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Vascular Plant Structure, Growth, and Development

Plants’ Hierarchical Organization

  • Cell: fundamental unit of life
  • Tissue: group of cells consisting of 1+ cell types that perform a specialized function
  • Organ: consists of several type of tissues that carry out particular functions

Vascular Plant Organs

  • Vascular Plants: plants that can transport things up and down

  • Root system: underground

    • Roots rely on sugar produced by photosynthesis in the shoot system (cannot make sugar by themselves)
    • ==Root==: organ with important functions
    • anchoring the plant
    • absorbing minerals and water
    • storing carbs (excess made in photosynthesis
      • because plants aren’t always guaranteed to have sunlight
    • ==Lateral roots==: roots that emerge from the sides
    • Help anchor
    • Spread, help absorb nutrients
    • ==Primary root:== first root to emerge from the seed
    • ==Taproot System==: one main root and branches
    • ==Taproot==: develops from primary root & anchors plant in soil
    • ==Fibrous root system:== lots of small roots (no need for strong anchor)
    • ==Adventitious roots==: side roots that arise from the stem & give rise to branching lateral roots
    • ==Root hairs==: extensions of epidermal cells forming near root tip
    • Increase surface area, more absorption
    • ==Mycorrhizal associations:== roots have fungus living in it that help them transform something they cannot use into something they can
    • ex/ Break down unusable nitrogen (symbiotic relationship)
  • Types of Roots

    • ^^Prop roots:^^ more anchorage
    • ^^Storage roots:^^ store carbs
    • ^^Pneumatophores^^: some can do gas exchange (similar to leaves)
    • ^^“Strangling” aerial roots:^^ add further anchorage
    • ^^Buttress roots^^: very tall and thin roots above ground, typically in forests and jungles (spread far and wide)
  • Shoot system: above ground

    • Shoots rely on water and minerals absorbed by the root system
    • ==Stem==: plant organ consisting of nodes & internodes
    • ==Nodes==: points at which leaves are attached
    • ==Internodes==: space between nodes
    • ==Apical bud==: causes elongation of a young shoot
    • where plant grows taller
    • ==Axillary bud:== structure that can form a lateral branch, thorn, or flower
    • not main bud for height, additional buds for leaves, flowers, etc.
  • ==Leaf==: main photosynthetic organ

    • Intercept light, exchange gases, dissipate heat, and defend the plant from herbivores and pathogens
    • ex/ chemical coat prevents fungus from attacking plant
    • Consist of blade and petiole
    • ^^Blade^^: flat part
    • ^^Petiole^^: stalk that joins leaf to node of stem
    • Chloroplasts in leaf
    • light shines & photosynthesis occurs
    • Why are leaves flat?
    • To maximize surface area and sun exposure
    • ==Monocot==: 1 thing comes out of seed
    • Parallel veins
    • Vascular bundles are scattered throughout the ground tissue
    • ==Eudicot==: 2+ things come out of seed
    • Branching veins
    • Vascular tissue consists of vascular bundles arranged in a ring
    • ==Vein==: the vascular tissue of leaves
    • How do we categorize plants?
    • Determine if monocot or eudicot
    • What do the flowers look like?
    • What do the leaves look like?
    • ==Simple leaf==: 1 leaf
    • ==Compound leaf:== has several individual ^^leaflets^^

    Evolutionary Adaptations of Leaves

    • ^^Tendrils^^: help anchor it above the ground
    • ^^Spines^^: defend
    • ^^Storage leaves^^: store carbs
    • ^^Reproductive leaves:^^ leaves can reproduce

Dermal, Vascular, and Ground Tissues

  • Tissue system: interconnected leaf, stem, and root
    • Dermal tissue system
    • Nonwoody plants
      • Dermal tissue system: protection of the plant, consists of epidermis
      • ==Cuticle==: waxy coating that helps prevent water loss from the epidermis (too hot from the sun)
      • No periderm
    • Woody plants
      • ==Periderm==: outer layer protective tissues that replace epidermis in older regions of stems and roots
      • They have both periderm & epidermis
    • ==Guard cells==: epidermal cells that facilitate gas exchange in shoots
      • too hot→ close
      • prevent water loss
      • cold→ open
    • ==Trichomes==: outgrowths of the shoot epidermis that can reduce water loss and reflect light, also defend against insects
  • Vascular tissue system: facilitates transport of materials through the plant & provides mechanical support
    • ==Xylem==: conducts water & dissolved minerals upwards from roots to shoots
    • ==Phloem==: transports sugars from leaves (where they are made) to storage/sites of growth (down)
    • ==Stele==: everything except the leaves
    • In angiosperms, stele of root is central solid ==vascular cylinder== (in the middle)
    • ==Angiosperms==: plants that make flowers
  • Ground tissue system: tissues that are neither dermal nor vascular
    • Cells specialized for storage, photosynthesis, support, and transport (everything else)
    • ==Pith==: ground tissue inside vascular tissue
    • ==Cortex==: ground tissue outside vascular tissue
  • Stem cells are not differentiated
    • Structure changes depending on function

Different Meristems Generate New Cells for Primary & Secondary Growth

  • Indeterminate growth: plant grows until death
  • Determinate growth: stop growing after certain age/size
  • Meristems: unspecialized tissues made of dividing cells
    • how plants grow continuously
    • ==Apical Meristems:== located at tips of roots and shoots (up and down)
    • ^^Primary growth^^: growing vertically
    • ==Lateral Meristems==: grow sides/in diameter/in width (^^secondary growth)^^
    • ==Vascular Cambium==: adds layers of vascular tissue called secondary xylem (wood) and secondary phloem
    • ==Cork Cambium:== replaces epidermis with periderm (thicker and tougher)
  • ==Initials==: stem cells (new cells) that remain in meristem and produce more cells
    • unspecialized
    • the farther from meristem, they get specialized

Primary Meristems

  • ==Protoderm==: produces dermal tissue
  • ==Ground meristem==: produces ground tissue
  • ==Procambium==: produces vascular tissue

Classification of Angiosperms

  • ==Annuals==: complete life cycle in <=1 yr
  • ==Biennials==: require 2 growing seasons
  • ==Perennials==: live for many years
  • “Life cycle”- moment of sprouting→ reproduction
  • ==Root Cap==: collection of cells that protects root apical meristem as root pushes through soil

Primary Growth of Shoots

  • ==Shoot apical meristem==: dome-shaped mass of dividing cells at shoot tip
    • leaves of apical bud protect meristem
  • Axillary buds develop from ==meristematic cells== left at the bases of ==lead primordia== (collection of cells that were told to become leaves)
    • Kept dormant by chemical communication from apical bud (==apical dominance)==
    • If axillary bud + apical bud closer→ more inhibited
    • Released if shoot tip removed or shaded

Leaf Growth and Anatomy

  • Leaves develop from lead primordia along sides of shoot apical meristem
  • Epidermis (outer dermal tissue) in leaves interrupted by ==stomata== (pores that allow CO2 and O2 exchange between air and photosynthetic cells)
    • Major evaporative loss of water, so located on bottom of leaf
    • Each stomatal pore has 2 guard cells to regulate opening and closing
  • ==Mesophyll==: where photosynthesis occurs; ground tissue of leaf
    • Between upper and lower epidermis
    • 2 layers:
    • ^^Palisade mesophyll:^^ upper part of leaf
    • ^^Spongy mesophyll:^^ lower part of leaf (loose for gas exchange)
  • Veins are leaf’s vascular bundles; skeleton of leaf
    • Each vein enclosed by protective ==bundle sheath== (xylem & phloem)
    • Why?
      • If xylem and phloem damaged, no transport can occur

Secondary Growth

  • Growth in thickness produced by lateral meristems
    • Occurs in ==gymnosperms== (don’t make flowers) & many eudicots; RARE in monocots
  • Occurs in stems and roots of woody plants; RARELY in leaves
    • Consists of growth of tissues produced by vascular cambium and cork cambium

Growth, Morphogenesis, and Cell Differentiation Produce the Plant Body

  • Developmental plasticity: effect of environment on development
    • Plant can change form depending on environmental triggers
  • ==Morphogenesis==: form body shapes
  • Growth is irreversible
  • Cell differentiation
    • Certain genes become accessible/inaccessible for the lifetime of the cell

Model

  • Arabidopsis
    • Why is it a model?
    • Small size, short generation time, abundant seed production, small genome size
    • Appearance changes depending on environment (genes on/off)
    • Easily transformed with transgenes (genes from different organism)

Cell Division

  • Increases # of cells & potential for growth
  • Cell elongation: process responsible for increase in plant size
    • ==Prophase==: nucleus dissolves
    • ==Metaphase==: chromosomes line up in middle
    • ==Anaphase==: chromosomes separated by microtubules
    • ==Microtubules==: pull apart chromosomes; become concentrated into ring called ^^preprophase^^ band that predicts future plane of cell division
    • ==Telophase==: cell dividing
    • ==Cytokinesis==: pinching off middle part
  • Transverse cell division: left and right
  • Longitudinal cell division: up and down
  • Symmetry of cell division & distribution of cytoplasm affects cell fate
    • Concentration of chemicals→ different genes turn on
    • Axis of division is predetermined
  • ==Polarity==: condition of having structural/chemical differences at opposite ends of organism
    • Asymmetrical cell divisions
    • Uneven distribution of molecules→ determine what is the root and what is the shoot
  • Plant cells grow fast by intake and storage of water in large central vacuole
    • Cellulose microfibrils in cell wall restrict direction of cell elongation (prevent from lysing)

Morphogenesis & Pattern Formation

  • Pattern formation: development of specific structures in specific locations
    • ==Lineage-based mechanisms==: cell fate determined early in development and passed on to daughter cells (animals)
    • ==Position-based mechanisms:== cell fate determined by final position (plants)
  • Homeotic (Hox) Genes: affect # and placement of appendages in embryos
    • MADS-box genes: position & # of plant organs
    • KNOTTED-1: development of lead morphology (3d shape)
  • Different cells have different proteins
    • Different structure→ different function
  • Gene activation/inactivation depends on cell-to-cell communication (hormones and signals)
  • ==Phase changes:== developmental stages
    • Juvenile phase→ adult vegetative stage→ adult reproductive stage
    • Occur within shoot apical meristem

Genetic Control of Flowering

  • ==Flower Formation:== phase change from vegetative growth→ reproductive growth
    • Triggered by environmental cues & internal signals
    • ex/ sudden increase in water
    • Associated with switching on of ==floral meristem identity genes==
    • takes energy to flower
  • Order of primordium’s emergence determines its fate:
    • ==Sepal==: supports flower after blooming
    • ==Petal==: attract pollinators
    • ==Stamen==: male reproductive part; make pollen
    • ==Carpel==: female reproductive part
  • 3 classes of floral organ identity genes
    • ABC Hypothesis: how floral organ identity genes direct the formation of the four types of floral organs
    • ==A==→ sepal
    • ==A+B==→ petal
    • ==B+C==→ stamen
    • ==C==→ carpel