Cells & Simple Tissues – Lecture 3

Microscopy Techniques

  • Polarized-light microscopy

    • Microscope fitted with two polarizing filters (one before and one after the specimen).
    • When the filters are “crossed,” the overall field appears dark; only structures that can rotate the plane of light (i.e., crystalline substances) glow.
    • Plant examples that light up:
    • Starch granules in potato tuber cells.
    • Cellulose-rich secondary cell walls in fibres or stone cells (sclereids).
    • Mineral inclusions such as calcium-oxalate crystals.

  • Compound (transmitted-light) microscope

    • Single objective lens, condenser below specimen.
    • Provides high-magnification, thin-section imaging; standard instrument in course labs.

  • Binocular / dissecting microscope

    • Two objective lenses inside the head → true stereoscopic view.
    • Lower magnification; ideal for dissection and sample preparation before transferring to a compound scope.

  • Hand sectioning + staining

    • Free-hand razor slicing of fresh tissue; essential skill revisited in upcoming labs.
    • Relies heavily on differential stains that target cell-wall chemistry.

  • Fluorescence & confocal microscopy

    • Epifluorescence setup: excitation light delivered from above through the objective; barrier filters prevent eye exposure to excitation wavelength.
    • Fluorochrome absorbs high-energy light and re-emits lower-energy light → specimen self-glows against dark background.
    • Important plant auto-fluorophore: lignin (bright white/blue under UV).
    • Confocal laser scanning (mentioned last lecture) is an advanced fluorescence mode that generates optical sections.

Workshop Context & Real-World Applications

  • Upcoming Workshop 1 (Monday)

    • Goal: diagnose practical problems through plant anatomy.
    • Students brainstorm appropriate sectioning/staining techniques, target cell types, and expected observations.

  • Case study 1 – New Zealand native flax (harakeke)

    • Certain iwi-selected cultivars yield easily extracted fibres suitable for weaving, whereas others are better for full-leaf plaiting.
    • Underlying cause = anatomical differences in leaf tissues/cells.

  • Case study 2 – Kiwifruit PSA outbreak

    • Pathogen: PSA bacterium with cell-wall-degrading enzymes.
    • Infection path: epidermis → phloem → xylem → systemic vascular blockage → shoot death.
    • Industry response: breeding/selection of resistant cultivars.

  • Case study 3 – Kauri (Agathis australis) water-use study

    • Installation of Sap-Flow probes demands precise knowledge of stem anatomy to avoid non-conducting zones.
    • Leaf cross-sections stained for lignin (red dye) confirm sclerophyllous, lignified tissues → indicates tough, nutrient-efficient foliage.

Meristems and Primary Tissue Systems (Revision)

  • Apical meristems generate primary meristems:
    • Protoderm
    • Ground meristem
    • Procambium
  • These in turn differentiate into the three primary tissue systems:
    • Dermal
    • Ground
    • Vascular
  • Developmental gradient: apical initials (tip) → primary meristems (slightly older) → differentiated tissues (even further back).
    • Observing a series of cross-sections down a shoot/root reveals sequential stages.

Cellular Differentiation: Definition & Hallmarks

  • Differentiation = process by which initially similar daughter cells acquire distinct structures/functions during maturation.
  • Diagnostic features:
    • Change in overall cell size/shape (often elongation).
    • Cytoplasmic modifications: vacuole enlargement, plastid conversion (e.g., chloroplast ↔ amyloplast).
    • Cell-wall alterations in thickness, layering, and chemical composition.
  • Example maturation series (illustrated in lecture): vessels, sieve-tube elements, fibres, generic parenchyma.

Plant Cell-Wall Composition & Histochemistry

  • Cellulose

    • Synthesised at the plasma membrane by six-membered cellulose-synthase “rosettes.”
    • Rosettes track cortical microtubules; extrusion likened to toothpaste from a moving nozzle → yields oriented microfibrils.

  • Pectins

    • Hydrophilic polysaccharides (gel-like\text{gel-like}).
    • Impart plasticity to primary walls; abundant in soft fruits and jams.

  • Hemicelluloses

    • Diverse backbone-and-side-chain polysaccharides that tether adjacent cellulose microfibrils.
    • Degree of cross-linking modulates wall extensibility.

  • Proteins & glycoproteins

    • Enzymes, structural glycine-rich proteins, signalling molecules; evidence that the wall (apoplast) is metabolically active.

  • Lignin

    • Complex phenolic polymer → rigidity, compressive strength, hydrophobic barrier (waterproofing).
    • Key to wood (lignified secondary walls), drought resistance, pathogen defence.

  • Waxy/fatty polymers

    • Cutin, suberin, outer waxes → restrict water loss & entry of pathogens.
Staining & Metachromasy
  • Toluidine Blue O (TBO)
    • Metachromatic: gives different colours on one slide depending on wall chemistry.
    • Pink-purple → pectin-rich primary walls.
    • Blue/teal/green → lignified or highly cross-linked walls.
    • Excellent for fresh, hand-cut sections in teaching labs.
Wall Layers & Pits
  • Middle lamella (outermost) – pectin-rich “glue” between cells.
  • Primary wall – formed first; thin, flexible.
  • Secondary wall – deposited internal to primary; thicker, often lignified.
  • Pits: thin regions in secondary wall; pit-pair flanks a pit membrane (primary wall + middle lamella) that retains plasmodesmata for symplastic continuity.

Basic Plant Cell Types

1 – Parenchyma
  • Only primary walls (thin).
  • Living at maturity.
  • Functions: photosynthesis, storage, secretion, wound repair.
  • Examples:
    • Potato tuber parenchyma packed with starch-filled amyloplasts.
    • Onion cortex = large vacuolated parenchyma cells.
2 – Collenchyma
  • Unevenly thickened primary walls; wall remains stretchable.
  • Provides tensile support during primary growth (young stems, petioles).
  • Two common subtypes:
    • Angular (corners thickened).
    • Lamellar (inner/outer tangential walls thickened).
3 – Sclerenchyma
  • Thick, lignified secondary walls; cells dead at functional maturity.
  • Mechanical, protective function.
  • Two main forms:
    • Fibres – long, slender; bundled; economic sources of paper, textiles, rope.
    • Sclereids (stone cells) – variable shapes; short/brachysclereids give gritty texture in pears.

Simple vs Complex Tissues & Tissue Systems

  • Simple tissue = composed of a single cell type (e.g.
    • Parenchyma tissue in pith
    • Collenchyma strands in herbage)
  • Complex tissue = mixed cell types performing a collective role (most notably vascular tissue).
Three Tissue Systems (organised concentrically in most organs)

  1. Dermal system

    • Outermost protective layer; mostly unspecialised epidermal cells.
    • Special cells embedded: guard cells, subsidiary cells, trichomes, secretory hairs.
    • Cuticle: extracellular layer of cutin + waxes → hydrophobic leaf surface (water droplets bead).
    • Trichome diversity: single-cell, multicellular, branched (illustrated example).

  2. Ground system

    • The “packing” or bulk matrix.
    • Cortex = dermal → vascular; Pith = central core; Mesophyll = photosynthetic leaf ground tissue.
    • Usually parenchyma; may include collenchyma (support) or sclerenchyma caps (strength).

  3. Vascular system (complex tissue)

    • Xylem: tracheids, vessel elements, fibres, parenchyma → transports H2O\mathrm{H_2O} & minerals.
    • Phloem: sieve-tube elements, companion cells, fibres, parenchyma → transports sucrose + signalling molecules\text{sucrose + signalling molecules}.
    • Origin = procambium; arrangement changes root ↔ stem ↔ leaf.

Key Stains & Identification Workflow

  1. Hand-section fresh sample.
  2. Apply Toluidine Blue O (rapid polychrome).
  3. Under compound scope, observe colour & thickness:
    • Pink → primary wall (pectic parenchyma).
    • Aqua/blue → lignified sclerenchyma/xylem.
    • Greenish rim → partially lignified collenchyma.
  4. Correlate location to expected map: epidermis outside, cortex next, vascular ring/core.
  5. Cross-confirm with polarized light (crystals, secondary walls) or fluorescence (lignin).

Conceptual Take-Home Messages

  • Only three basic cell types (parenchyma, collenchyma, sclerenchyma) + three primary tissue systems (dermal, ground, vascular) build the immense diversity of plant anatomy.
  • Differentiation is tracked chiefly by cell-wall chemistry and architecture; therefore, stains & microscopy are indispensable.
  • Understanding tissue layout lets researchers:
    • Diagnose disease pathways (e.g., PSA bacteria).
    • Optimize industrial fibre extraction (flax, wood).
    • Position sensors accurately (sap-flow in Kauri).
  • Mastery of sectioning, staining, and microscopic interpretation will be practiced in labs and assessed in the upcoming workshop.