Unit 4 Ch 35 Vascular Plant Structure, Growth, and Development Flashcards

Hierarchical Organization: Organs, Tissues, and Cells

  • Vascular plants exhibit a hierarchical organization consisting of organs, tissues, and cells. At every level, structure has been molded by natural selection to fit specific functions.
  • Fundamental Units:
    • Cell: The fundamental unit of life.
    • Tissue: A group of cells consisting of one or more cell types that together perform a specialized function.
    • Organ: Consists of several types of tissues that together carry out particular functions.
  • Basic Morphology:
    • Vascular plant shape reflects their evolutionary history as terrestrial organisms inhabiting two environments: below ground and above ground.
    • Subterranean resources: Absorption of water and minerals.
    • Aerial resources: Absorption of CO2CO_2 and light.
    • The organs form two distinct systems: the root system and the shoot system (consisting of stems and leaves).
    • Relationship between systems: Roots are generally non-photosynthetic and depend on photosynthates (sugars/carbohydrates) imported from the shoot system. Conversely, the shoot system depends on water and minerals absorbed by the root system from the soil.

Vascular Plant Organs: Roots, Stems, and Leaves

  • Roots: An organ that anchors the plant, absorbs minerals and water, and often stores carbohydrates/reserves.
    • Primary Root: The first organ/root to emerge from a germinating seed. It branches into lateral roots.
    • Taproot System: Found in tall, erect plants with large shoot masses. Consists of one main vertical root (the taproot) developed from the primary root. Primarily functions in anchorage; absorption is restricted to tips of lateral roots. Allows plants to grow taller for better light and better pollen/seed dispersal.
    • Fibrous Root System: Common in small or trailing vascular plants (and most monocots). The primary root dies early, and adventitious roots (roots arising from unusual locations like stems) emerge, each forming its own lateral roots. This thick mat is excellent for preventing soil erosion.
    • Root Hairs: Thin, finger-like extensions of root epidermal cells located near the tips of elongating roots. They increase the surface area enormously for absorption. A four-month-old rye plant can have 1414 billion root hairs, which if laid end-to-end would cover 10,000 km10,000\,km.
    • Mycorrhizal associations: Symbiotic interactions with soil fungi that enhance mineral absorption.
    • Specialized Root Adaptations:
      • Buttress roots: Tall tropical trees with shallow systems use aerial roots for architectural support.
      • Prop roots: Aerial, adventitious roots (e.g., maize) that support top-heavy plants.
      • Pneumatophores (Air roots): Produced by trees in tidal swamps (e.g., mangroves) to obtain oxygen above waterlogged mud.
      • Storage roots: Store food and water (e.g., beets).
      • Strangling aerial roots: Germinate in crevices of host trees, grow to the ground, and eventually shade out and kill the host (e.g., strangler fig).
  • Stems: Plant organs bearing leaves and buds. Functions include elongating/orienting the shoot to maximize photosynthesis and elevating reproductive structures.
    • Components: Nodes (leaf attachment points) and Internodes (segments between nodes).
    • Buds:
      • Apical Bud: Located at the growing shoot tip where growth is concentrated.
      • Axillary Bud: Located in the upper angle (axil) of each leaf; can form lateral branches, thorns, or flowers.
    • Modified Stems:
      • Rhizomes: Horizontal shoots growing just below the surface.
      • Stolons: Horizontal "runners" (e.g., strawberries) that allow for asexual reproduction.
      • Tubers: Enlarged ends of rhizomes or stolons specialized for food storage (e.g., potatoes; the "eyes" are axillary buds).
  • Leaves: The main photosynthetic organ. Functions include light interception, gas exchange, heat dissipation, and defense.
    • Anatomy: Consists of a flattened blade and a stalk called the petiole (joins the leaf to the stem at a node). Grasses/monocots lack petioles and have a sheath instead.
    • Venation Patterns: Monocots have parallel major veins. Eudicots have a branched network of veins arising from a midrib.
    • Leaf Morphologies: Simple leaves (single undivided blade) vs. Compound leaves (blade divided into multiple leaflets; leaflets have no axillary bud at their base).
    • Leaf Adaptations:
      • Tendrils: Modified leaves (or stems) for clinging/support.
      • Spines: Modified leaves for protection (e.g., cacti); photosynthesis happens in the fleshy stem.
      • Storage leaves: Bulbs (e.g., onions) have modified leaves that store food.
      • Reproductive leaves: Produce adventitious plantlets that take root (e.g., Kalanchoë daigremontiana).

Dermal, Vascular, and Ground Tissue Systems

  • Dermal Tissue System: Protective outer covering.
    • Epidermis: Single layer of tightly packed cells in nonwoody plants.
    • Cuticle: Waxy coating preventing water loss in most stems and leaves.
    • Periderm: Replaces epidermis in older regions of woody stems/roots.
    • Specialized Cells: Guard cells (gas exchange), Trichomes (outgrowths for defense, light reflection, or reducing water loss).
  • Vascular Tissue System: Facilitates material transport and mechanical support.
    • Xylem: Conducts water and dissolved minerals upward from roots.
    • Phloem: Transports sugars from sources (leaves) to sinks (roots/growth sites).
    • Stele: Collective name for the vascular tissue of a root or stem. Root steles are solid cylinders; stem/leaf steles consist of vascular bundles.
  • Ground Tissue System: Tissue that is neither dermal nor vascular.
    • Pith: Ground tissue internal to the vascular tissue.
    • Cortex: Ground tissue external to the vascular tissue.
    • Functions: Storage, photosynthesis, support, and short-distance transport.

Specialized Plant Cell Types

  • Parenchyma cells: Thin, flexible primary walls; generally lack secondary walls. Large central vacuole. Perform photosynthesis (in leaves) and storage (amyloplasts store starch in roots/stems). Retain the ability to divide and differentiate (totipotent).
  • Collenchyma cells: Grouped in strands, support young parts of the shoot. Elongated cells with unevenly thickened primary walls. Living at maturity and flexible, allowing growth.
  • Sclerenchyma cells: Rigid supporting elements with thick secondary walls containing lignin (25%25\% of dry mass of wood). Mature cells cannot elongate and are often dead at functional maturity.
    • Sclereids: Boxy, irregular shape; found in nutshells, seed coats, and pear grit.
    • Fibers: Long, slender, tapered strands (e.g., flax for linen, hemp for rope).
  • Water-conducting cells of Xylem: Dead and lignified at functional maturity.
    • Tracheids: Long, thin cells with tapered ends; water moves through pits (thinner regions with only primary walls).
    • Vessel elements: Wider, shorter, thinner walled; aligned into long pipes (vessels). End walls have perforation plates for free water flow.
  • Sugar-conducting cells of Phloem: Alive at functional maturity.
    • Sieve cells: Found in seedless vascular plants and gymnosperms.
    • Sieve-tube elements: Lack a nucleus, ribosomes, and distinct vacuoles to facilitate nutrient pass-through. Ends have sieve plates with pores.
    • Companion cells: Nonconducting cells connected to sieve-tube elements by plasmodesmata; their nucleus/ribosomes serve the sieve-tube member. Also help load sugars.

Meristematic Growth and Life Cycles

  • Indeterminate Growth: Plants grow throughout their life due to undifferentiated tissues called meristems.
  • Determinate Growth: Growth stops after reaching a certain size (e.g., leaves, flowers).
  • Types of Meristems:
    • Apical Meristems: Located at root/shoot tips; provide cells for Primary Growth (increase in length).
    • Lateral Meristems: Vascular cambium (adds secondary xylem/wood and secondary phloem) and Cork cambium (replaces epidermis with periderm). Responsible for Secondary Growth (increase in diameter).
  • Life Cycles:
    • Annuals: Complete cycle in 11 year or less (wildflowers, staple crops like rice/wheat).
    • Biennials: Require 22 growing seasons (e.g., turnips).
    • Perennials: Live many years (trees, shrubs, buffalo grass estimated at 10,00010,000 years old).

Primary Growth of Roots and Shoots

  • Roots: Growth occurs in three overlapping zones.
    • Zone of cell division: Includes root apical meristem and stem cells (initials). Protected by the root cap which secretes polysaccharide slime.
    • Zone of elongation: Cells lengthen up to 1010 times their original size, pushing the tip into the soil.
    • Zone of differentiation (maturation): Cells become distinct types. Protoderm becomes epidermis; Ground meristem becomes cortex; Procambium becomes the vascular cylinder.
    • Tissue arrangement: In eudicot roots, xylem is starlike in cross-section. In monocot roots, vascular tissue is a ring surrounding a parenchyma core.
    • Pericycle: Outermost cell layer in the vascular cylinder; the origin site for lateral roots.
  • Shoots: Derived from the shoot apical meristem.
    • Leaf primordia: Projections along the sides of the apical meristem that develop into leaves.
    • Apical dominance: Chemical communication (hormones) where the apical bud inhibits axillary bud growth. Pruning disrupts this, leading to bushier growth.
    • Stem Anatomy: Eudicots have vascular bundles arranged in a ring; monocots have scattered vascular bundles.
    • Leaf Anatomy:
      • Stomata: Pores flanked by guard cells for gas/water control.
      • Mesophyll: Palisade mesophyll (tightly packed for light capture) and Spongy mesophyll (loose air spaces for gas circulation).
      • Bundle sheath: Layer of cells enclosing veins.

Secondary Growth in Woody Plants

  • Vascular Cambium: A cylinder of meristematic cells one cell thick. Adds secondary xylem (to the inside) and secondary phloem (to the outside).
    • Vascular Rays: Radial files of parenchyma cells connecting xylem and phloem for transport and storage.
    • Early Wood (Spring): Large diameter, thin walls to maximize water delivery.
    • Late Wood (Summer): Thick-walled cells for support.
    • Dendrochronology: Analyzing tree growth ring patterns to study climate history.
  • Heartwood vs. Sapwood:
    • Heartwood: Older, central layers of secondary xylem that no longer transport water; dark due to resins/defense compounds.
    • Sapwood: Newest, outer layers that still transport xylem sap.
  • Cork Cambium and Periderm:
    • Tissues external to the vascular cambium are called Bark (includes secondary phloem and periderm).
    • Cork cells: Mature cells deposit suberin (waxy/hydrophobic) and die. Protects against water loss and pathogens.
    • Lenticels: Small, raised areas in the periderm where space between cork cells allows for gas exchange.

Growth, Morphogenesis, and Cell Differentiation

  • Development: The series of changes by which cells form tissues/organs. Influenced by genetics and external environment.
  • Developmental Plasticity: Ability to alter form in response to conditions (e.g., Cabomba aquatica produce different leaves above vs. below water).
  • Model Organism: Arabidopsis thaliana.
    • Advantages: Small size, short generation time (66 weeks), small genome (27,00027,000 genes, 55 pairs of chromosomes).
  • Processes of Development:
    • Growth: Irreversible increase in size via expansion. Plant cells expand using water uptake in vacuoles (90%90\% of expansion). Orientation of cellulose microfibrils in the cell wall determines the axis of expansion.
    • Morphogenesis: Pattern formation determined by position-based mechanisms (signaling from neighbors) rather than just cell lineage.
    • Cell Differentiation: Regulation of gene expression. Example: The homeotic gene GLABRA−2GLABRA-2 prevents root hair formation in epidermal cells contacting only one cortical cell.

The Genetic Control of Flowering

  • Flower formation is a phase change from vegetative to reproductive growth, triggered by environmental cues (day length) and internal signals (hormones).
  • Basic Flower Structure: Four whorls of organs:
    1. Sepals (outermost)
    2. Petals
    3. Stamens
    4. Carpels (innermost)
  • ABC Hypothesis: Three classes of genes determine organ identity:
    • Whorl 1 (Sepals): Class A only.
    • Whorl 2 (Petals): Class A + Class B.
    • Whorl 3 (Stamens): Class B + Class C.
    • Whorl 4 (Carpels): Class C only.
    • Mutual Inhibition: If Gene A is suppressed, Gene C is expressed in all whorls (and vice-versa).

Questions and Discussion

  • Scientific Skills Exercise: Studies on Acer rubrum show that leaf "toothiness" (size and number) is determined by both genotype and phenotype. Northern populations had higher tooth numbers than southern populations. Growing southern seeds in the north increased tooth number, demonstrating phenotypic plasticity.
  • Concept Check 35.2 Question 3: Is leaving carrots in the ground for a second year a good idea? No. Since carrots are biennials, the second year is dedicated to flowering and fruiting, which consumes the energy stored in the taproot, making it woody and less palatable.
  • Critical Observation: Tree growth rings indicate that a thick ring signifies a warm/wet year, while a thin ring signifies a cold/dry year.
  • Tissue Destruction Sequence: To drill to the center of a tree, one would destroy: Periderm (cork, cork cambium), Secondary Phloem, Vascular Cambium, Sapwood (secondary xylem), Heartwood (secondary xylem), and Pith.