Explanation Flower: A flower is a modified shoot. The internodes are highly condensed and the floral leaves are borne on the crowded nodes that form the receptacle of the flower. A flower is borne on the tip of a stalk called pedicel. A typical flower has four whorls of floral leaves-sepals, petals, stamens and carpels arranged on the receptacle or thalamus. A bract is a leaf-like structure, which bears a flower in its axil. Sometimes the pedicel may bear hairy or leaf-like structures called bracteoles. The four whorls of the flower are calyx, corolla, androecium and gynoecium. Calyx: Calyx is the outermost whorl of the flower. It is made up of sepals. Sepals are green in colour and carry out photosynthesis, besides protecting the flower in the bud stage. In some flowers, the sepals are not green but brightly coloured like the petals and are called petaloid sepals, as seen in the flowers of Mussaenda and Clerodendron. When the sepals are free from each other in the calyx, the calyx is referred to as polysepalous and when the sepals are fused with each other, the calyx is called gamosepalous. Sepals usually fall off after fruit formation and are called deciduous sepals. But some sepals that remain even after fruit formation are called persistent sepals, as seen in tomato, brinjal and chilly. Corolla: Corolla is the second whorl of the flower and is made up of petals. Petals are usually brightly coloured and help to attract pollinating agents like birds and insects. When the petals are free from each other, the corolla is called polypetalous and when the petals are fused with each other, the corolla is called gamopetalous. Corolla of some flowers exhibits distinct shapes like cruciform, papilionaceous, ligulate, tubular and bilabiate. The corolla shaped like a cross is called the cruciform corolla, as seen in the mustard flower. Papilionaceous corolla has five petals, in which, the large anterior petal called standard or vexillum petal overlaps the two side petals called wing or alae petals, which in turn overlap the two fused petals called keel or carina petals. Papilionaceous corolla is a distinctive feature of the pea family. A strap shaped or tongue shaped corolla is called a ligulate corolla, as seen in the ray florets of sunflower. A tube shaped corolla is called tubular corolla, as seen in the disc florets of sunflower. A two-lipped corolla is called a bilabiate corolla, as seen in the flower of salvia. When the sepals and petals look similar and cannot be distinguished, the whorls are referred to as perianth and the floral leaves are called tepals, as seen in the flowers of onion and lily. Androecium: Androecium is the third whorl of the flower and is made up of stamens. Stamens are the male reproductive structures of the flower. Sterile stamens are called staminodes. The different parts of the stamen are filament, connective and anther. The anther has the pollen grains that contain the male gametes. When the androecium has two pairs of stamens and one pair is longer than the other pair, it is referred to as didynamous stamens, as seen in the flower of sacred basil. When the androecium is made up of six stamens and the four stamens in the inner whorl are longer than the two stamens in the outer whorl, the condition is called tetradynamous stamens, as seen in the flower of mustard. When there is fusion among the filaments of the stamens but the anthers remain free, the condition is called adelphy. When all the filaments of the androecium fuse to form one bundle, the condition is called monadelphous stamens, as seen in shoe-flower. When the filaments fuse to form two bundles, it is called diadelphous stamens, as seen in pea flower. When the filaments fuse to form many bundles, the condition is called polyadelphous stamens, as seen in Bombax. When the anthers unite but the filaments remain free, the condition is called syngenesious, as seen in the androecium of disc florets of sunflower. When the stamens are completely fused with each other, it is called synandrous condition, as seen in pumpkin. When the stamens unite with the petals, the condition is called epipetalous stamens, as seen in the flower of petunia. When the stamens or the anthers unite with the gynoecium, the condition is called gynandrous, as seen in orchids. Gynoecium: The innermost whorl of the flower is called the gynoecium and is composed of carpels. A typical carpel has three parts- ovary, style and stigma. When the carpels in the gynoecium are fused with each other, it is referred to as syncarpous gynoecium, as seen in the flower of mustard. When the carpels are free from each other it is called apocarpous gynoecium, as seen in the flower of lotus. The ovary may have one or more chambers called locules. Depending on the number of chambers, the ovary may be unilocular or one chambered; bilocular or two chambered; and multilocular or many chambered. The ovary contains ovules, which develop on a special tissue called placenta. The arrangement of placenta in the ovary is called placentation. Placentation is specific for a species or family. The different types of placentation include marginal placentation, parietal placentation, basal placentation, axile placentation and free central placentation. The style is a thread-like structure that develops over the ovary. When the style develops from the upper part of the ovary, it is called a terminal style as seen in mustard. When the style develops from the side of the ovary, it is called a lateral style as seen in mango. When the style develops from the base of the ovary, it is called a gynobasic style, as seen in sacred basil. The terminal part of the gynoecium is called the stigma. It receives the pollen grains. The stigma secretes a sugary substance that facilitates the germination of pollen grains. Variations are seen in the structure of the stigma in different flowers. Capitate or round stigma is seen in shoe-flower, feathery stigma is seen in grass, forked stigma is seen in Tridax, commissural stigma is seen in mustard and wheel shaped stigma is seen in poppy.