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Structure of flower
The structure of typical flower consists of four whorls namely, calyx, corolla, androecium and gynoecium.
Figure 1 Parts of a flower
Calyx: The calyx is the outermost whorl of the flower. The members of the calyx are called as sepals, which are small green, leaf-like structures protecting the flower during its bud stage. The calyx may be gamosepalous (united sepals) or polysepalous (free sepals).
Corolla: The second inner whorl is the corolla. The members of the corolla are called as petals, which are brightly colored to attract insects during pollination. Corolla may be free (polypetalous) or united (gamopetalous). The shape of the corolla may be tubular, bell-shaped, funnel-shaped or wheel shaped.
Aestivation: Aestivation is defined as the mode of arrangement of petals or sepals in the floral bud with respect to the other members of the same whorl. Four main types of aestivation are valvate, twisted, imbricate and vexillary.
Valvate: In this type, the sepals or petals in a whorl just touch one another at the margin. E.g Calotropis
Twisted: In this type, one margin of the appendage overlaps that of the next one. E.g China rose
Imbricate: In this type, the margins of sepals or petals overlap but not necessarily in a specific direction. E.g Cassia
Vexillary: In this type of flowers, there are five petals, the largest petal also known as standard overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel). E.g Pea flowers
Figure 2 Types of aestivation in Corolla
A. Valvate B. Twisted C. Imbricate D. Vexillary
The androecium is the male reproductive organ of flower and is composed of stamens. Each stamen consists of a stalk which is made up of filament and anther. The anther is usually bi-lobed and each lobe has two chambers consisting of pollen sacs. The pollen grains are produced in pollen sacs. A sterile stamen is called as staminode.
In some plants, the stamens are attached to petals, hence they are known as epipetalous (E.g. Eggplant). If the stamens are attached to perianth, then they are known as epiphyllous (E.g Flowers of lily). The stamens may either be free (polyandrous) or united into one bundle (monadelphous; Eg China Rose) or two bundles (diadelphous; Eg. Pea) or more than two bundles (Polyadelphous; E.g Citrus)