Botany Lecture Flashcards

Foundations and History of Botany

Definition of Botany:
- Botany is the scientific study of plants.
- Etymology: Derived from the Greek word BOTANE, meaning herb or plants. It originates from three Greek words meaning "graze," "plant," and "cattle."
- It represents one of the oldest branches of science.
The Scientific Method:
- Defined as a process for experimentation used to explore observations and answer questions.
- Aristotle ( $384\text{--}322\,\text{BC}$ ): Recognized as the First Scientist.
- Sir Francis Bacon ( $1561\text{--}1626$ ): Credited with being the first to define the scientific method and is known as the Father of Modern Scientific Method.
Historical Timeline of Botanical Milestones:
- Pre-17th Century:
  - $4\text{th century BCE}$ : Contributions by Aristotle and Theophrastus.
  - $\text{A.D. } 60$ : Pedanius Dioscorides documented medicinal plants.
- 17th Century:
  - Early 17th Century: Search for botanical knowledge became temporarily stagnant.
  - Johannes van Helmont ( $1640$ ): Investigated water intake and weight increase in plants, concluding that trees gain most of their mass from water.
  - Robert Hooke ( $1665$ ): Associated with the Hooke Microscope; published Micrographia ( $1665$ ), coined the term "cell," and is known as the Father of Cytology.
  - Anton van Leeuwenhoek ( $1674$ ): Conducted groundbreaking microscopic observations, observing living cells in a drop of pond water.
  - John Ray ( $1686$ ): Published Methodus Plantarum.
- 18th Century:
  - Stephen Hales ( $1727$ ): Initiated the scientific study of plant physiology.
  - Carolus Linnaeus / Carl Linnaeus ( $1758$ ): Devised binomial nomenclature and published Systema Naturae; recognized as the Father of Modern Taxonomy.
  - $1760$ : Widespread global practice of Botany emerged.
  - Later Part of 18th Century: Joseph Priestley laid the foundation for the chemical analysis of plant metabolism.
- 19th Century:
  - $1818$ : Discovery of Chlorophyll.
  - $1840$ : Expansion in the systematic study of plants.
  - $1847$ : Elucidation of Photosynthesis.
  - Charles Darwin ( $1859$ ): Formulated the theory of evolution by natural selection and published On the Origin of Species; known as the Father of Evolution.
  - Gregor Mendel ( $1862/1865$ ): Established principles of inheritance of traits and classical genetics.
- 20th Century to Present:
  - Early 20th Century: Research into Nitrogen Fixation and nitrification.
  - $1903$ : Discovery that there are $2$ types of Chlorophyll.
  - Alexander Oparin ( $1936$ ): Conducted foundational origin of life studies.
  - $1940$ : Integration of Plant Ecology as a major discipline.
Prominent Historical Figures in Botany and Biology:
- Charles Darwin: Author of On the Origin of Species, Father of Evolution, established natural selection.
- Robert Hooke: Author of Micrographia ( $1665$ ), coined the term "cell," Father of Cytology.
- Leonardo da Vinci: Renaissance polymath who produced anatomical illustrations and detailed biological observations.
- Theophrastus: Author of Historia Plantarum, Father of Botany, created early plant classification systems.
- George Ledyard Stebbins: Author of Variation and Evolution in Plants, integrated plant evolution and genetics, pioneered modern evolutionary botany.
- Richard Spruce: Explored the Amazon, conducted extensive plant collections, contributed to ethnology and zoology.
- Carlos Muñoz Pizarro: Documented Chilean flora, advanced plant studies and Chilean natural history.
- Robert Brown: Discovered and named the cell nucleus in $1831$ through plant cell studies.
- Carl Linnaeus: Father of Modern Taxonomy, author of Systema Naturae, established binomial nomenclature.
- Pedanius Dioscorides: Author of De Materia Medica, specialized in medicinal plants and pharmacognosy.
- David Douglas: Explored North America and China, introduced numerous plant species to Europe (including the Douglas fir).
- Luther Burbank: Pioneer in agricultural horticulture and selective breeding, developing hundreds of plant varieties.
- Claudius Galen: Developed influential historical medical theories, anatomy, and physiology.
- Augustin Pyramus de Candolle: Author of Natures War and Théorie élémentaire de la botanique, established plant classification theories.
- Hugo von Mohl: Coined and explained protoplasm, advanced plant cell biology.
- Matthias Schleiden: Discovered that plants are entirely composed of cells; co-founded Cell Theory ( $1838$ ).
- Theodor Schwann: Developed animal cell theory; co-founded Cell Theory.
- Antonie van Leeuwenhoek: First to observe single-celled organisms in pond water using custom micro-lenses.

Branches of Science Involved in Botany

Agronomy: Study of field crops and agricultural management.
Anatomy / Plant Anatomy: Study of the internal structure of plants.
Morphology / Plant Morphology: Study of plant form and external structure.
Bryology: Study of bryophytes, including mosses, liverworts, and hornworts.
Economic Botany: Study of the economic importance, commercial uses, and applications of plants.
Ethnobotany: Study of the relationships and interactions between human cultures and plants.
Forestry: Study, management, and conservation of forests and timber resources.
Horticulture: Cultivation and management of garden plants, flowers, and food crops.
Lichenology: Study of lichens (symbiotic associations between fungi and algae/cyanobacteria).
Paleobotany: Study of fossil plants and plant evolution through geological time.
Palynology: Study of pollen grains and spores.
Phycology: Study of algae.
Phytochemistry: Study of chemical compounds, secondary metabolites, and molecules found in plants.
Phytopathology: Study of plant diseases, pathogens, and immunity.
Plant Ecology: Study of interactions between plants and their abiotic/biotic environment.
Plant Genetics: Study of genes, genetic variation, and heredity in plants.
Plant Physiology: Study of physical, chemical, and metabolic functions and processes in plants.
Plant Systematics: Study of plant diversity, evolutionary relationships, and phylogenetics.
Taxonomy: Identification, naming, and systematic classification of organisms.

Modern Tenets of the Cell Theory

All known living things are composed of one or more cells.
The cell is the fundamental structural and functional unit of all living organisms.
All cells arise from pre-existing cells through cell division.
Cells contain hereditary information (DNA) that is passed from cell to cell during cell division.
All cells are basic and similar in chemical composition.
All energy flow (metabolism and biochemistry) of life occurs inside cells.

Major Plant Tissue Systems and Cell Categories

Major Tissue Systems:
1. Dermal Tissue System: Outer protective skin layer.
2. Ground Tissue System: Primary interior fundamental body layer.
3. Vascular Tissue System: Internal conducting transport network.
Categorization by Cellular Makeup:
1. Meristematic Tissues: Undifferentiated, actively dividing tissues.
2. Permanent Tissues: Differentiated, specialized tissues.
Dermal Tissue System:
- Epidermis:
  - Covers roots, stems, leaves, and fruits in the primary plant body.
  - Cuticle: A continuous outer layer made of fatty material called cutin, located on the outer surface of epidermal cells.
  - Stomata: Specialized epidermal structures bordered by guard cells that control air exchange ( $\text{CO}_2$ and $\text{O}_2$ ) and limit water loss by adjusting the size of the stomatal pore.
  - Stomatal Pore: Enables gas exchange between atmospheric air and internal plant tissues.
  - Functions of Epidermis:
    1. Provides mechanical support.
    2. Protects against desiccation (drying out) and infection by virulent pathogenic organisms or insects.
    3. Facilitates gas exchange.
    4. Restricts water loss by evaporation (transpiration) via stomata.
    5. Stores water and minerals.
- Periderm:
  - Outermost cell layer covering woody stems and roots.
  - Formed during secondary growth, replacing the primary epidermis.
Ground Tissue System:
- Located interior to dermal tissue and composed of simple tissues: Parenchyma, Collenchyma, and Sclerenchyma.
- In leaves, ground tissue forms the mesophyll, bounded by the upper and lower epidermis.
Vascular Tissue System:
- Main conducting elements: Xylem (transports water and minerals) and Phloem (transports food and organic solutes).
- Arrangement: May be scattered throughout ground tissue or arranged in a ring.
  - In ring arrangements: Cortex lies outer to vascular tissue; Pith lies inner to vascular tissue.

Meristematic and Embryonic Tissues

Characteristics of Meristematic Cells:
- Actively dividing cells producing new cells via mitosis.
- Isodiametric in shape, rich in dense cytoplasm, possess small or no vacuoles, and remain undifferentiated.
- Located at shoot tips, root tips, vascular cambium, and cork cambium.
- Differentiate into the specialized functional cells of dermal, ground, and vascular tissue systems.

Meristematic vs Permanent Comparison

Meristematic Types Summary

Types of Meristematic Tissues:
- Apical Meristem:
  - Location: At tips (apex) of stems, roots, and shoots.
  - Function: Responsible for primary growth (increase in plant length).
- Intercalary Meristem:
  - Location: Basal portion of internodes.
  - Function: Responsible for elongation of internodes, particularly prominent in plants lacking vascular or cork cambium (e.g., family Poaceae).
- Lateral Meristem:
  - Location: Vascular cambium and cork cambium.
  - Function: Responsible for secondary growth (increase in plant stem/root diameter).

Simple Permanent Tissues

Definition: Composed of cells of similar structure, function, and origin that have matured into specific, irreversible forms.
Parenchyma:
- The fundamental, versatile tissue found throughout every part of the plant body (pith, stem/root cortex, leaf mesophyll, fruit flesh, floral parts, xylem, and phloem).
- Functions: Storage of starch, sucrose, protein, water, phenol derivatives, and mineral substances; active site for respiration and protein synthesis.
- Specializations of Parenchyma:
  1. Aerenchyma: Characterized by large, numerous intercellular air spaces with reduced cell surface area; provides structural support, aeration, and buoyancy to aquatic plants.
  2. Transfer Cells: Possess irregular inward extensions of primary cell walls to maximize plasma membrane surface area; found in floral structures and carnivorous plants.
  3. Chlorenchyma: Parenchyma cells containing abundant chloroplasts specialized for photosynthesis; located in leaves and young green stems.
Mesophyll:
- Internal photosynthetic parenchyma of leaves containing chloroplasts.
- Palisade Mesophyll: Columnar, elongated cells arranged compactly near the upper surface.
- Spongy Mesophyll: Loosely, irregularly arranged cells with large intercellular air spaces.
Collenchyma:
- Living cells with unevenly thickened primary cell walls due to localized deposition of pectin.
- Narrower cell lumens than parenchyma; provides flexible, elastic support to growing stems and leaves.
- Primary wall components: Cellulose, hemicellulose, pectin, and water.
- Location: Sub-epidermal position, cortex, and surrounding vascular bundles of petioles and stems.
- Primary supporting tissue in stems, leaves, and floral structures of dicots; usually absent in monocots (where sclerenchyma provides support instead).
Sclerenchyma:
- Dead at functional maturity; provides rigid mechanical support via heavily lignified secondary cell walls containing high concentrations of cellulose and lignin.
- Fibers:
  - Elongated, slender, spindle-shaped cells with tapering ends and simple pits.
  - Occur in xylem, phloem, along leaf margins/veins, and surrounding stem vascular bundles.
- Sclereids:
  - Shorter than fibers, variable in shape; occur individually or in clusters.
  - Found in fruit walls, seed coats, epidermal scales, cortex, pith, and mesophyll.
  - Types include: Asterosclereids, Macrosclereids, Osteosclereids, and Brachysclereids.
  - Note: Gritty texture in pears is produced by sclereids (stone cells).
Endodermis:
- Single layer of tightly packed parenchyma cells surrounding the central vascular core of roots.
- Radial and end walls possess suberized and lignified thickenings forming a continuous waterproof band called the Casparian Strip.

Complex Permanent Tissues

Definition: Composed of multiple cell types differing in structure and shape that function together to perform a unified transport or protective role.

Xylem vs Phloem and Tracheids vs Vessel Elements

Xylem (Water-Conducting Tissue):
- Tracheids:
  - Elongated, dead cells at maturity with tapering ends, lignified secondary walls, and bordered wall pits.
  - Lack open perforation plates (contain pits, NO perforations).
  - Aligned side-by-side; primary water-conducting element in gymnosperms and seedless vascular plants (secondary in angiosperms).
- Vessel Elements:
  - Elongated, dead cells with lignified secondary walls and bordered pits.
  - Aligned end-to-end to form long continuous tubes (vessels).
  - Possess open perforation plates at end walls (single large opening or parallel slits).
  - Primary water-conducting element in angiosperms; xylem sap flows vertically through perforations.
- Bordered Pits:
  - Depressions in primary cell walls called primary pit fields.
  - Bordered pit apertures in secondary walls consist of a pit chamber and a permeable pit membrane.
- Xylem Parenchyma: Thin-walled, living cells functioning in storage and tissue repair.
- Xylem Fibers: Elongated, thick-walled dead cells providing structural non-conducting support.
Phloem (Food-Conducting Tissue):
- Sieve Cells:
  - Found in gymnosperms (conifers) and primitive seedless vascular plants (ferns).
  - Elongated, thin-walled cells living at maturity; possess primary cell walls with unspecialized sieve areas.
  - Lack a nucleus at maturity; cytoplasm contains dense networks of tubular endoplasmic reticulum.
  - Function in transporting dissolved carbohydrates.
- Sieve-Tube Elements:
  - Found in advanced flowering plants (angiosperms).
  - Elongated cells with primary cell walls only; living at maturity but enucleate (lacking a nucleus).
  - End walls specialized into sieve plates containing large sieve pores.
  - Aligned end-to-end to form continuous sieve tubes.
- Companion Cells:
  - Specialized, elongated living parenchyma cells with primary cell walls and intact nuclei.
  - Derived from the exact same mother cell as their associated sieve-tube element.
  - Connected to sieve-tube elements via protoplasmic connections called plasmodesmata.
  - Delivers ATP, proteins, and signal molecules to sustain the metabolism of enucleate sieve-tube elements.
  - Pathway: Companion cell $\rightarrow$ Plasmodesmata $\rightarrow$ Sieve-tube element $\rightarrow$ ATP + Signals.
Secretory Tissues and Cells:
- Derived from specialized parenchyma forming ducts, cavities, or trichomes.
- Secrete compounds such as metabolic waste materials, nectar, plant hormones, latex, essential oils, mucilage, and resins.
- Structures: Nectaries (nectar in flowers), Hydathodes (water secretion), Secretory hairs (mucilage in sundews, oils in citrus), and Laticifers (latex production).

High-Yield Comparative Summaries

Meristematic vs. Permanent Tissues:
- Meristematic: Actively dividing; produces new cells via mitosis; undifferentiated; small or no vacuoles.
- Permanent: Cells fully matured; specific and irreversible forms; specialized; possess large central vacuoles.
Apical vs. Intercalary vs. Lateral Meristems:
- Apical: Located at root/shoot tips; increases plant length (primary growth).
- Intercalary: Located at internode bases; elongates internodes.
- Lateral: Located at vascular and cork cambium; increases diameter (secondary growth).
Xylem vs. Phloem:
- Xylem: Transports water and inorganic minerals upward.
- Phloem: Transports dissolved sucrose, carbohydrates, and organic solutes.
Tracheids vs. Vessel Elements:
- Tracheid: Possesses pits, NO perforations; arranged side-by-side.
- Vessel Element: Possesses end perforation plates; arranged end-to-end.
Sieve Cells vs. Sieve-Tube Elements:
- Sieve Cell: Found in gymnosperms and ferns; unspecialized sieve areas.
- Sieve-Tube Element: Found in angiosperms; specialized sieve plates on end walls.
Palisade vs. Spongy Mesophyll:
- Palisade: Columnar, elongated, compactly packed.
- Spongy: Irregularly shaped, loosely arranged with large air spaces.
Collenchyma vs. Sclerenchyma:
- Collenchyma: Living cells; uneven pectin wall thickenings; flexible elastic support.
- Sclerenchyma: Dead cells; heavily lignified secondary walls; rigid mechanical support.
Fibers vs. Sclereids:
- Fibers: Long, narrow, spindle-shaped, tapering ends.
- Sclereids: Short, variable shape, occurs singly or in clusters.
Epidermis vs. Periderm:
- Epidermis: Primary outer covering of non-woody plant organs.
- Periderm: Secondary outer corky covering replacing epidermis in woody roots/stems.

Root Biology and Structure

Functions of Roots:
1. Absorption: Uptake of water and dissolved soil minerals.
2. Anchorage: Secures the plant firmly in soil substrate.
3. Conduction: Transports water and minerals upward to shoot systems.
4. Reproduction: Vegetative propagation via adventitious buds.
5. Food Storage: Accumulates starches and carbohydrates.
6. Photosynthesis: Specialized function in select aerial photosynthetic roots.
General Characteristics of Roots:
- Cylindrical structure, typically colorless or drab.
- Distinctly lack nodes, internodes, terminal/lateral buds, leaves, and flowers.
Root System Types:
- Diffuse or Fibrous Root System: Consists of several or many main roots that are slender, roughly equal in length and diameter, with extensive lateral branches.
- Taproot (Primary) System: Features a dominant main root developed directly from the seed embryo's radicle that penetrates deeply into the soil.
- Fleshy Root System: A taproot system modified where the main root becomes greatly enlarged for food/water storage (e.g., carrot, radish, turnip).
- Fascicled Root System: Roots featuring clusters of swollen bulb-like storage structures.
- Adventitious Roots: Roots arising from non-root tissue, such as stems or leaves. (The radicle is the embryonic root originating directly from the seed).
Four Regions of the Root (Longitudinal Section, from Tip Upward):
1. Root Cap:
  - Thimble-shaped protective mass of parenchyma cells covering the root apical meristem.
  - Dictyosomes secrete mucigel, a slimy lubricant aiding root passage through soil.
  - Contains specialized statocytes with amyloplasts (starch grains) acting as gravity sensors (gravitropism).
2. Region of Cell Division (Meristematic Region):
  - Active mitotic cell division occurring every $12\text{--}36\,\text{hours}$ .
  - Cells are small, dense, and cuboidal.
  - Apical Initials give rise to primary meristems:
    - Protoderm: Gives rise to epidermis.
    - Ground Meristem: Gives rise to cortex parenchyma.
    - Procambium: Gives rise to primary xylem and phloem.
  - Quiescent Center: A central zone of dormant cells with extremely slow division rates.
3. Region of Elongation:
  - Cells elongate significantly along the vertical axis, widen, and form large vacuoles.
  - Consists of primary embryonic tissues: Protoderm (dermatogen), Ground Meristem (periblem), and Procambium (plerome).
4. Region of Maturation (Region of Differentiation / Root Hair Zone):
  - Cells differentiate into specialized mature cell types:
    - Protoderm $\rightarrow$ Epidermis.
    - Ground Meristem $\rightarrow$ Cortex and Pericycle.
    - Procambium $\rightarrow$ Primary Xylem, Primary Phloem, and Pith.
  - Root Hairs: Tubular extensions of epidermal cells located in the youngest region of this zone; adhere to soil particles to maximize absorptive surface area.
  - Cortex: Storage region situated between the epidermis and pericycle.
  - Pericycle: Layer situated between endodermis and vascular tissue; retains meristematic potential to produce lateral branch roots and vascular cambium.
Internal Root Anatomy (Cross Section):
1. Epidermis: Single cell layer lacking cuticle; absorbs water/minerals.
2. Cortex: Composed of Ground Meristem derivatives. Largest area in root cross-section.
  - Hypodermis: Outermost layer with suberized walls.
  - Parenchyma: Storage cells packed with starch.
  - Endodermis: Innermost boundary layer possessing the Casparian Strip.
3. Stele: Central vascular cylinder derived from procambium.
  - Pericycle: Outermost layer of stele.
  - Vascular Tissues: Xylem and phloem.
  - Pith: Central parenchyma present in monocot roots, absent in dicot roots.
Monocot vs. Dicot Root Cross-Section:
- Monocot Root: Central pith present; xylem bundles arranged in a distinct ring.
- Dicot Root: Central pith absent; xylem forms a central star or x-shape pattern.
Physiology of Root Absorption:
- Water Absorption: Occurs passively via osmosis. Takes place only when water potential/concentration inside root cells is lower than that of the external soil environment.
- Mineral Absorption: Occurs via active transport requiring metabolic energy. Accumulation of minerals maintains cell hypertonicity, driving continued osmotic water uptake.
- Water Pathways:
  - Apoplastic Pathway: Movement through interconnected non-living cell walls and intercellular spaces.
  - Symplastic Pathway: Movement through continuous living cytoplasm connected by plasmodesmata.
Secondary Growth in Roots:
- Occurs in dicotyledons and gymnosperms; absent in monocots.
- Growth patterns: Determinate (growth stops after full organ expansion) vs. Indeterminate (tissues added indefinitely, e.g., perennial trees).
Modified Roots and Specialized Functions:
- Fleshy/Storage Roots: Expanded cortex parenchyma storing starch/sucrose (e.g., carrot, turnip, radish, sweet potato).
- Adventitious Propagation Roots: Produce adventitious buds developing into aerial shoots.
- Prop/Brace Roots: Grow from lower stem nodes into soil to provide structural support (e.g., corn).
- Aerial Roots: Entirely above ground; includes climbing roots, photosynthetic roots, and velamen roots of orchids (sponge-like multiple epidermis absorbing moisture and preventing water loss).
- Pneumatophores: Upward-growing knee-shaped roots containing aerenchyma; facilitate oxygen gas exchange in submerged muddy environments.
- Aquatic Roots: Highly feathery root structures providing buoyancy (e.g., water hyacinth).
- Photosynthetic Roots: Green chloroplast-containing aerial roots (e.g., certain epiphytic orchids).
- Contractile Roots: Pull plant bulbs or corms deeper into the soil via vertical contraction (e.g., Gladiolus).
- Buttress Roots: Large, plank-like stability roots extending from trunks of large trees in shallow tropical soils.
- Parasitic/Haustorial Roots: Possess peg-like projections called haustoria that penetrate host plant tissues to draw water/nutrients directly from host xylem/phloem.
- Symbiotic Roots:
  - Rhizobium Nodules: Mutualism between legumes (beans) and Rhizobium bacteria; bacteria fix atmospheric nitrogen into usable ions in exchange for plant carbohydrates.
  - Mycorrhizae: Mutualism between root systems and fungi; fungus increases absorptive surface area (especially phosphorus uptake) and protection, while plant supplies sugars and amino acids.
Human Relevance of Roots:
- Direct food sources (carrots, turnips, cassava, ube, sweet potatoes, radishes).
- Spices and flavorings (sarsaparilla, licorice, turmeric).
- Sources of dyes, industrial materials, and ornamental goods.
- Medicinal compounds (aconite, ipecac, gentian, reserpine, ginseng, nicotine).
- Natural insecticides (rotenone extracted from barbasco roots).
- Erosion prevention, soil stabilization, water conservation, and phytoremediation (using plants to extract heavy metals from polluted soils).

Stem Biology and Morphology

Definition and Function of Stems:
- Central axis of the shoot system, continuous with the root system.
- Main Functions:
  1. Conduction of water, nutrients, and photosynthetic products.
  2. Production, support, and spatial orientation of leaves and reproductive organs.
  3. Storage of water, starch, and nutrients.
Terminology and Origin:
- Epicotyl: Embryonic shoot stem developing above the cotyledons in a seed.
- Shoot: A stem together with its attached leaves.
- Shoot System: Total aerial structure comprising stems, branches, and leaves.
Stem Habit Categories:
- Aerial Stems: Grow above ground. Types include Erect (upright), Climbing (ascends structures), and Prostrate (trails horizontally along ground).
- Subterranean Stems: Modified stem structures growing below the soil surface.

Herbaceous vs Woody Stems Comparison

Plant Forms:
- Tree: Large woody perennial with a single main central trunk.
- Shrub: Woody perennial characterized by several major stems of roughly equal size arising near ground level.
External Anatomy of Stems:
- Node: Precise point on a stem where leaves or buds emerge.
- Internode: The stem segment extending between two successive nodes.
- Terminal/Apical Bud: Bud situated at the apex tip of a stem or twig.
- Lateral/Axillary Bud: Bud situated in the leaf axil along the side of a stem.
- Adventitious Bud: Buds arising out of normal sequence from locations other than leaf axils (e.g., trunks, roots).
Classification Systems for Buds:
- By Location: Terminal, Axillary, or Adventitious.
- By Status:
  - Accessory: Secondary buds formed alongside a primary bud.
  - Resting: Formed at the end of a growth season, remaining dormant until the next season.
  - Dormant/Latent: Inactive buds whose growth has been delayed for an extended period.
  - Pseudoterminal: An axillary bud that assumes the functional role of a dead/lost terminal bud.
- By Morphology: Scaly/Covered (protected by modified scales), Naked (devoid of scales), or Hairy (protected by dense hairs).
- By Function: Vegetative (develops into leafy shoots), Reproductive (develops into flowers), or Mixed (contains embryonic leaves and flowers).
External Stem Markings and Scars:
- Lenticels: Raised pores on woody stems enabling gaseous exchange.
- Leaf Scars: Marks left on a stem after leaf abscission.
- Bundle Scars: Tiny dots within a leaf scar representing broken vascular connections.
- Bud Scars: Concentric rings of narrow scars left by fallen bud scales; used to determine annual shoot growth.
- Twig Scars: Circular scars marking where branch twigs have shed.
- Fruit Scars: Marks left by detached fruit pedicels.
Specialized Aerial Stems:
- Tendrils: Coiling, climbing stem modifications (e.g., grapes, Boston ivy, ampalaya).
- Thorns: Sharp, modified protective stems (e.g., honey locust, rose, cactus spines).
- Storage Stems: Fleshy, green photosynthetic stems specialized for water/food storage (e.g., cactus).
- Aerial Bulbs: Bulbil structures used in vegetative reproduction (e.g., onion aerial bulbils).
- Runners/Stolons: Horizontal surface stems that root and produce new daughter plants at nodes (e.g., strawberry).
- Cladophylls: Flattened, leaf-like photosynthetic expansion of stems (e.g., Ruscus / butcher's broom).
Specialized Subterranean Stems:
- Rhizomes: Horizontal underground stems growing at or beneath soil level; serve for storage and perennial reproduction (e.g., Iris, ginger, potato rhizomes).
- Tubers: Swollen terminal ends of rhizomes specialized for dense starch storage (e.g., potato).
- Bulbs: Globose perennial structures featuring a small central stem axis surrounded by thick, fleshy, overlapping scale leaves (e.g., onion, tulip).
- Corms: Solid, globose stem tissues encased in thin papery scale leaves (e.g., Gladiolus).

Internal Anatomy and Secondary Growth of Woody Stems

Gross Structural Divisions (Cross Section):
1. Bark: All tissues external to the vascular cambium.
2. Wood (Secondary Xylem): All tissues internal to the vascular cambium.
3. Cambium: Cylinder of meristematic cells responsible for secondary lateral growth.
4. Pith: Central parenchyma core.
Primary vs. Secondary Stem Tissues:
- Primary Tissues: Produced during initial growth by apical meristems (Protoderm $\rightarrow$ Epidermis; Procambium $\rightarrow$ Primary Xylem/Phloem; Ground Meristem $\rightarrow$ Cortex/Pith).
- Secondary Tissues: Produced by lateral meristems (Vascular Cambium and Cork Cambium).
Layers of Primary/Secondary Woody Stems:
- Epidermis: Outer single cell layer, cutinized and waterproof; replaced by periderm in secondary growth.
- Cortex: Contains collenchyma (support), parenchyma (storage), and fibers/sclereids.
- Stelar Tissues:
  - Pericycle: Parenchyma and supporting fiber bundles.
  - Phloem: Sieve tubes, companion cells, fibers, and parenchyma.
  - Vascular Cambium: Continuous meristematic ring dividing inward to form secondary xylem and outward to form secondary phloem.
  - Xylem: Vessels, tracheids, fibers, and xylem parenchyma.
  - Rays: Radial bands of parenchyma cells extending transversely across the stem for lateral transport.
Growth Rings and Wood Anatomy:
- Annual Growth Ring: Formed by seasonal variations in secondary xylem production (Spring Wood + Summer Wood = $1\,\text{year}$ of growth).
  - Spring Wood (Early Wood): Produced during rapid spring growth; characterized by wide vessel lumens and thin cell walls.
  - Summer Wood (Late Wood): Produced during drier summer months; features narrow vessel lumens and dense, thick-walled cells.
- Heartwood: Non-functional, dark, dry central wood providing structural support; impregnated with resins and tannins.
- Sapwood: Functional, lighter outer wood actively conducting xylem sap.
Bark and Cork Cambium:
- Cork Cambium (Phellogen): Secondary lateral meristem originating in the cortex or secondary phloem.
- Divides to produce suberized, waterproof cork cells (phellem) outward; cork cells die at maturity, conserving water and resisting pathogens.
- Lenticels: Porous breaks in cork tissue that facilitate internal gas exchange.

Plant Transport Mechanisms

Phloem Transport (Translocation):
- Movement of dissolved organic compounds (sucrose, amino acids, proteins, ions, hormones) from source to sink tissues.
- Source: Plant organ producing or exporting sugar (e.g., mature photosynthetic leaf).
- Sink: Plant organ consuming or storing sugar (e.g., developing root, fruit, or bud).
- Pressure-Flow (Mass Flow) Hypothesis (Ernst Münch, $1930$ ):
  1. Sugar is actively loaded into sieve-tube elements at the source.
  2. Water follows passively into sieve tubes from xylem via osmosis.
  3. High turgor hydrostatic pressure builds up inside sieve tubes at the source end.
  4. Pressure differential drives mass movement of phloem sap toward the sink.
  5. Sugar is actively unloaded at the sink, and water exits back into the xylem via osmosis.
Xylem Transport:
- Passive upward movement of water and minerals driven by the Transpiration Pull (Tension-Cohesion) Theory (proposed by Dixon and Jolly).
- Transpiration (evaporation of water through leaf stomata) creates negative pressure tension pulling water upward.
- Cohesion (attraction between water molecules) forms a continuous unbroken water column, while Adhesion (attraction of water to tracheary wall surfaces) resists gravitational pull.

Plant Propagation and Biotechnology

Propagation Modes:
- Sexual Propagation: Regeneration via seed production.
- Asexual / Vegetative Propagation: Reproduction using vegetative organs (leaves, stems, roots, buds), producing genetically identical clones.
Vegetative Propagation Techniques:
- Cuttings: Detached vegetative segments (stem, root, leaf) induced to form root/shoot systems.
- Layering: Inducing root formation on a stem while still attached to the parent plant before detachment (Ground Layering or Air Layering / Marcotting).
- Grafting (Graftage): Joining vegetative tissues of two plants (Scion attached to Stock); types include splice grafting, whip and tongue grafting, and approach grafting.
Micropropagation and Tissue Culture:
- In vitro technique of growing plant cells, tissues, or organs on sterile, artificial nutrient media under controlled environmental conditions.
- Explants used: Shoot tips, root tips, callus tissues, leaf discs, seed embryos, and anthers.
- Key Phytohormones Used in Culture:
  - Auxins: Induce cell division, cell elongation, and callus tissue formation.
  - Cytokinins: Promote RNA synthesis and stimulate protein and enzyme metabolic activity.
  - Gibberellins: Used to induce plantlet formation from adventive embryos.