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