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Flower is a modified shoot. It is the reproductive unit  of the plant. It is the means of sexual reproduction.
A typical flower
Each flower usually has four whorls arranged successively on the swollen end of the receptacle called the thalamus. The parts of a typical flower are :

i. Calyx – It forms the outermost whorl of a flower and contains sepals. Sepals are green, leaf-like structures that cover and protect the flowers during the bud stage. When the sepals of a flower are free, they are called polysepalous, when fused sepals they  are called gamosepalous.

ii. Corolla – It is the layer that lies inside the calyx. It contains petals which brightly coloured and help in attracting insects for pollination. When the petals are free, they are called polypetalous, while fused petals are called gamopetalous. Shape of corolla varies in different plants and may tubular. Bell shaped, funnel shaped or wheel shaped.

iii. Androecium – It is the male reproductive part of a flower. It is composed of stamens. A stamen has a stalk or filament and an anther. Each anther is bilobed, with each lobe having two chambers called the pollen sacs. The pollen sacs have the male gamete or the pollen grains.

iv. Gynoecium - represents the female reproductive part of a flower. It is made up of carpels. A carpel is made up of three parts namely stigma style and ovary.Ovary is the enlarged part at the base.  The ovary is connected by a long tube called style. The stigma is usually the sticky tip at the end of the style.  Each ovary bears one or many ovules attached to the placenta.