Flower

Introduction to Flowers

  • Flowers are the most attractive and vital parts of a plant.
  • Characteristics of flowers:
    • Varied in colour, arrangement, shape, size, and arrangement of parts.

Definition of a Flower

  • Flowers serve as reproductive organs of plants.
  • They are responsible for producing seeds from which new plants arise.
  • A flower is described as a specialized shoot with leaves modified into floral whorls.
  • It is situated at the end of a stalk known as the pedicel or peduncle.
  • Pedicellate Flowers: Flowers with a pedicel.
  • Sessile Flowers: Flowers without a pedicel.
  • The pedicel may have two small green, leaf-like structures known as bracteoles.
  • The upper part of the flower stalk swells to form a cup-shaped structure called the thalamus or receptacle, characterized by highly condensed nodes and internodes.

Structure of a Bisexual Flower

  • Floral parts are arranged on the thalamus in four whorls:
    1. Calyx: First whorl made up of green sepals.
    2. Corolla: Second whorl of large, brightly-coloured petals.
    3. Androecium: Third whorl containing stamens (male parts).
    4. Gynoecium: Fourth whorl that consists of one or more pistils (female parts). Each pistil may comprise a single carpel or multiple fused carpels.
  • Complete Flowers: Have all four whorls (calyx, corolla, androecium, and gynoecium).
  • Incomplete Flowers: One or more whorls are absent.

Essential and Nonessential Whorls of a Flower

  • Essential Whorls: Directly involved in reproduction:
    • Androecium: Male parts (stamens).
    • Gynoecium: Female parts (carpels).
  • Nonessential Whorls: Accessory structures not directly involved in reproduction:
    • Calyx: Protects the reproductive parts during the bud stage and attracts pollinators when the flower is open.
    • Corolla: Also attracts pollinators.

Bracts and Nectaries

  • Bracts: Leaf-like structures in the axil of flowers; they can be green or large and colourful (e.g. in Bougainvillea).
  • Nectaries: Special glands at the base of the pistil or petals; secrete sweet nectar to attract insects for pollination.

Sexuality in Flowers

  • Bisexual (Hermaphrodite) Flower: Contains both male (androecium) and female (gynoecium) parts.
    • Examples: Mustard, China Rose, Petunia.
  • Unisexual (Imperfect) Flower: Has only one reproductive whorl (either androecium or gynoecium).
    • Staminate Flower: Contains only androecium.
    • Pistillate Flower: Contains only gynoecium.
    • Examples: Papaya, Palm, Luffa.
  • Neuter Flower: Both androecium and gynoecium are absent.

Structure of a Bisexual Flower: Details

Calyx

  • The outermost whorl, consisting of sepals.
  • Sepals are typically green but may be coloured (petaloid) in some flowers (e.g. Gulmohar and Nasturtium).
  • Polysepalous Calyx: Sepals are free (e.g., Mustard, Radish).
  • Gamosepalous Calyx: Sepals are fused (e.g., China Rose).
  • Additional structures called epicalyx may be present (e.g., Hibiscus).
  • Deciduous Calyx: Sepals fall off at fruit formation or withering (e.g., Brassica).
  • Persistent Calyx: Sepals remain attached during fruit development (e.g., Guava, Brinjal, Pea).
Functions of Calyx
  • Protects the flower in bud stage.
  • Provides photosynthesis via green sepals.
  • Attracts pollinators when sepals are brightly coloured along with petals.

Corolla

  • The second whorl composed of petals, typically brightly-colored.
  • Polypetalous Corolla: Free petals (e.g., Mustard, Rose).
  • Gamopetalous Corolla: Fused petals (e.g., Datura, Petunia).
Functions of Corolla
  • Protects the essential organs in bud stage.
  • Attracts insects for pollination with colour and fragrance.

Concept Check 1

  1. A typical flower has four whorls? True.
  2. The calyx and corolla are nonreproductive whorls? True.
  3. A bract is a leaf-like structure that arises in the axil of a flower? True.
  4. In Bougainvillea, bracts are large and green? False; they are large and colourful.
  5. Sepals and petals are absent in neuter flowers? True.
  6. In polypetalous corolla, the petals are united? False; they are free.

Androecium

  • The third whorl of a flower; consists of the male reproductive organs, called stamens, with numbers varying across different species.
  • Each stamen comprises three parts:
    • Filament: The slender stalk that supports the anther.
    • Connective: Connects two lobes of the anther.
    • Anther: Usually two-lobed, each lobe containing two pollen sacs, producing pollen grains (male gametes) within pollen sacs.
Functions of Androecium
  • Produces pollen grains containing male reproductive cells.
  • Positions anther optimally for pollen transfer through filament support.

Perianth

  • In some flowers, sepals and petals are indistinguishable (collectively referred to as perianth).
  • A single member of the perianth is termed tepal.
  • Green Perianth: Called sepaloid.
  • Non-Green Perianth: Called petaloid.
  • Found in flowers such as Salvia, Onion, Lilies, Gloriosa.

Gynoecium

  • Innermost whorl of the flower, centrally located on the thalamus.
  • Made up of one or more carpels.
  • Each carpel has three parts:
    • Stigma: The sticky or feathery top of the carpel that receives pollen grains.
    • Style: A slender stalk that connects stigma to the ovary.
    • Ovary: Swollen basal part with one or more locules (chambers) containing ovules.
  • Functions: Ovary develops into fruit, ovules develop into seeds.
  • Ovary can be classified as:
    • Unilocular: One chamber.
    • Multilocular: Multiple chambers.

Inflorescence

  • Arrangement of flowers on a branch or floral axis, varying across plant species.

Placentation

  • The part of the ovary to which ovules attach is called placenta (plural: placentae).
  • Arrangement of placentae within the ovary is referred to as placentation.