Flower is a modified and condensed shoot useful for reproduction. Normally in an angiospermic flower the apical part of pedicel is called `thalamus or torus'. On this thalamus the nodes and internodes are condensed and at each node modified leaves are present. These are called floral leaves. They are arranged in whorls at the nodes. In a typical flower, there are four types of floral leaves. They are sepals, petals, stamens and carpels.
Based on the sexuality of flowers the sex distribution is of three types:
Fig : Parts of a Flower
It is the condition in which male and female flowers are borne on the same plant (e.g. Cocos nucifera).
It is the condition in which male and female flowers are borne on different plants (e.g. Vallisneria).
If unisexual and bisexual flowers are borne on the same plant it is called polygamous (e.g. Mangifera).
Flowers are classified into three types based on their structural symmetry:
- Actinomorphic Flower
Various organs of each whorl in a flower are arranged similarly so that the flower can be divided longitudinally into two equal halves on any plane (e.g. Hibiscus).
- Zygomorphic Flower
The floral organs in any whorl of a flower are arranged in a different way so that the flower can be cut into two equal halves only in one plane (e.g. Ocimum).
- Asymmetric Flower
The flower cannot be cut into two equal halves in any plane because of the arrangement of floral organs in various whorls in different ways (e.g. Canna indica).
Position of Gynoecium on the Thalamus
Based on the position of gynoecium on the thalamus, flowers are of three types;
Thalamus is convex or conical. The gynoecium is arranged at the apex of the thalamus. The remaining floral parts like calyx, corolla and androecium are arranged at the base of the gynoecium. This type of ovary is called `superior ovary' and the flower is hypogynous. (e.g. Annona)
Thalamus is concave or flattened (saucer shaped) with a centrally located gynoecium. The remaining floral parts like calyx, corolla and androecium are arranged along the margins. This type of ovary is `half-superior' or `half-inferior' and the flower is perigynous. (e.g. Crotalaria)
In this flower the thalamus is a deep cup like structure, inside which the gynoecium is arranged. The walls of the thalamus and gynoecium are fused. The remaining floral parts are arranged along the margin of the thalamus i.e., above the level of gynoecium. So, the ovary is inferior and the flower is epigynous. (e.g. Musa)
Description of Flower
The condensed leaf-like structures on the peduncle are called `bracts'. Flowers are borne only in the axils of bracts. These bracts protect the flowers in bud condition.
- Thalamus or Receptacle or Torus
The condensed axis of the flower is called thalamus. It contains four condensed nodes, at which calyx, corolla, androecium and gynoecium are developed in four whorls.
Calyx and corolla together constitute the perianth e.g. Banana.
They are thin and green coloured. They provide protection to the flower during bud condition. If the sepals are free they are called `polysepalous' and if united they are called `gamosepalous' (e.g. Hibiscus).
They are coloured and attractive. They help in cross-pollination by attracting the insects. The petals may be polypetalous or gamopetalous.
The mode of arrangement of perianth lobes during bud condition is called `aestivation'. It is of different types:
Perianth lobes are free and are arranged with wide gaps between them (e.g. Calyx of Brassica).
In this type of aestivation the perianth lobes are arranged closely with small gaps between them. They remain free or united (e.g. Calyx of Annona).
In this type of aestivation the perianth lobes overlap each other i.e. one margin of the perianth lobe is covered by the next one. In this way in all the perianth lobes, one margin is inside and the other one is outside (e.g. Corolla of Hibiscus).
It is a type of twisted aestivation found in corolla. The overlapping of the petals is in 1+1+3 manner i.e. the two margins of one perianth lobe are completely overlapped and the other one is external and in the remaining three perianth lobes one margin is overlapped by the next one. It is otherwise called as vexillary aestivation e.g. Pea.
It is the third whorl of the flower containing stamens. These are the male reproductive organs:
Parts of a Stamen
Each stamen has 3 main parts: Filament, Anthers and Connective.
Length of Filaments
Based on the length of the filament, stamens are of two types: Didynamous and tetradynamous.
Union of Stamens
In some flowers, stamens are united. The union is of two types:
It is the fusion among filaments, or anther lobes or both. It is of 3 types:
- Monadelphous: The filaments of all the stamens are united to form a single bundle. e.g. Hibiscus.
- Diadelphous: Out of 10 stamens in a flower, 9 stamens are fused to form a single bundle and the 10th remains free as the second bundle. e.g. Dolichos.
- Polyadelphous: The stamens are united to form many bundles.
- AdhesionFusion of stamens with other parts of the flower arranged in different whorls like calyx, corolla and gynoecium is called `adhesion'.
It is known as female reproductive organ. It consists of carpels.
The gynoecium has three parts: Ovary, Style and Stigma.
The ovary consists of one to many carpels. It is the basal swollen part of gynoecium. If the carpels present on the thalamus remain free, they are called apocarpous. If the carpels are partly fused they are called as sub-apocarpous. If the carpels are fused as a single structure, and they are called syncarpous ovary.
The arrangement of ovules on the placenta is called 'placentation'.
Types of placentation (a) marginal (b) Axile (c) Parietal (d) Free-central (e) Basal
It is an elongated, tubular, stalk like structure developing from the gynoecium.
It is the terminal part of the style, which receives pollen grains at the time of pollination.