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 the 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.
Structure of flower
A flower consists of the following parts-
Calyx: It is the outermost whorl of a flower. It comprises of units called sepals. In the bud stage calyx, encloses the rest of the flower. They are usually green in color in some cases they may be a color like petals. This condition of Calyx is termed as petaloid. Calyx can either be prominent or absent.
Corolla: It consists of a many numbers of petals and it is the second whorl of the flower. These petals are sometimes fragrant. They are colored, thin and soft that would help in the process of pollination as they would attract animals and insects.
Androecium: It is the male reproductive organ consisting of stamens and it is the third whorl. Each stamen comprises of two parts namely anther and filament. The filament supports anther at its tip. 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.
Gynoecium: It is the female reproductive organ and the last whorl of the flower. It is composed of carpels and it occupies the central position of the thalamus. The stigma, style, and ovary form the components of the carpels. Ovary produces ovules internally. Through meiosis, ovules produce megaspores which in turn develops into female gametophytes. As a result egg cells are produced.
- 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 pollens from the stigma to the ovary and holding the stigma in place.
Pollination is the process where mature pollens are transferred to the stigma for the purpose of sexual reproduction in flowering plants. It can happen in two ways-
- Cross-Pollination: If pollens from the flower of one plant lands on the stigma of another flower of a different plant, it is called Cross-pollination or Birds. Bees, insect and the wind are some of the agents of cross-pollination.
- Self-Pollination: If the pollen of a flower is transferred to its own ovary, it is known as Self-pollination or fertilization.
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.
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 a 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 calyx.
Microsporogenesis involves the formation and differentiation of microspores from microspore mother cells by reductional division. It is well studied in following cases.
- The structure of anther: The anther is the fertile portion of stamens. Each anther is connected by two lobes. Each anther lobe comprises of two pollen chambers. They are usually placed longitudinally. Each pollen chamber is filled with a large number of pollen microspores.
The pollen sacs are surrounded by four layers. They are stated below:
- 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 thin-walled cells. They are situated just below the endothelium.
- Endothecium: This layer is inner to the epidermis. Some cell develops cells of endothecium.
- 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.
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