Sexual Reproduction in Flowering Plants

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.

Table of Contents

• Structure
• Reproductive Structure
  • Androecium
    • Microsporogenesis
  • Gynoecium
    • Megasporogenesis
• Pollination
• Fertilization
• Functions

Explore sexual reproduction in flowering plants notes to learn about the reproductive structure of the flower and the process of pollination.

A flower comprises 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, at some other instances, they may be a colour like petals. This state 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.

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It is the male reproductive part of a flower, comprising 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.

Microsporogenesis involves the formation of microspores from microspore mother cells by meiotic division.

The structure of anther: The anther is the breeding part of stamens. Each anther is connected by two lobes. Each anther lobe comprises two pollen chambers located longitudinally. The microsporangium is sufficiently supplied with a number of pollen microspores.

Structure of Microsporangia

Microsporangia consist of:

1. Tapetum: The cells are multinucleate and this layer is one of the innermost layers of the wall.
2. Middle layer: It is composed of three to four layers of thin-walled cells. They are located just beneath the endothelium.
3. Endothecium: This layer is inner to the epidermis. Some cell develops cells of the endothelium.
4. 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 a homogenous mass of parenchymatous cells engirdled by the epidermis. Each archesporial initially present in hypodermal cells divides into the inner primary sporogenous cell and outer primary parietal cell. Each primary sporogenous cell splits to produce microsporocytes. Each microspore mother cell meiotically divides to form pollen grains.
• Microgametogenesis (Development of Male Gametophyte): Microspore is partially developed at this stage and it is the first cell of the male gametophyte. The wall of the pollen grain comprises exine (outer layer) and inline (inner layer). The outer layer comprises sporopollenin and the inner layer is made up of pectose and cellulose.
• Pre-pollination development: The microspores begin to germinate in situ. A smaller generative cell lies next to the spore wall. A callose layer is placed around the generative cell. Then, the callose layer dissolves. The anther then sheds the pollen grains. The released pollen grains are passed to the stigma through pollination. The pollen grain present on the stigma absorbs water and within some minutes, swells up. The wall of pollen tube secretes hydrolytic enzymes and exogenous pectinases creating a passage to enter. The generative and vegetative cells are supplied by the pollen tube. The generative cell splits forming 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.

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 of the megaspore mother cell. The megaspore mother cell divides meiotically. One of the three megaspores is utilized while the others 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 uniting with the eggs results in the production of a zygote, that gradually develops to be an embryo. The second gamete that unites with the polar nuclei results in the formation of endosperm nucleus. It supplies nourishment to the embryo. The fertilization ultimately turns ovules into seeds while the ovary develops into the fruit.

Explore more Fertilization in Plants

• 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 differently, helping 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 development of the pollen tube, base germination of pollen, the formation of gametes and fertilization. The ovarian portion of the carpel is supplied into the fruit. The ovules are converted into seeds after fertilization. The gametophytes also develop inside the spores in case of heterosporous plants.
• The flowers that lack one or the other reproductive organ are called unisexual or imperfect. In ovaries, the different modification helps in the dispersal of seeds and fruits 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.