Introduction to Plants: Angiosperm Anatomy, Growth, and Physiology

Introduction to Angiosperms and Plant Body Architecture

Taxonomic Focus: The vast majority of study in Kingdom Plantae centers on the Angiosperms (Phylum Anthophyta), which constitute greater than $90\%$ of all species in the Plant Kingdom.
Key Angiosperm Innovations:
- Flowers and Fruits: Specialized structures for efficient pollination, seed protection, and seed dispersal.
- Advanced Seeds: Highly adapted reproductive structures housing a dormant embryo with a nutrient supply.
- Vascular Tissues: Highly specialized conducting tissues (xylem and phloem) providing superior fluid transport and structural support.
The Plant Body Plan: Composed of two main systems and three primary vegetative organs:
- Shoot System:
  - Stem: Structural axis that produces leaves and branches, bearing reproductive organs (flowers).
  - Leaves: Flattened organs specialized for capturing solar energy and conducting photosynthesis.
- Root System:
  - Roots: Anchors the plant in soil, absorbs water and dissolved minerals, and stores food and water reserves.
Structural Hierarchy of a Mature Plant:
- $\text{Specialized Cells} \rightarrow \text{Tissues} \rightarrow \text{Organs} \rightarrow \text{Organ Systems (branches, buds, flowers, seeds, fruits)} \rightarrow \text{Shoot and Root Systems} \rightarrow \text{Whole Plant (Sporophyte Organism)}$

Plant Body Diagram

Alternation of Generations and Plant Life Cycles

Universal Plant Life Cycle: All plants and plant-like organisms exhibit Alternation of Generations, alternating between two distinct multicellular forms:
- Sporophyte Generation (Diploid, $2N$ ):
  - Multicellular, spore-producing plant form.
  - Represents the large, dominant, independent "plant" in flowering plants.
  - Produces haploid spores ( $1N$ ) via meiosis (reduction division) within the specialized organs of the flower.
- Gametophyte Generation (Haploid, $1N$ ):
  - Multicellular, gamete-producing plant form.
  - Microscopic and dependent in flowering plants.
  - Grows and develops entirely inside the flower tissues of the sporophyte.
  - Produces haploid gametes (sperm and egg) via mitosis and cytokinesis.
Evolutionary Trends in Kingdom Plantae:
- Primitive plants (e.g., mosses / bryophytes) possess a dominant, independent gametophyte and a small, dependent sporophyte.
- Advanced flowering plants (e.g., oak trees) feature a dominant, independent, complex sporophyte ( $2N$ ) and a microscopic, dependent gametophyte ( $1N$ ) reduced to just a few cells within the flower.

Moss vs Oak Life Cycle Comparison

Schematic Sequence of Alternation of Generations:
1. Diploid Sporophyte ( $2N$ ) produces Spore Mother Cells.
2. Spore Mother Cells undergo meiosis / cytokinesis to produce haploid Meispores ( $1N$ ).
3. Meispores undergo mitosis / cytokinesis to form the Gametophyte ( $1N$ ):
  - Male gametophyte: Pollen grain
  - Female gametophyte: Embryo sac
4. Gametophytes undergo mitosis / cytokinesis to produce gametes:
  - Male gametes: Sperm
  - Female gametes: Egg
5. Syngamy (Fertilization) fuses egg ( $1N$ ) and sperm ( $1N$ ) to yield a diploid Zygote ( $2N$ ).
6. Zygote undergoes mitosis / cytokinesis to form an Embryo ( $2N$ ) inside a seed.
7. Embryo germinates and grows via mitosis / cytokinesis into a mature Sporophyte ( $2N$ ).

Alternation of Generations Diagram

Plant Meristems, Primary Growth, and Primary Tissues

Meristems ("Cell Factories"):
- Regions of perpetually undifferentiated cells that divide continuously by mitosis to generate new plant tissues.
- Seed embryos possess dormant meristems at the shoot and root tips, which activate upon seed germination.
- Mature plants maintain active Shoot Apical Meristems (SAM) and Root Apical Meristems (RAM) at their terminal tips.
Indeterminate Growth:
- Plants increase in size continuously as long as they remain alive.
- Growth occurs via three distinct processes: increase in cell number (mitosis/cytokinesis), increase in cell size (water uptake), and increase in overall dry weight.
Primary Growth ( $1^\circ$ Growth):
- Refers to the elongation of plant organs (roots, stems, leaves).
- Occurs in ALL plant species.
- Driven by apical meristems (SAM and RAM) at organ tips.
Six Primary Tissues Produced by Apical Meristems:
1. Primary Xylem: Vascular tissue specialized for conducting water and inorganic minerals upward.
2. Primary Phloem: Vascular tissue specialized for conducting organic food compounds and solutes.
3. Epidermis: Dermal tissue forming the outermost protective cell layer of non-woody organs.
4. Parenchyma: Most abundant ground tissue; functions in food/water storage and metabolic processes; fills the cortex and central pith.
5. Collenchyma: Ground tissue providing flexible mechanical support and protection to actively elongating stems and petioles.
6. Sclerenchyma: Non-elongating ground tissue reinforced with thick secondary walls for rigid protection and support within non-growing organ regions.

Secondary Growth and Wood Anatomy in Eudicots

Secondary Growth ( $2^\circ$ Growth):
- Refers to the lateral expansion (increase in diameter/girth) of plant organs.
- Occurs in roots and stems, but NEVER in leaves.
- Present in most eudicots; absent in monocots.
- Driven by two lateral meristems arranged in concentric rings that produce secondary (woody) tissues late in the plant's first year of growth.

Trunk Cross Section Diagram

The Two Lateral Meristems:
1. Vascular Cambium (VC):
  - A cylinder of meristematic cells (2–3 layers thick) located between primary xylem and primary phloem.
  - Divides to produce Secondary Xylem (Wood) toward the interior of the stem and Secondary Phloem (Inner Bark) toward the exterior.
2. Cork Cambium (CC):
  - Forms within the outer cortex beneath the epidermis.
  - Divides to produce rings of Periderm (Outer Bark), composed largely of dead cork cells that replace the original epidermis and cortex for external protection.
Composition of Bark:
- $\text{Bark} = \text{Inner Bark (Secondary Phloem)} + \text{Outer Bark (Periderm)}$

Eudicot Stem Micrograph After Secondary Growth

Comparative Organ Anatomy: Monocots versus Eudicots

Taxonomic Species Abundance:
- Eudicots: Greater than $240{,}000$ species (e.g., oaks, hickories, pansies, roses).
- Monocots: Greater than $60{,}000$ species (e.g., grasses, lilies, tulips).
Leaf Anatomy Differences:
- Eudicots: Leaves feature netted venation (pinnate or palmate branching patterns) providing structural support. Leaves are composed of primary tissues only (upper/lower epidermis, mesophyll, and vascular bundles containing primary xylem on top and primary phloem on bottom).
- Monocots: Leaves feature parallel venation, with leaf blades attaching to the stem via a surrounding sheath.

Leaf Venation Patterns

Root System Differences:
- Eudicots: Possess a central taproot system. In cross-section, eudicot roots feature an epidermis, broad cortex, endodermis, pericycle, and a central solid core of primary xylem (star-shaped) with primary phloem in arms; no pith is present.
- Monocots: Possess a fibrous root system. In cross-section, monocot roots feature an epidermis, cortex, endodermis, pericycle, and a central pith surrounded by alternating ring-like strands of primary xylem and phloem; xylem does not occupy the innermost core.
Stem Architecture Differences:
- Eudicots: Vascular bundles are arranged in a distinct ring pattern, dividing the ground tissue into an outer cortex and a central pith. Undergo both primary and secondary growth.
- Monocots: Vascular bundles are scattered throughout the stem; lack a distinct pith and cortex. Undergo primary growth only.

Root System Anatomy and Root Growth Zones

Major Functions of Roots: Absorption of water and inorganic minerals, anchorage in soil, storage of carbohydrates and nutrients.
Three Zones of Root Growth (Longitudinal View):
1. Region of Cell Division:
  - Houses the Root Apical Meristem (RAM) and protective Root Cap.
  - RAM cells divide constantly by mitosis.
  - Root cap cells produce and secrete lubricating mucigel to ease passage through soil particles.
2. Region of Elongation:
  - Cells absorb water into vacuole, causing dramatic longitudinal extension/enlargement.
3. Region of Maturation:
  - Cells undergo differentiation into specialized primary tissues ( $1^\circ$ tissues).
  - Characterized externally by root hairs (dermal outgrowths that maximize surface area for water/mineral absorption; absent in older root zones).
Internal Root Structures:
- Epidermis: Outer dermal boundary layer.
- Root Cortex: Parenchyma cylinder inner to epidermis; stores starch; contains intercellular air spaces.
- Endodermis: Innermost layer of the root cortex that selectively regulates mineral and water entry into the vascular system.
- Meristematic Pericycle: Layer directly interior to endodermis that encloses root vascular tissue and retains cell division capacity to produce lateral (branch) roots.

Shoot System Anatomy and Leaf Structure

Modular Organization of Shoots: Shoots develop as repeating modules comprising four distinct components:
1. Stem Node: Swollen location where leaves or lateral branches originate.
2. Internode: Stem segment between consecutive nodes responsible for vertical shoot elongation.
3. Leaf: Photosynthetic blade connected via a petiole (eudicots) or sheath (monocots).
4. Axillary Meristem: Meristematic region in leaf axils generating axillary (lateral) buds, which can grow into vegetative branches or reproductive flowers. Each branch tip bears a new SAM.
Shoot Tip Architecture:
- Terminal Bud: Tip of the primary stem housing the SAM, flanked by protective primordial leaves and primordial buds.
Internal Leaf Microanatomy:
- Upper and Lower Epidermis: Protective outer cellular layers coated with a waxy cuticle.
- Stomata: Pore complexes located primarily on the lower leaf epidermis for controlled gas exchange ( $CO_2$ absorption, $O_2$ and water vapor release).
- Mesophyll: Photosynthetic ground tissue packed with chloroplasts between the epidermal layers.
- Vascular Bundles (Veins): Contain primary xylem (positioned superiorly) and primary phloem (positioned inferiorly).

Plant Growth Regulation and Phytohormones

Phytohormones: Chemical messengers produced in low concentrations that regulate growth, physiological responses, and development.
Transport Mechanism: Most hormones are moved long-distance within phloem sap. ALL active hormone transport requires ATP expenditure by the plant.
Broad Functional Categories:
- Growth-Promoting Hormones: Predominantly active during spring and summer.
- Growth-Inhibiting Hormones: Predominantly active during autumn and winter when environmental conditions are harsh.
Growth-Promoting Hormones:
- Auxins:
  - First group described. Produced in shoot tips, seeds, fruits, leaves, and stems (NOT in roots).
  - Effects: Promotes cell elongation, shoot lengthening, secondary xylem (wood) production, and fruit development; inhibits lateral bud growth (apical dominance) and prevents leaf/flower/fruit abscission.
- Cytokinins:
  - Originally discovered in coconut milk. Produced in seeds, fruits, and roots.
  - Effects: Promotes cell division (cytokinesis) and lateral bud development; inhibits leaf senescence (degradation of chlorophylls).
- Gibberellins (Gibberellic Acids):
  - Largest class of hormones. Found throughout the plant body, concentrated in seeds.
  - Effects: Promotes stem elongation by driving cell division and cell elongation; stimulates food reserve breakdown in germinating seeds.
- Brassinosteroids:
  - Effects: Promotes cell expansion, shoot elongation, xylem differentiation, stress responses, and inhibits leaf abscission.
Growth-Inhibiting Hormones:
- Abscisic Acid (ABA):
  - Present in high concentrations in seeds, mature leaves, and dormant buds.
  - Effects: Inhibits cell elongation and $\alpha$ -amylase enzyme production; promotes seed storage carbohydrate synthesis, leaf senescence, and bud/seed dormancy.
- Ethylene:
  - A gaseous hormone produced via incomplete metabolic processes.
  - Effects: Accelerates fruit ripening; promotes leaf, flower, and fruit abscission; interacts with growth-promoting hormones to determine final cell dimensions.

Seed Germination and Seedling Development

Physiology of Grain Seed Germination (e.g., Corn Kernel):
1. Imbibition: Seed absorbs water, causing hydration and tissue swelling.
2. Embryo Activation: The embryo synthesizes and secretes gibberellins.
3. Hormone Diffusion: Gibberellins diffuse to the specialized aleurone layer cells surrounding the endosperm.
4. Enzyme Synthesis: Aleurone layer secretes the hydrolytic enzyme $\alpha$ -amylase.
5. Endosperm Digestion: $\alpha$ -Amylase breaks down stored endosperm starch into soluble glucose molecules.
6. Energy Generation: The embryo respires glucose to generate ATP to fuel seedling growth.
7. Evolutionary Advantage: The embryo directly controls the precise timing of germination relative to moisture availability.
General Germination Sequence:
- Requires breaking of dormancy via combined internal factors (hormonal shifts, water uptake) and external environmental cues (light, temperature, moisture, day length).
- Seed coat cracks under internal pressure.
- Radicle (embryonic root) emerges first and grows downward into soil.
- Shoot axis emerges second and grows upward toward light.

Essential Plant Nutrients and Mineral Nutrition

Nutritional Requirements: Plants require inorganic raw materials for photosynthesis, biomass assembly, and cellular metabolism.
Total Essential Elements: 16 elements essential for life:
- Non-mineral Elements (3): Carbon ( $C$ ), Hydrogen ( $H$ ), Oxygen ( $O$ ) — acquired directly from atmospheric $CO_2$ and soil $H_2O$ .
- Mineral Elements (13): Acquired from soil water solution via roots and transported upward in the xylem stream.
Soil Macronutrients (6) (Required in concentrations $\ge 1\,\text{g/kg}$ of dry mass):
1. Nitrogen ( $N$ ): Component of proteins, amino acids, nucleic acids, and chlorophyll.
2. Potassium ( $K$ ): Osmotic regulator; maintains turgor pressure and governs guard cell opening/closing.
3. Phosphorus ( $P$ ): Structural constituent of nucleic acids, ATP, and membrane phospholipids.
4. Calcium ( $Ca$ ): Crucial component of the middle lamella and structural cell wall integrity.
5. Sulfur ( $S$ ): Essential component of specific amino acids (cysteine, methionine) and coenzymes.
6. Magnesium ( $Mg$ ): Central atom of chlorophyll molecules; indispensable enzyme activator.
Soil Micronutrients / Trace Elements (7) (Required in concentrations $\le 0.1\,\text{g/kg}$ of dry mass):
1. Molybdenum ( $Mo$ ): Essential enzyme cofactor for nitrogen metabolism.
2. Copper ( $Cu$ ): Redox enzyme cofactor in electron transport pathways.
3. Zinc ( $Zn$ ): Enzyme cofactor necessary for auxin synthesis.
4. Manganese ( $Mn$ ): Structural component of photosystem II required for the water-splitting reaction.
5. Chlorine ( $Cl$ ): Photosynthetic water-splitting reaction participant; involved in ion balance.
6. Boron ( $B$ ): Structural role in cell wall formation and nucleic acid synthesis.
7. Iron ( $Fe$ ): Cytochrome cofactor; essential for chlorophyll biosynthesis pathways.

Plant Transport Systems and Water Potential

Overview of Whole-Plant Transport:
- Xylem System: Conducts water and dissolved inorganic minerals unidirectionally from soil to roots, stems, and leaves.
- Phloem System: Conducts organic sap (sucrose dissolved in water) bidirectionally between sugar sources and sugar sinks.

Plant Transport Overview

Fundamental Properties of Water:
- Polar molecule forming extensive hydrogen bonding networks.
- Displays high cohesion (water sticking to water) and adhesion (water sticking to polar cell walls).
- Acts as the ultimate biological solvent and thermal stabilizer; average plant cells are composed of $\sim 90\%$ water.
Principles of Physical Movement:
- Bulk / Mass Flow: Movement of fluid driven by pressure differentials, gravity, or both (significantly faster than diffusion).
- Diffusion: Movement of solutes down concentration gradients (simple vs. facilitated via protein channels/transporters).
- Osmosis: Diffusion of water across a selectively permeable membrane in response to solute concentration differences.
- Aquaporins: Plasma membrane protein channels that facilitate rapid water transport into expanding cells.

Mechanisms of Cellular Fluid Transport

Cellular Water Turgor Dynamics:
- Turgid Cell: High water content, hydrostatic turgor pressure increases, pressing cytosol firmly against the cell wall.
- Plasmolyzed Cell: Severe water loss results in loss of turgor pressure; plasma membrane detaches and shrinks away from the cell wall.

Turgid vs Plasmolyzed Plant Cell

Water Potential ( $\Psi$ ): Measurement of the free energy/potential energy of water. Water always flows spontaneously from regions of higher water potential to regions of lower water potential.

Stomatal Regulation and the Transpiration Stream

Transpiration: Thermal energy from sunlight heats leaves, driving the evaporation of water vapor through open stomatal pores into the atmosphere. Represents the primary long-distance engine for water ascent.
Pathway of the Transpiration Stream:
- $\text{Soil } H_2O \rightarrow \text{Root Epidermis} \rightarrow \text{Root Cortex} \rightarrow \text{Endodermis} \rightarrow \text{Root Xylem} \rightarrow \text{Stem Xylem} \rightarrow \text{Leaf Xylem} \rightarrow \text{Leaf Mesophyll} \rightarrow \text{Substomatal Cavity} \rightarrow \text{Atmosphere}$
The C-A-T Mechanism:
- Cohesion: Hydrogen bonding holds water molecules together in a continuous liquid column inside xylem elements.
- Adhesion: Hydrogen bonding attaches water molecules to hydrophilic cellulose microfibrils in xylem walls, preventing downward gravitational slipping.
- Tension: Transpirational water loss from mesophyll cell surfaces creates negative pressure (pulling force) at the top of the column.
- Energy Investment: Zero ATP expended by the plant; driven passively by ambient solar heat.
Stomatal Guard Cell Physiology:
- Stomata account for greater than $90\%$ of all plant water loss.
- Stomatal Opening Mechanism (Daylight):
  1. Sun exposure lowers leaf internal $CO_2$ concentration.
  2. Guard cells actively pump Potassium ions ( $K^+$ ) inside across their plasma membranes (requires ATP).
  3. Increased intracellular $K^+$ concentration dramatically lowers guard cell water potential.
  4. Water enters guard cells from surrounding cells via osmosis.
  5. Guard cells swell, bowing outward due to radially oriented cellulose microfibrils and thickened inner cell walls, opening the stoma.
- Stomatal Closing Mechanism: $K^+$ is actively pumped out of guard cells; water follows out by osmosis, causing guard cells to flaccidly collapse and close the pore.

Stomatal Guard Cell Micrograph and Opening Mechanism

Phloem Transport and the Pressure-Flow Hypothesis

Translocation: The long-distance transport of soluble organic carbohydrates (primarily non-reducing sucrose) dissolved in water via the phloem from a Source (photosynthetic or storage tissue) to a Sink (growth or storage site).
Comparison of Transpiration vs. Translocation:
- Transpiration: Conducted in Xylem; strictly unidirectional (upward); zero metabolic energy expended by plant (powered by solar heat).
- Translocation: Conducted in Phloem; bidirectional movement; requires metabolic ATP expenditure by companion cells.
Cellular Structure of Phloem:
- Sieve-Tube Elements (STMs): Conducting cells stacked end-to-end; lose nuclei and most organelles at maturity to remove structural resistance to bulk fluid flow; end walls form porous sieve plates.
- Companion Cells: Nucleated helper cells connected to STMs via plasmodesmata; execute metabolic functions and perform active solute transport.

Phloem Micrograph and Sieve-Tube Structure

Steps of the Pressure-Flow Hypothesis:
1. Phloem Loading at Source: Companion cells actively load sucrose into sieve-tube elements against a concentration gradient (requires ATP).
2. Water Uptake: High solute concentration inside STMs lowers water potential. Water moves from adjacent high-potential xylem into STMs via osmosis.
3. Hydrostatic Pressure Generation: Influx of water creates high hydrostatic pressure at the source end of the sieve tube.
4. Bulk Flow: Pressure differential drives the mass flow of phloem sap from high-pressure source regions toward low-pressure sink regions.
5. Phloem Unloading at Sink: Companion cells actively unload sucrose from STMs into sink cells (requires ATP), where sucrose is metabolized or converted to insoluble starch.
6. Water Recycling: Loss of solute increases water potential in STMs. Water moves out of phloem STMs back into adjacent xylem vessel elements via osmosis.

Sexual Reproduction in Angiosperms: Floral Anatomy

The Flower: A highly modified, short-term reproductive shoot system exhibiting determinate growth.
Basic Attachment Structure:
- Pedicel: Specialized stem axis/flower stalk.
- Receptacle: Swollen stem tip at the apex of the pedicel to which four concentric whorls of modified leaves attach.
The Four Concentric Floral Whorls:
1. Calyx: Outermost whorl composed of individual sepals; protects internal floral tissues during bud development.
2. Corolla: Whorl composed of individual petals; frequently pigmented and scented to attract animal pollinators.
3. Androecium: Male reproductive whorl composed of stamens:
  - Filament: Elongated structural stalk.
  - Anther: Terminal bilobed sac containing four internal microsporangia (pollen chambers).
4. Gynoecium: Innermost female reproductive whorl composed of one or more carpels/pistils:
  - Stigma: Sticky terminal surface specialized to catch pollen grains.
  - Style: Elongated neck connecting stigma to ovary.
  - Ovary: Swollen basal chamber enclosing one to many protective ovules (future seeds).

Detailed Diagram of Flower Anatomy

Gametogenesis and Double Fertilization

Microsporogenesis (Male Gametophyte Formation):
- Occurs inside the four pollen chambers of the anther.
- Diploid Microspore Mother Cells ( $2N$ ) undergo meiosis / cytokinesis to yield four haploid Microspores ( $1N$ ).
- Each microspore undergoes mitosis / cytokinesis to yield a two-celled Pollen Grain (immature male gametophyte), consisting of:
  - Tube Cell: Generates the pollen tube.
  - Generative Cell: Divides via mitosis to generate two non-motile sperm cells.
Megasporogenesis & Embryo Sac Formation (Female Gametophyte):
- Enclosed inside the protected ovule (Angiospermy).
- A single diploid Megaspore Mother Cell ( $2N$ ) undergoes meiosis / cytokinesis to produce four haploid Megaspores ( $1N$ ).
- Three megaspores degenerate; one Functional Megaspore ( $1N$ ) survives.
- The functional megaspore undergoes three sequential rounds of mitosis without immediate cytokinesis, culminating in a 7-celled, 8-nucleate Embryo Sac (Female Gametophyte):
  - 3 Antipodal Cells ( $1N$ ): Positioned at the chalazal end (opposite micropyle).
  - 1 Large Central Cell: Contains 2 Polar Nuclei ( $1N$ each).
  - 2 Synergid Cells ( $1N$ ): Flank the egg at the micropyle entrance.
  - 1 Egg Cell ( $1N$ ): Positioned centrally at the micropyle end.

Embryo Sac 7-Celled 8-Nucleate Architecture

Pollination and Fertilization Sequence:
1. Pollination: Transfer of pollen from anther to stigma (Self-pollination vs. Cross-pollination via wind, water, or animal vectors).
2. Germination: Pollen tube grows down through style tissues toward the ovule, guided by chemical signals.
3. Entry: Pollen tube enters the ovule via the micropyle pore and penetrates one synergid cell, which ruptures to discharge content.
4. Double Fertilization:
  - First Syngamy Event: One sperm cell ( $1N$ ) fuses with the Egg cell ( $1N$ ) to produce the Diploid Zygote ( $2N$ ).
  - Second Syngamy Event: The second sperm cell ( $1N$ ) fuses simultaneously with the two Polar Nuclei ( $1N + 1N$ ) in the central cell to produce a Triploid Primary Endosperm Cell ( $3N$ ).

Post-Fertilization Development, Fruit Formation, and Seed Dispersal

Post-Fertilization Morphological Changes:
- Zygote ( $2N$ ) undergoes mitosis to form the multicellular Embryo (young sporophyte).
- Primary Endosperm Cell ( $3N$ ) undergoes rapid mitosis to form Endosperm Tissue, a nutrient-rich food store for the developing embryo.
- Ovule Integuments harden to form protective Seed Coats; the entire ovule matures into a Seed.
- Ovary Wall expands and accumulates water and sugars to mature into a Fruit, which encloses the seed(s).
Seed Dispersal Mechanisms:
- Enclosed seeds are dispersed away from the parent plant to prevent resource competition.
- Dispersal Vectors:
  1. Wind: Lightweight seeds with wings or plumes (e.g., dandelions, maples).
  2. Water: Buoyant fruits/seeds adapted for floating.
  3. Animals: Primary vector; plants utilize edible fleshy fruits as rewards or hooked barbs for external attachment.
Seed Germination and Seedling Establishment:
- Mature seeds enter a period of dormancy until favorable environmental conditions return.
- Germination is triggered by combination of imbibition (water uptake), internal hormonal shifts (high gibberellins, low ABA), warm temperatures, and oxygen.
- Radicle emerges downwards to establish the primary root system; shoot apex grows upwards to establish the primary photosynthetic shoot system.