The flower can be defined as a modified shoot bearing nodes and modified floral leaves. Flowers exhibit a wide variety of shape, size, colour and arrangement that have the same basic plan. It consists of the following:
1. Calyx (group of sepals).
2. Corolla (group of petals).
3. Androecium (group of stamens).
4. Gynoecium (group of pistils).
1. Calyx : It is the outermost whorl of the flower. This is green, leaf-like structure. It may be polysepalous (sepals free), gamosepalous (sepals united), calyx may be regular or irregular.
2. Corolla : It is the second whorl of the flower inside the sepals. The petals (units of corolla) are usually brightly coloured. The insects are attracted due to the attractive colour of the petals. The narrow stalk like lower portion of the petal is called as 'claw', and the upper extended portion is known as limb. The corollas may be polypetalous (petals not united) or gamopetalous (petals united).
3. Androecium : It is the third whorl of the flower inside the petals. It represents as male reproductive whorl. Its units are stamen. In each stamen there are three parts viz :
(a) Anther: Knob-like bilobed structure containing the pollen grains. Each lobe contains two chambers called pollen sac.
(b) Connective : A strip of tissue, which connect the anther lobe is called connective.
(c) Filament : A slender stalk by which anther lobes are attached is called filament.
If the stamen is sterile then it is called 'staminode'.
4. Gynoecium : It is the female part of the flower. It is made up of 'carpels'. A typical carpel is made up of three parts :
(a) Stigma : Upper part which receives pollen grains.
(b) Style : The stalk between stigma and ovary.
(c) Ovary: Basal part containing ovules.
The sterile gynoecium is known as 'pistillode'.