Plant Biology Notes

Plant Diversity

• Alternation of Generations:

  • Gametophyte (n, multicellular): Produces gametes via mitosis.
  • Fertilization: Results in a Zygote (2n).
  • Sporophyte (2n, multicellular): Develops and produces spores (n) via meiosis.
  • Spores germinate: Give rise to new gametophytes.
  • Evolutionary Trend: Gametophyte dominance (bryophytes) to Sporophyte dominance (seed plants).
    • Sporophyte dominance provides greater genetic buffering and independence from water.

• Evolutionary Advantages of Seeds:

  • Protection: Seeds encase the embryo in a protective, nutrient-rich package.
  • Dormancy: Seeds can remain dormant until conditions are ideal.
  • Dispersal: Seeds enable long-distance dispersal via wind, water, or animals.
  • Ecological Advantage: Reduces competition with the parent plant.

Angiosperm Anatomy

• Meristems:
  • Apical Meristem:
    • Location: Root and shoot tips.
    • Products: Primary tissues (↑ length).
    • Function: Protected by root cap; generates protoderm (dermal tissue), procambium (vascular tissue), ground meristem (ground tissue), intercalary (length in internode).
  • Lateral Meristem:
    • Location: Stems and roots.
    • Products: Secondary tissues (thickness).
    • Contains: Cork and Vascular Cambium
    • Vascular cambium (Lateral):
      • Cylindrical ring in stems/roots
      • Secondary xylem (inside) + secondary phloem (outside)
      • Creates wood & inner bark; responsible for growth rings
    • Cork cambium (Lateral)
      • Outer cortex of woody plants
      • Cork + sometimes phelloderm (periderm)
      • Replaces epidermis with waterproof, pathogen‑resistant bark
• Primary vs Secondary Growth
  • Primary → elongation; all plants; forms primary plant body.
  • Secondary → radial thickening; only woody eudicots & gymnosperms; forms bark & wood.

Anatomy of Leaves, Roots, and Stems

• Leaf Anatomy and Function

  • Function: Main site of photosynthesis, gas exchange, and transpiration.
  • Key Structures:
    • Cuticle:
      • Description: Waxy outer layer.
      • Function: Prevents water loss
    • Upper epidermis:
      • Description: Transparent layer.
      • Function: Allows light in, protects inner tissues
    • Palisade mesophyll:
      • Description: Tightly packed cells rich in chloroplasts
      • Function: Primary site of photosynthesis
    • Spongy mesophyll:
      • Description: Loosely packed with air spaces
      • Function: Gas exchange ($CO<em>2$, $O</em>2$)
    • Veins (vascular bundles):
      • Description: Xylem and phloem
      • Function: Transport water, minerals, and sugars
    • Stomata (with guard cells):
      • Description: Openings mainly on lower surface
      • Function: Regulate gas exchange and transpiration

• Root Anatomy and Function

  • Function: Anchorage, water and mineral absorption, and sometimes storage.
  • Key Structures:
    • Root hairs:
      • Description: Extensions of epidermal cells
      • Function: Increase surface area for absorption
    • Epidermis:
      • Description: Outer protective layer
      • Function: Protection and water intake
    • Cortex:
      • Description: Parenchyma cells
      • Function: Storage and movement of nutrients
    • Endodermis:
      • Description: Layer surrounding vascular tissue
      • Function: Regulates what enters the xylem via Casparian strip
    • Xylem:
      • Description: Vascular tissue
      • Function: Transports water and minerals upward
    • Phloem:
      • Description: Vascular tissue
      • Function: Transports sugars from leaves to root

• Stem Anatomy and Function

  • Function: Support, transport, and in some cases, storage or photosynthesis.
  • Key Structures:
    • Epidermis:
      • Description: Protective outer layer
      • Function: Reduces water loss and protects stem tissues
    • Cortex:
      • Description: Layers of supportive/storage cells
      • Function: Storage and mechanical support
    • Vascular bundles:
      • Description: Xylem and phloem (in rings or scattered)
      • Function: Transport of water (xylem) and food (phloem)
    • Pith (in dicots):
      • Description: Central core of parenchyma cells
      • Function: Storage and internal support

• Layers in a cross section of a woody stem (from center to outside):

  1. Secondary xylem (wood)
  2. Vascular cambium
  3. Secondary phloem
  4. Cork cambium
  5. Cork (outer bark)
  • Bark = secondary phloem, cork cambium, cork + phelloderm
    • Lenticels are pores in bark for gas exchange
  • Periderm (outer bark) = cork cambium + cork + phelloderm

Plant Transport

• Overview of transport:

  • Water moves unidirectionally UPWARDS
  • Carbohydrates move bidirectionally in the plant both UP and DOWN

• Cohesion-Tension Theory of Water Transport in the Xylem

  • Explains how water moves upward from roots to leaves through the xylem in plants without the use of energy (ATP).

  • Key Steps in the Process

    1. Transpiration:
       • Water evaporates from stomata in leaves (mainly during the day).
       • This creates negative pressure (tension) in the leaf’s air spaces and xylem.
    2. Sticky Column:
       • Cohesion (Water sticks to water):
         • Water molecules form hydrogen bonds with each other.
         • This cohesion forms a continuous water column in the xylem vessels.
       • Adhesion (Water sticks to walls of xylem):
         • Water also adheres to the hydrophilic walls of xylem vessels.
         • This helps resist gravity and keeps the water column stable.
    3. Water is Pulled Upward:
       • As water exits the leaves, more is pulled up to replace it—like sucking on a straw.
       • The tension created by transpiration drives this upward movement.

  • Water Potential Gradient: Highest in roots (soil), lowest in leaves (air).

  • Cavitation Risk: Air bubbles can break columns; mitigated by pit connections and narrow conduits (tracheids/vessels).

• Phloem Transport (Pressure‑Flow Hypothesis)

  1. Sources & Sinks

     • Sources: Mature leaves (photosynthesis) or storage tissues.
     • Sinks: Growing tips, roots, developing fruits.

  2. Phloem Loading

     • Active transport of sucrose into sieve-tube elements (requires companion-cell ATP). In the source
     • Because the sieve tube is now “salty”, water from nearby xylem moves in osmotically. Lowers solute potential in phloem ⇒ water influx from xylem ⇒ turgor pressure ↑.

  3. Bulk Flow

     • Pressure gradient (High to low) drives sap toward sinks.

  4. Phloem Unloading

     • Active removal of solutes at sink from phloem⇒ water follows by osmosis; some returns via xylem. Turgor pressure drops.
  • Continuous circle. In essence: plants load sugars into the phloem at sources, water follows and builds pressure, and this pressure pushes the sugary sap toward sinks where sugars are removed and water exits—producing a solar‑powered, plant‑wide conveyor belt for food distribution

Plant Nutrition and Defense

• Nutritional Requirements: Chris (C) HOPKNS CaFe is Mighty good (Mg)

• Cation exchange: the chemical handshake between root‑released cations(protons) and nutrient cations held on negatively charged soil particles

• Nutrient Acquisition

  • By actively acidifying their micro‑environment (negative ions stay in solution surrounding root, creating a charge gradient that tends to pull positive ions out of root cells), plants swap H⁺ for essential minerals (Active Transport is required to acquire and maintain K+ and other positive ions in the root), releasing them into solution for immediate uptake.

• Special Nutrient‑Acquisition Strategies

  • Nitrogen‑fixing symbiosis (rare - mostly in plants from bean family)
    • Mutualism
    • Legumes + Rhizobium (root nodules)
    • N2 -> NH3 (Fixation) NH₃ -> NO₃
    • Nitrate and ammonia supply for amino & nucleic acids
    • Energy cost of nodulation; O₂ tightly regulated by leghemoglobin.
  • Mycorrhizal Fungi (very common)
    • Mutualism
    • Fungi that live in association with plant roots
    • Massive surface area boost → P, and water uptake; drought resistance
    • photosynthate given to fungus.
  • Carnivory (rare)
    • Direct eat
    • Direct N (and some P) from digested insects via enzyme‑rich leaves
    • Restricted to acidic, N‑poor soils; photosynthetic.
  • Parasitism
    • Exploits host
    • Steals water, minerals, or sugars from host vasculature
    • Photosynthetic - obtain inorganic nutrients and water from other plants.
    • Nonphotosynthetic -obtain manufactured sugars

• Static (Always there) Defenses

  • Physical barriers(First line)
    • Dermal tissue:
      • Cuticle, epidermis (waxy covering)
      • Suberin in endodermis & bark (contains fatty acids)
      • Specialized dermal protective structures: trichomes, thorns, and bark
    • Block entry, reduce water loss, seals wounds

Plant Responses

• Plant responses to light

  • Photosynthesis

    • Chlorophyll used to capture light energy
    • Light energy used to synthesize organic compounds

  • Photomorphogenesis

    • Morphogenesis-development of form
    • Growth and development of plants in response to light (not directional).
    • Phytochrome: Red/far-red light

  • Phototropism

    • Tropism-turn or move in response to a stimulus
    • Directional growth in response to light.
    • Blue light receptors: phototropin 1 and 2
    • Shoots positively phototropic

  • Photoperiodism

    • Using light to measure day-length.
    • Phytochrome: Red/far-red light

• Phytochrome System

  Property	Pr	Pfr
  Absorbs	Red	Far‑red (≈ 730 nm)
  Activity	Inactive	Active – enters nucleus or triggers kinase cascade
  Fate	Converts to Pfr in sunlight	Converts back to Pr in shade; degraded via proteasome

  • Relative amounts of Pr versus Pfr provide estimates of day length.
  • Full sunlight is high in red light.
  • Filtered sun is higher in far-red light.
  • Long night: no Pfr at dawn
  • Short night: more Pfr at dawn

• Key Pfr‑mediated responses

  • Seed germination – promoted by red light, inhibited by far‑red.
  • Etiolation vs. de‑etiolation – dark‑grown seedlings elongate; Pfr accumulation restores normal morphology.
  • Shade‑avoidance (crowding) – low Pfr / high Pr triggers rapid stem elongation.

• Phototropism

  • Directional growth in response to light.
  • Shoots positively phototropic

• Gravitropism

  Response	Sensor / Hormone	Adaptive Benefit
  Gravitropism – roots (+) and shoots (–)	Response to gravitational field Amyloplast(starch containing plastids)	Ensures roots reach soil, shoots reach light

  • Germination - seed awakens from dormancy and grows into a new place

Angiosperm Reproduction

• Floral Anatomy & Terminology (Angiosperms)

  Whorl	Part(s)	Primary Function	Notes
  Calyx	Sepals	Protects unopened bud	Often green, leaf‑like
  Corolla	Petals	Attracts pollinators (colour, scent)	Showy in animal‑pollinated species
  Androecium	Stamens (filament + anther)	Produces microspore mother cells → pollen	“Male” structures
  Gynoecium	Carpels (ovary, style, stigma)	Houses ovules; receives pollen	One or many carpels may fuse (Female)

  • Complete flower = all four whorls; Incomplete = ≥ 1 whorl missing.
  • Fusion (flowers that were separated) and part‑number reduction (#’s of flower parts) are trends in floral specialization.
  • Bilateral symmetry (e.g., orchids/ normal) is generally derived from radial symmetry (e.g., snapdragon/mutant).

• Alternation of Generations & Gametophyte Development

  • Microgametophyte (pollen grain - diploid)

    1. Microspore mother cells (2n) in anther → meiosis → 4 microspores (n).
    2. Each microspore → mitosis → tube cell + generative cell (later divides → 2 sperm).

  • Megagametophyte (embryo sac - haploid)

    1. Megaspore mother cell (2n) in ovule → meiosis → 4 megaspores (n); 3 degenerate.

    2. Remaining megaspore → three mitoses → 8 nuclei / 7‑celled embryo sac:

       • 3 antipodal cells
       • 2 polar nuclei (central cell)
       • 2 synergids flanking the egg cell

  • These gametophytes are highly reduced and remain inside the sporophyte tissues.

• Mutualism: plants get pollen, pollinators get food
  * Plant characteristics that are important: timing of opening, color patterns, and flower shape and size.
  Coevolution with Pollinators

• Development of the Ovary into the Fruit

  • After fertilization in a flowering plant:

    1. Ovary transforms into the fruit:

       • The ovary, which houses the ovules, begins to swell and mature after fertilization.
       • The ovules develop into seeds.
       • The ovary wall becomes the pericarp, which encloses the seeds.

    2. Fruit maturation:

       • Hormones like auxin and gibberellins regulate the growth of the fruit.
       • The fruit may develop into a fleshy or dry structure, depending on the plant species.

  • The pericarp (fruit wall) has three layers:

    1. Exocarp – outer skin
    2. Mesocarp – middle, often fleshy part
    3. Endocarp – inner layer, often hard or papery

• Seed & Fruit Dispersal Strategies

  1. Ingestion & defecation – coloured, fleshy fruits (e.g., berries).
  2. Adhesion – hooks/spines cling to fur or feathers.
  3. Wind – parachute‑like pappus (dandelion), winged samaras.
  4. Water – buoyant fruits/seeds (coconut).
  • Dispersal reduces competition and allows colonization of new habitats.