Science: Reproduction: Modes and Sexual Reproduction in Plants (For CBSE, ICSE, IAS, NET, NRA 2022)

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Reproduction may be defined as the biological process by which organisms give rise to their own kind.

Reproduction may occur in two ways:

1. Asexual reproduction

2. Sexual reproduction

  • Bacterial and protozoan offspring may be produced by single individuals. This is termed asexual reproduction.
  • Certain animals and many plants reproduce asexually.
  • When two individuals are involved in reproduction, it is termed sexual reproduction. In sexual reproduction, male gamete fuses with female gamete to mark the beginning of a new individual. This is a more common mode of reproduction in plants and animals.

Modes of Asexual Reproduction

Organisms choose to reproduce asexually by different means. Some of the asexual methods are binary fission, bacteria) , budding (e. g. Hydra) , fragmentation (e. g. Planaria) , spore formation (e. g. ferns) (e. g. Onion)

Fission

  • Fission means division. During asexual reproduction, the parent cell divides into two or more cells. cellular organisms show different patterns of cell division according to their cell structure. For example, amoeba can divide itself into two at any plane but the division in euglena is longitudinal.
  • Fission can be of two types, namely, binary fission and multiple fission. In binary fission, parent cell divides into two equal halves called daughter cells. Daughter cells are identical to each other and to their parent cell. Organisms like the amoeba, bacteria, euglena, etc. exhibit binary fission.
Asexual Reproduction: Fission

During multiple fission, organism divides itself into numerous daughter cells. Examples of multiple fission are sporozoans and algae.

Fragmentation

Fragmentation is another mode of asexual reproduction. Multi-cellular organisms like planaria, spirogyra, etc. reproduce by fragmentation. The parent body divides into two or more fragments. Later, each fragment develops into a new individual.

Regeneration

When a lizard loses its tail, it grows a new one. This is known as regeneration. In many organisms, there are specialized cells, which can differentiate and grow into a new organism. Organisms like hydra and planaria also show regeneration. In these organisms, when the cell divides into numerous pieces, each piece proliferates and differentiates to regenerate new organisms.

Budding

Some organisms develop buds on their body. These buds develop into a new individual. This is known as budding. An example is a hydra. From the parent hydra, a bud arises which eventually matures into a new hydra. Once it gets mature, it detaches from the parent body.

Vegetative Propagation

Plants reproduce asexually through their vegetative parts such as leaves, and buds. This is known as vegetative propagation. For example, onion bulbs, tubers of potato, runners/stolon, etc. vegetative propagation is much faster than the sexual reproduction in plants. This can be done artificially as well, which is widely employed in horticulture.

Sexual Reproduction in Plants

Self-Pollination and Cross-Pollination

  • In angiosperms, pollination is defined as the placement or transfer of pollen from the anther to the stigma of the same flower or another flower. In gymnosperms, pollination involves pollen transfer from the male cone to the female cone.
  • Upon transfer, the pollen germinates to form the pollen tube and the sperm for fertilizing the egg. Pollination has been well studied since the time of Gregor Mendel. Mendel successfully carried out self- as well as cross-pollination in garden peas while studying how characteristics were passed on from one generation to the next.
  • Today՚s crops are a result of plant breeding, which employs artificial selection to produce the present-day cultivars. A case in point is today՚s corn, which is a result of years of breeding that started with its ancestor, teosinte. The teosinte that the ancient Mayans originally began cultivating had tiny seeds — vastly different from today՚s relatively giant ears of corn. Interestingly, though these two plants appear to be entirely different, the genetic difference between them is miniscule.

Pollination takes two forms:

  • self-pollination and
  • cross-pollination.

Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination occurs in flowers where the stamen and carpel mature at the same time and are positioned so that the pollen can land on the flower՚s stigma. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.

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