • Characteristics:

  • Class of complex organic polymers (NOT a polysaccharide).

  • Second most abundant natural polymer, after cellulose.

  • Deposited in cell walls; fills spaces among polysaccharides (cellulose, hemicellulose, & pectin).

  • Provides structural strength to wood & bark.

  • Found in both woody and non-woody plants (e.g., palm trees, bamboo).

  • Human Uses:

    • Wood constitutes 20–33% lignin by dry weight.

    • Used in the production of rope, clothing, and rugs through sclerenchyma fibers.

    • In papermaking, lignin is removed to isolate cellulose.

    • Artificial vanillin can be synthesized from lignin.

Phloem: Sieve-Tube Elements

  • Functions:

    • Transport sugars (especially sucrose), organic compounds, and some minerals.

  • Features:

    • Alive at maturity, associate with companion cells (parenchyma).

Meristems and Plant Growth

  • Definition of Meristems:

    • Undifferentiated tissues responsible for plant growth; retain the ability to divide throughout the life of the plant.

  • Types of Meristems:

    1. Apical Meristems:

    • Responsible for primary growth in roots and shoots.

    • Include:

      • Root apical meristem

      • Shoot apical meristem

    1. Lateral Meristems:

    • Vascular cambium & cork cambium responsible for secondary growth.

Apical Meristem

  • Functions:

    • Source of primary growth for both root and shoot.

    • Forms three other meristems:

    1. Protoderm (dermal tissue)

    2. Procambium (vascular tissue)

    3. Ground meristem (ground tissue)

Root and Shoot Structure

  • Root Primary Growth:

    • Involves three zones:

    • Zone of cell division

    • Zone of elongation

    • Zone of maturation

  • Features of root structure include:

    • Epidermis, cortex, vascular cylinder (stele), and pericycle.

Root Hairs

  • Function: Increase surface area for absorption.

  • Example: A 4-month-old rye plant can have approximately 14 billion root hairs covering about 400 square meters total surface area, cumulatively extending over 10,000 km when laid end to end.

Cross-Section Observations

  • Young Stems:

    • Differences between eudicot and monocot structures, specifically regarding vascular bundles.

  • Young Roots:

    • Eudicot roots have an endodermis as the innermost cortex layer; the stele is composed entirely of vascular tissue (xylem & phloem) resulting from procambium.

Lateral Roots

  • Origin: Arise in the pericycle, the outermost layer of the vascular cylinder, and extend outward through the cortex and epidermis.

Summary of Root Structure and Development

  • Key Components of Roots:

    • Root cap for protection, root hairs to enhance absorption, and the vascular cylinder (stele) comprising xylem and phloem.

    • Pericycle acts as the origin for lateral roots.

Practice Questions

  • Differences in Plant Reproduction:
    1) Unlike mosses, ferns have a dominant sporophyte.
    2) In angiosperms, double fertilization produces one embryo and one endosperm.
    3) Cell walls provide strength due to a complex mixture of polysaccharides.

Review Areas

  • Cell Types and Functions: Familiarize with dermal, ground, and vascular tissues along with their specific cell types and functions.

  • Meristem Functionality: Understand the role of meristems in plant growth and tissue differentiation.

  • Structural Differences: Compare and contrast the anatomical features between eudicot and monocot plants.