2.1 The Flower: A Marvel of Reproduction

Introduction: What is a Flower?

  • The reproductive unit of an angiosperm (flowering plant).
  • Modified shoot with condensed internodes, floral structures are modified leaves.
  • Primary purpose: Sexual reproduction and seed production.

Structure of a Bisexual Flower

  • Contains both male (androecium) and female (gynoecium) parts.
  • Arranged in four concentric whorls on the receptacle:
    1. Calyx (outermost)
    2. Corolla
    3. Androecium
    4. Gynoecium (innermost)
  • Complete Flower: Possesses all four whorls.
  • Incomplete Flower: Missing one or more whorls.

The Whorls of a Flower: Parts and Functions

Non-Essential (Accessory) Whorls

Whorl Individual Unit Function(s)
Calyx Sepal Protection of bud, sometimes photosynthesis.
Corolla Petal Attraction of pollinators.
  • Perianth: Calyx and corolla are undifferentiated (e.g., Lily); individual units are tepals.

The Whorls of a Flower: Parts and Functions (Continued)

Essential (Reproductive) Whorls

Whorl Part Description Function(s)
Androecium Stamen Male part (anther + filament) Anther produces pollen grains (male gametes).
Gynoecium (Pistil) Carpel Female part (stigma + style + ovary) Stigma receives pollen; Ovary contains ovules (develops into fruit, ovules into seeds).

Inflorescence: The Arrangement of Flowers

  • Definition: Arrangement of flowers on a floral axis (peduncle).
  • Main Types:
    1. Racemose: Main axis grows indefinitely, flowers in acropetal succession (older at base, younger at top). (e.g., Mustard).
    2. Cymose: Main axis terminates in a flower, limited growth, flowers in basipetal succession (older at top, younger below). (e.g., Jasmine).

Placentation: Arrangement of Ovules

  • Definition: Arrangement of ovules within the ovary, attached to the placenta.
  • Types:
    • Marginal: Placenta forms a ridge, ovules in two rows (e.g., Pea).
    • Axile: Ovules attached to central axis in multilocular ovary (e.g., Tomato).
    • Parietal: Ovules on inner wall of one-chambered ovary (e.g., Mustard).
    • Basal: Single ovule at the base of the ovary (e.g., Sunflower).
    • Free Central: Ovules on central axis, no septa (e.g., Dianthus).

Flower Symmetry

  • Actinomorphic (Radial Symmetry): Divisible into two equal halves by any radial plane through center (e.g., Mustard).
  • Zygomorphic (Bilateral Symmetry): Divisible into two similar halves by only one vertical plane (e.g., Pea).
  • Asymmetrical/Irregular: Cannot be divided into two equal halves (e.g., Canna).

Aestivation: Arrangement of Sepals/Petals

  • Valvate: Just touch at margin (e.g., Calotropis).
  • Twisted: One margin overlaps next (e.g., China rose).
  • Imbricate: Overlap, but not in particular direction (e.g., Cassia).
  • Vexillary (Papilionaceous): Largest petal (standard) overlaps two wings, which overlap two keel petals (e.g., Pea).

Cohesion and Adhesion of Floral Parts

  • Cohesion (Fusion of Similar Parts):
    • Polyandrous: Free stamens.
    • Monadelphous: Stamens in one bundle.
    • Diadelphous: Stamens in two bundles.
    • Polyadelphous: Stamens in more than two bundles.
    • Syncarpous: Fused carpels.
    • Apocarpous: Free carpels.
  • Adhesion (Fusion of Dissimilar Parts):
    • Epipetalous: Stamens attached to petals.
    • Epiphyllous: Stamens attached to perianth.