Flowers are the modified shoots bearing modified leaves that serve as the sexual reproductive organs of angiosperms. This strategy for reproduction has been so successful that angiosperms now dominate the plant world, and accordingly there are many variations on the basic structure of a flower.
Flowers are organs of sexual reproduction produced by the angiosperms (phylum Anthophyta), the largest phylum of photosynthetic organisms, with roughly 250,000 species. This large number represents a great diversity of flower types, but all flowers have some common structural elements.
Parts of flower
This is the stalk of the flower. Plants that have a stalk are known as pedicellate flowers whereas those that do not have a stalk are known as sessile flowers.
This is the base of the flower and lies above the pedicel. It is actually a modified shoot that forms the floral axis and holds the layers of the flower.
This layer actually forms the first whorl (layer) in the flower structure. They are said to be modified leaves. ‘Calyx’ is the word given for a collection of sepals. The sepals or calyx are often green in colour. Their main function is to protect the flower while it is still in the bud stage.
This forms the second whorl of the flower structure. ‘Corolla’ is the term given for a collection of petals. The petals are the colourful parts of the flower to which the pollinators are attracted to. In many flowers, the Corolla is scented to further make the flower attractive. Like the sepals, the petals are also said to be modified leaves. The petals are normally arranged in radial symmetry which means the flower can be divided into three equal parts. This type of symmetry is known as actinomorphic symmetry. In other flowers, the petals are arranged bilaterally symmetrical or an irregular pattern and this kind of an arrangement are called zygomorphic symmetry. The androecium and gynoecium often follow the symmetry seen in the calyx and corolla. The calyx and corolla are collectively called the perianth.
The androecium is a term given to the male reproductive system in a flowering plant. It forms the third whorl in a flower. It is made up of one or more stamens. Each stamen is made up of the following parts:
It is a long slender tube-like structure that holds the anther at the top of it.
It is the pollen producing part of the plant. It is made up of four chambers or segments known as pollen sacs. The anthers give rise to microspores during their development and each of these microspores forms a pollen grain that carries the male gamete. The nucleus in the pollen grain divided mitotically to form two male nuclei.
The pollen grain has double layered protective coverings. The inner layer is called the intine whereas the outer layer is called the exine. The pollen grains are released by the anthers and they get pollinated to the same flower(self- pollination) or to a different flower(cross-pollination).
The petals, sepals and the stamens often show fusion to form tubes: Petals fuse to form a tube and this condition is called gamopetalous. Stamens fuse to form a tube called the staminal tube.
It forms the fourth whorl of the flower and is often found towards the centre of the flower. This is the female reproductive part of a flower. The gynoecium is a term given to a collection of pistils or carpals. It is made up of the following structures:
It is the sticky end of the style which is responsible for catching pollen when they are pollinated.
It is a thin tube-like structure that holds the stigma and is attached to the ovary at its base.
It is the swollen basal portion of the flower which houses the ovules which contain the female gamete.The wall of the ovary, called the pericarp, is typically green, and the hollow space in the ovary is called the locule. Within the locule are one or more tiny globular ovules, each containing an egg nucleus and thus functioning as the female structure in sexual reproduction.
Based on the position of the ovary with respect to the other flower whorls, it can be classified into three types:
Hypogynous (superior ovary): This type of ovary lies above all the whorls of the flower.
Perigynous (intermediate ovary): This type of ovary is present in between the other flower layers.
Epigynous (inferior ovary): This type of ovary is present below the other whorls of the flower.It is often surrounded by the receptacle.
Functions of Flower Parts
Flowers and their parts function to achieve sexual reproduction, including pollination and seed formation. After pollination is finished, the flower begins the process of seed and finally fruit formation. During pollination, pollen grains are released from the anther and carried to the stigma, either by animals (such as insects, birds, and bats) or by wind.
Animals, attracted by the flower’s colors or aromas, visit flowers to obtain food—either the pollen itself or the nectar, a sugary liquid produced by small glands called nectaries at the base of the flower. The animal brushes up against the anthers, which deposit pollen on the animal’s body.
The animal transfers the pollen to the stigma of either the same flower (self-pollination) or a second flower (cross-pollination). During wind pollination, the anthers release their pollen, which is then borne by air currents. Some of the grains are deposited on a stigma of the same or another flower.
Each pollen grain germinates and produces a slender thread of protoplasm that grows downward through the style and into the ovary. This thread, the pollen tube, contains the sperm and grows toward an ovule, where it deposits its sperm.
The sperm then fuses with the egg, achieving fertilization as the first cell of the new generation is produced. The ovule matures to form a seed. At the same time, the surrounding ovary enlarges greatly, becoming a fruit as other parts of the flower recede and die off.
Because the stamens and pistils are intimately involved in reproduction, botanists refer to these as essential parts. The sepals and petals are termed nonessential parts, though in fact they remain important.
The sepals and petals are sometimes called the perianth because they are found on the periphery of the anthers. A complete flower is one that has all four sets of parts. A perfect flower is one that has both androecium and gynoecium and is thus bisexual.