Plant Life Cycle and Alternation of Generations

Alternation of Generations Definition

“Alternation of generations is a type of life cycle in which subsequent generations of plants alternate between diploid and haploid organisms.”

What is Alternation of Generations?

Alternation of generations is common in plants, algae, and fungi. This can be compared to the sexual reproduction in animals where both haploid and diploid cells are found in every generation.

Plants alternate between the diploid sporophyte and haploid gametophyte, and between asexual and sexual reproduction. Therefore, the life cycle of plants is known as alternation of generations. The ability of the plants to reproduce sexually and asexually helps them to adapt to different environments.

The alternation of generations depends upon the type of the plant. In Bryophytes, the dominant generation is haploid and the gametophyte comprises the main plant. In tracheophytes, the dominant generation is diploid and the sporophyte comprises the main plant.

The plants’ life cycle in one of the two generations is dominant over the other. The plants in the dominant generation grow larger and live longer. The plants in the non-dominant generations are small and hardly visible. On the contrary, the dominant generations are seen in the form of ferns, trees or other plants.

The dominant generation in vascular plants is the sporophyte, while in the non-vascular plants is the gametophyte.

Alternation of Generations – Life Cycle

Alternation of Generations

The alternation of generations include the following stages:

• The diploid sporophyte has a structure called sporangium.
• The sporangium undergoes meiosis and forms haploid spores.
• The spore develops into a gametophyte which is haploid in nature.
• The gametophyte has the reproductive organs which undergo mitosis to form haploid gametes.
• The gametes fertilize to form a haploid zygote which matures into a mature sporophyte. This cycle keeps repeating.

Stages of Alternation of Generations

Following are the two stages of alternation of generations:

Sporophyte Generation

Two haploid gametes fuse together to form a diploid zygote. This results in a sporophyte.

The sporophyte is formed by multiple rounds of mitosis and is a multicellular organism. On reaching maturity, the sporophyte develops reproductive organs known as sporangia. This is one key point in the alternation of generations.

These sporangia are used to create haploid spores. These spores are released and carried away by air and water and when the conditions are favourable they develop into a gametophyte.

Also Read: Gymnosperms

Gametophyte Generation

This is the next generation in the alternation of generations. The spore is newly formed and has half the DNA as the parent organism. This spore undergoes mitosis multiple times to form a gametophyte.

The gametophyte generation creates gametes. These gametes are produced by gametangia. These gametes are then transferred between plants or spread into the environment.

When a gamete encounters a gamete of the opposite sex, it fuses with it to form a zygote which eventually becomes a sporophyte.

This is the simplest version of alternation of generations. This is widely found in ferns.

Life Cycle Events in a Flowering Plant

A flowering plant undergoes the following events during its life cycle:

• Germination: A plant undergoes germination and begins to grow from seed. The roots are formed below the soil while the leaves, roots, and stem appear above the soil.
• Pollination: Pollens are carried by wind or insects to another flower. This is called pollination.
• Fertilization: The pollen travels to the ovary of the flower where the fusion of the male and gametes take place. This is called fertilization.
• Dispersal: The seeds are scattered by the wind and animals. Some of these seeds emerge into a new plant.

Thus we see how a plant life cycle begins with a seed. The seed sprouts to form a seedling. The seedling gets converted into a new plant which forms new seeds and the cycle continues.

Also Read: Sexual Reproduction in Plants

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