Flowering plants also called angiosperms, use a sexual mode of reproduction. Reproduction in plants mainly revolves around the flower, which has both the male and the female gametes. All parts of a flower aid in the process of reproduction, although some of them are sterile. Therefore, to understand the process of reproduction in flowering plants, we need to look at the different parts of the flower and their functions.
Explore sexual reproduction in flowering plants notes to learn about the reproductive structure of flower and the process of pollination.
Structure of flower
A flower consists of the following parts-
Calyx: It is the outermost whorl of a flower. It comprises units called sepals. In the bud stage calyx, encloses the rest of the flower. They usually exhibit green colouration in some cases they may be a colour like petals. This condition of Calyx is termed as petaloid. Calyx can either be prominent or absent.
Corolla: It consists of many numbers of petals and it is the second whorl of the flower. These petals are sometimes fragrant. They are coloured, thin and soft that would help in the process of pollination as they would attract animals and insects.
Reproductive Structure of a Flower
It is the male reproductive part of a flower, consisting of stamens and it is the third whorl. Each stamen comprises two parts, namely, anther and filament. The tip of the anther is supported by the filament. Here pollens are produced by meiosis and disappear eventually.
- Anther: This is a four-lobed sac-like structure responsible for pollen formation.
- Filaments: These thread-like structures are attached to the anther and they keep the anther in place.
The transverse section of an anther is microsporangia that further forms a pollen sac. The pollen sac contains pollen grains.
Microsporangia and Microsporogenesis
Microsporogenesis involves the formation of microspores from microspore mother cells by meiotic division.
The structure of anther: The anther is the breeding portion of stamens. Each anther is connected by two lobes. Each anther lobe comprises two pollen chambers placed longitudinally. The microsporangium is filled with a number of pollen microspores.
Structure of Microsporangia
Microsporangia consist of:
- Tarentum: The cells are multinucleate and this layer is one of the innermost layers of the wall.
- Middle layer: It is composed of three to four layers of thin-walled cells. They are situated just below the endothelium.
- Endothecium: This layer is inner to the epidermis. Some cell develops cells of the endothelium.
- Epidermis: It is a protective outermost single-layer. Cells of the epidermis in Arceuthobium develop a fibrous thickening and the epidermis is designated as exothecium.
- Formation of Pollen grains: The young anther soon becomes four-lobed as it comprises of a homogenous mass of parenchymatous cells surrounded by the epidermis. Each archesporial initial present in hypodermal cells divides into the inner primary sporogenous cell and outer primary parietal cell. Each primary sporogenous cells divide to produce microsporocytes. Each microspore mother cell divides meiotically to form pollen grains.
- Microgametogenesis (Development of Male Gametophyte): Microspore is partially developed at this stage and it is the first cell of male gametophyte. The wall of the pollen grain is made up of exine ( outer layer) and inline ( inner layer). The outer layer is made up of sporopollenin and the inner layer is made up of pectose and cellulose.
- Pre-pollination development: The microspores start germinating in situ. A smaller generative cell lies next lying spore wall. A callose layer is placed around the generative cell. The callose layer then dissolves. Then the pollen grains are shed from the anther. The liberated pollen grains are transferred to the stigma through pollination. The pollen grain present on the stigma absorbs water and swells within a few minutes. The wall of pollen tube secretes hydrolytic enzymes and exogenous pectinases to create a passage for its entry. The generative and vegetative are carried by the pollen tube. The generative cell divides to form two non-motile male gametes. tube nucleus may disintegrate as there is no important function.
It is the female reproductive organ and the last whorl of the flower. It is composed of pistil and occupies the central position of the thalamus. The stigma, style, and ovary are the components of the pistil. The ovary produces ovules internally. Through meiosis, ovules produce megaspores which in turn develops into female gametophytes. As a result egg cells are produced.
Gynoecium can be:
Monocarpellary: The gynoecium consists of a single pistil. For eg., peas and beans.
Multicarpellary: Here, gynoecium comprises more than one pistil.
Syncarpous: It is the gynoecium with combined pistil. For eg., Tomato, cucumber.
Apocarpous: It is the gynoecium with free pistil. For eg., Lotus Vinca.
Pistil- Each pistil has three parts:
- Ovary: The ovary is a chamber where ovules (eggs) are stored, waiting for fertilization.
- Stigma: It is attached to the top of the carpel, where the pollen from other flowers lands.
- Style: It is a tubular structure that connects the ovary and the stigma. It is responsible for the transportation of pollen from the stigma to the ovary and holding the stigma in place.
Megasporangia and Megasporogenesis
The ovule is connected to the placenta with a stalk called funicle. It yields megasporocytes that form megaspores. Megasporogenesis is the process of arrangement of megaspores fro megaspore mother cell. The megaspore mother cell divides meiotically. One of the three megaspores is utilized while the other three degenerate. The functional megaspore forms the female gametophyte.
Also refer: Flower – Parts And Functions
Pollination is an ecological process carried out by all flowering plants. In this process, the matured pollen grains are transferred from the anther to the stigma for the purpose of sexual reproduction in flowering plants.
There are two types of pollination:
- Self-Pollination: This process occurs when the pollen grains from the anther is deposited on the stigma of the same flower, or another flower on the same plant.
- Cross-Pollination: This process occurs when the pollen grains are transferred from the anther of one flower into the stigma of another flower of different plants of the same species.
Also refer: Types of Pollination
After pollination, the pollens are transferred to the ovary through the pollen tube. After reaching the ovary, one of the male gametes mates with the ovule, or the female gamete (fertilization) and the other joins with the polar nuclei. The gamete fusing with the eggs produces a zygote, which eventually grows to be an embryo. The second gamete which fuses with the polar nuclei leads to the formation of endosperm nucleus, which looks after the nourishment of the embryo. In the end, the fertilized ovules turn into seeds and the ovary itself turns into fruit.
Explore more Fertilization in Plants
Functions of a Flower
- The main primary function of the flower is the reproduction of the individual and the species.
- They are the modifications of a shoot. Angiosperms of flowers are shaped variously to help diverse modes of pollination.
- Flowering plants are heterosporous. They mainly produce two types of spores. Megaspores are produced inside ovules and Microspores are produced by meiosis inside anthers.
- Flowers provide base germination of pollen, development of pollen tube, the formation of gametes and fertilization. The ovary part of the carpel gets transformed into fruit. The ovules are transformed into seeds after fertilization. The gametophytes also develop inside the spores in a case of heterosporous plants.
- The flowers lack one or the other reproductive organ and called unisexually or imperfect. The various modifications in ovaries help in the dispersal of fruits and seeds in some floral parts like a calyx.
Learn more about Flowering Plants, Sexual Reproduction in Flowering Plants, by referring to the reproduction in flowering plants notes @ BYJU’S Biology. Download BYJU’S app for further reference.
Frequently Asked Questions
Do all flowering plants show sexual reproduction?
Only angiosperms show sexual reproduction. The gymnosperms or non-flowering plants cannot undergo sexual reproduction because they have no reproductive organs.
How do flowers reproduce?
Flowers reproduce by the process of pollination. The pollen grains are transferred from the anther of the male flower to the stigma of the female flower, enabling fertilization resulting in the production of seeds.
What are the different stages of a plant life cycle?
The different stages of the plant life cycle include:
- Spreading of seeds
Which plants die after flowering?
Monocarpic flowers die after flowering. They flower, set seeds and fall. Alphonse De Candolle gave the term monocarpic flowers.
What is gregarious flowering?
Gregarious flowering is the most attractive pattern of flowering in bamboo. This occurs when all the flowers of a particular bamboo species bloom at once. This usually happens at an interval of 60-130 years.