Morphology of Flowering Plants: Comprehensive Study Notes

Introduction to Biological Description and Reductionism

  • Historical Perspective: The description of diverse life forms on Earth initially relied on observation through the naked eye, magnifying lenses, and microscopes. Descriptions focused on gross structural features, both external and internal, alongside observable and perceivable living phenomena.
  • Evolution of Botany/Zoology: Before the establishment of experimental biology or physiology, naturalists primarily described biology, making it synonymous with "natural history" for a significant period.
  • Utility of Detailed Descriptions: Detailed structural descriptions provided the basis for the later "reductionist biology." These descriptions became meaningful in framing research questions in physiology and evolutionary biology, as living processes eventually drew more scientific attention than mere structure.
  • Unit Focus: This unit covers the structural organization of plants and animals, providing the morphological and anatomical basis of physiological and behavioral phenomena. This is presented separately for plants (Chapter 5: Morphology; Chapter 6: Anatomy) and animals (Chapter 7: Structural Organisation).

Biography of Katherine Esau (1898 – 1997)

  • Early Life and Education: Born in Ukraine in 1898. She studied agriculture in Russia and Germany and received her doctorate in 1931 in the United States.
  • Research Focus: Reported that the curly top virus spreads through plant tissue via the food-conducting or phloem tissue.
  • Major Publications:
    • Plant Anatomy (19541954): Adopted a dynamic, developmental approach to plant structure; had a massive worldwide impact and revived the discipline.
    • The Anatomy of Seed Plants (19601960): Often referred to as the "Webster’s of plant biology" due to its encyclopedic nature.
  • Honors and Recognition:
    • Elected to the National Academy of Sciences in 19571957 (the sixth woman to receive this honor).
    • Received the National Medal of Science from President George Bush in 19891989.
  • Legacy: Peter Raven, director of Anatomy and Morphology at the Missouri Botanical Garden, noted that she "absolutely dominated" the field of plant biology even at the age of 9999.

Morphology of Flowering Plants (Angiosperms)

  • General Characteristics: Angiosperms exhibit a wide range of external structures but share the presence of roots, stems, leaves, flowers, and fruits.
  • Standardization: Successful classification and understanding necessitate the use of standard technical terms and definitions, accounting for variations representing adaptations to habitats, protection, climbing, and storage.
  • Plant Systems:
    • Root System: The underground part of the flowering plant.
    • Shoot System: The portion above the ground (stem, leaves, flowers, and fruits).

5.1 The Root

  • Origin: In most dicotyledonous plants, the radical elongates directly to form the primary root.
  • Branching: The primary root bears lateral roots of several orders (secondary, tertiary, etc.).
  • Types of Root Systems:
    • Tap Root System: Consists of the primary root and its branches; seen in the mustard plant.
    • Fibrous Root System: In monocotyledonous plants, the primary root is short-lived and replaced by many roots originating from the base of the stem; seen in the wheat plant.
    • Adventitious Roots: Roots arising from parts other than the radicle; found in grass, Monstera, and the banyan tree.
  • Main Functions of Roots:
    • Absorption of water and minerals from soil.
    • Providing proper anchorage to plant parts.
    • Storing reserve food material.
    • Synthesis of plant growth regulators (PGRs).

5.1.1 Regions of the Root

  • Root Cap: A thimble-like structure at the apex that protects the tender root tip as it penetrates the soil.
  • Region of Meristematic Activity: Located a few millimeters above the root cap. Cells are very small, thin-walled, contain dense protoplasm, and divide repeatedly.
  • Region of Elongation: Located proximal to the meristematic region. Cells undergo rapid elongation and enlargement, responsible for the root's growth in length.
  • Region of Maturation: Located proximal to the elongation zone; cells differentiate and mature here.
  • Root Hairs: Fine, delicate, thread-like structures formed by epidermal cells in the region of maturation; they absorb water and minerals.

5.2 The Stem

  • Definition: The ascending part of the plant axis that develops from the plumule of the embryo. It bears branches, leaves, flowers, and fruits.
  • Key Features:
    • Nodes: Regions where leaves are born.
    • Internodes: Portions between two nodes.
    • Buds: Can be terminal (tip) or axillary (axils).
    • Color: Generally green when young, often becoming woody and dark brown later.
  • Main Functions: Spreading branches, conducting water/minerals/photosynthates, food storage, support, protection, and vegetative propagation.

5.3 The Leaf

  • Development: A lateral, flattened structure originating from the shoot apical meristem and arranged in an acropetal order. It develops at the node and bears an axillary bud which may become a branch.
  • Parts of a Leaf:
    • Leaf Base: Attaches leaf to stem; may have two lateral stipules. In monocots, it expands into a sheath. In some legumes, it becomes swollen (pulvinus).
    • Petiole: Holds the blade to light; long, flexible petioles allow the blade to flutter in the wind, cooling the leaf and bringing fresh air.
    • Lamina (Leaf Blade): Green expanded part with veins and veinlets, featuring a prominent middle vein called the midrib. Veins provide rigidity and act as transport channels.

5.3.1 Venation and Types of Leaves

  • Venation Patterns:
    • Reticulate: Veinlets form a network; characteristic of most dicotyledonous plants.
    • Parallel: Veins run parallel within the lamina; characteristic of most monocotyledonous plants.
  • Leaf Types:
    • Simple Leaf: Lamina is entire or incisions do not touch the midrib.
    • Compound Leaf: Incisions reach the midrib, breaking it into leaflets. (Note: Buds are in the axil of the petiole, but NOT in the axil of leaflets).
    • Pinnately Compound: Leaflets on a common axis (rachis); example: neem.
    • Palmately Compound: Leaflets attached at a common point (tip of petiole); example: silk cotton.

5.3.3 Phyllotaxy

  • Definition: The pattern of arrangement of leaves on the stem/branch.
  • Types:
    • Alternate: A single leaf arises at each node in an alternate manner; examples: china rose, mustard, sun flower.
    • Opposite: A pair of leaves arises at each node opposite to each other; examples: Calotropis, guava.
    • Whorled: More than two leaves arise at a node to form a whorl; example: Alstonia.

5.4 The Inflorescence

  • Definition: The arrangement of flowers on the floral axis. A flower itself is a modified shoot where the shoot apical meristem changes to floral meristem.
  • Types:
    • Racemose: Main axis continues to grow; flowers are borne laterally in acropetal succession.
    • Cymose: Main axis terminates in a flower (limited growth); flowers are borne in basipetal order.

5.5 The Flower

  • Function: Reproductive unit in angiosperms for sexual reproduction.
  • Structure: Four whorls on the thalamus (receptacle) of the pedicel:
    • Accessory Organs: Calyx and Corolla.
    • Reproductive Organs: Androecium and Gynoecium.
    • Perianth: When calyx and corolla are not distinct (example: lily).
  • Sexuality: Bisexual (both organs present) or Unisexual (only stamens or carpels).
  • Symmetry:
    • Actinomorphic (Radial): Can divide into equal halves in any radial plane; examples: mustard, datura, chilli.
    • Zygomorphic (Bilateral): Can divide into similar halves in only one vertical plane; examples: pea, gulmohur, bean, Cassia.
    • Asymmetric (Irregular): Cannot be divided into two similar halves; example: canna.
  • Merosity: Trimerous (33), tetramerous (44), or pentamerous (55) appendages.
  • Bracts: Reduced leaf at the base of the pedicel (bracteate or ebracteate).

Floral Position and Parts

  • Ovary Position:
    • Hypogynous: Gynoecium highest; ovary superior; examples: mustard, china rose, brinjal.
    • Perigynous: Gynoecium in center, other parts on the rim at the same level; ovary half inferior; examples: plum, rose, peach.
    • Epigynous: Thalamus margin grows upward enclosing ovary; other parts above ovary; ovary inferior; examples: guava, cucumber, ray florets of sunflower.
  • Whorl Details:
    • Calyx: Members are sepals. Gamosepalous (united) vs. Polysepalous (free).
    • Corolla: Members are petals. Gamopetalous (united) vs. Polypetalous (free). Aestivation determines insect attraction. Shapes: tubular, bell-shaped, funnel-shaped, wheel-shaped.

5.5.1.2 Aestivation

  • Definition: Arrangement of sepals/petals in a floral bud relative to other members.
  • Types:
    • Valvate: Margins touch without overlapping; example: Calotropis.
    • Twisted: One margin overlaps the next; examples: china rose, lady’s finger, cotton.
    • Imbricate: Overlap is not in a particular direction; examples: Cassia, gulmohur.
    • Vexillary (Papilionaceous): 55 petals; largest (standard) overlaps two lateral (wings), which overlap two smallest anterior (keel); examples: pea, bean.

5.5.1.3 Androecium

  • Structure: Stamens (filament and anther). Anthers are usually bilobed with two pollen-sacs.
  • Staminode: A sterile stamen.
  • Attachments:
    • Epipetalous: Attached to petals (example: brinjal).
    • Epiphyllous: Attached to perianth (example: lily).
  • Cohesion:
    • Polyandrous: Free stamens.
    • Monoadelphous: One bundle (example: china rose).
    • Diadelphous: Two bundles (example: pea).
    • Polyadelphous: More than two bundles (example: citrus).
  • Variation: Different filament lengths; examples: Salvia, mustard.

5.5.1.4 Gynoecium

  • Structure: One or more carpels (stigma, style, ovary).
  • Type:
    • Apocarpous: Carpels are free; examples: lotus, rose.
    • Syncarpous: Carpels are fused; examples: mustard, tomato.
  • Placentation Types:
    • Marginal: Ridge along ventral suture; example: pea.
    • Axile: Multilocular ovary; example: china rose, tomato, lemon.
    • Parietal: Inner wall/periphery; false septum makes it two-chambered; examples: mustard, Argemone.
    • Free Central: On central axis, no septa; examples: Dianthus, Primrose.
    • Basal: Base of ovary, single ovule; examples: sunflower, marigold.

5.6 The Fruit and 5.7 The Seed

  • Fruit: Ripened ovary after fertilization (parthenocarpic if without fertilization).
  • Pericarp Layers: Epicarp (outer), Mesocarp (middle), Endocarp (inner).
  • Drupe: One-seeded, from monocarpellary superior ovaries.
    • Mango: Fleshy edible mesocarp, stony hard endocarp.
    • Coconut: Fibrous mesocarp.
  • Seed Structure:
    • Dicotyledonous: Seed coat (outer testa, inner tegmen), hilum (attachment scar), micropyle (pore). Embryo has radicle, plumule, and two cotyledons. Endospermic (example: castor) or Non-endospermous (examples: bean, gram, pea).
    • Monocotyledonous: Typically endospermic (except orchids). Seeds of cereals (e.g., maize) have a fused seed coat/fruit wall. Proteinous aleurone layer surrounds endosperm. Single cotyledon is the scutellum. Sheaths: coleoptile (plumule) and coleorhiza (radicle).

Semi-Technical Description of Flowering Plants

  • Sequence: Habit → Vegetative characters (roots, stem, leaves) → Floral characters (inflorescence, flower parts) → Floral formula → Floral diagram.
  • Symbols:
    • BrBr: Bracteate
    • KK: Calyx
    • CC: Corolla
    • PP: Perianth
    • AA: Androecium
    • GG: Gynoecium
    • ၤGၤ{G}: Superior ovary
    • ၢGၢ{G}: Inferior ovary
    • ♂♂: Male
    • ♀♀: Female
    • ☥☥: Bisexual
    • ⊕⊕: Actinomorphic
    • %\%: Zygomorphic
    • Fusion: Enclosed in brackets ()( ).
    • Adhesion: Line drawn above symbols.
  • Mustard Formula (Brassicaceae): 2˘295 2˘625 K2+2 C4 A2+4 1˘064G(2)\u2295 \text{ } \u2625 \text{ } K_{2+2} \text{ } C_4 \text{ } A_{2+4} \text{ } \u1064{G}_{(2)}

5.9 Solanaceae (Potato Family)

  • Distribution: Tropics, subtropics, and temperate zones.
  • Vegetative: Mostly herbs/shrubs; stem herbaceous/woody, aerial/underground (e.g., Solanum tuberosum); leaves alternate/simple/exstipulate/reticulate.
  • Floral characters:
    • Inflorescence: Solitary, axillary, or cymose (Solanum).
    • Flower: Bisexual, actinomorphic.
    • Calyx: 55 sepals, united, persistent, valvate.
    • Corolla: 55 petals, united, valvate.
    • Androecium: 55 stamens, epipetalous.
    • Gynoecium: Bicarpellary, syncarpous, superior ovary, bilocular, axile placentation (swollen placenta).
    • Fruit: Berry or capsule. Seeds endospermous.
  • Formula: 2˘295 2˘625 K(5) C(5) A5 1˘064G(2)\u2295 \text{ } \u2625 \text{ } K_{(5)} \text{ } C_{(5)} \text{ } A_5 \text{ } \u1064{G}_{(2)}
  • Economic Importance:
    • Food: Tomato, brinjal, potato.
    • Spice: Chilli.
    • Medicine: Belladonna, ashwagandha.
    • Fumigatory: Tobacco.
    • Ornamental: Petunia.