Flower

Flower is the reproductive part of a plant. It is a modified shoot with very short and condensed internodes and a number of floral appendages at each node. These floral appendages are modified leaves. A flower may be trimerous, tetramerous or pentamerous when the floral appendages are in multiple of 3, 4 or 5, respectively.

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  • Flowers can be Axilliary or terminal in position.
  • They are produced on the axils of modified leaf called bract.
  • Flowers with bract are called bracteates and those without bract are called ebractate.

Whorls of a Flower

  • The floral appendages are arranged in whorls on the swollen end of the pedicel called thalamus or receptacle.
  • A complete flower contains four whorls: Calyx, Corolla, Androecium and Gynoecium.
  • The flower is incomplete when one or more whorls are missing.
  • The whorls are classified into 2 groups.
  • Accessory whorls: They are the outer whorls – calyx and corolla. Calyx (sepals) is a protection organ while corolla (petals) is the attractive organ. In some flowers, the calyx and corolla are not distinct, and is known as the perianth.
  • Essential whorls: They are the inner two whorls – Androecium (stamens) and Gynoecium (ovary). They are the reproductive structures of the flower. If a flower has both androecium and gynoecium, it is called a bisexual flower. If either one of these is only present in the flower, it is called a unisexual.

Symmetry in Flowers

  • Flowers that can be divided into two equal halves along any median longitudinal plane are called Actinomorphic (or regular) flowers. (e.g. Hibiscus, Ixora)
  • If the flowers can be divided into two equal halves only in one vertical plane, they are called Zygomorhic flowers. (e.g. Pisum)
  • Flowers that cannot be divided into two similar halves are called asymmetrical flowers.

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Based on the position of the floral parts on the thalamus, flowers are grouped into three types.

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Hypogynous flowers:

  • In this flower, the gynoecium occupies the highest position.
  • The thalamus may be conical or flat.
  • The ovary is above the level of thalamus.
  • The rest of the whorls – sepals, petals and stamens, are arranged below the ovary.
  • The ovary is thus called superior ovary. (e.g. Hibiscus)

Perigynous flower:

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  • The thalamus grows upwards forming a cup-shaped structure enclosing the ovary.
  • The other floral parts are located at the rim of the thalamus.
  • Thus, the ovary is called half-superior or half-inferior ovary. (e.g. Rose)

Epigynous flower:

  • The margin of the thalamous grows upwards, and encloses the ovary completely.
  • From the margin, other floral parts will arise.
  • Thus, the ovary is called inferior ovary. (e.g. Ixora)

 

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