Plant Life Cycle

What is the Alternation of Generations?

Alternation of generations can be defined as a type of life cycle in which a number of generations of plants differentiate between diploid and haploid organisms. Alternation of generations is a common factor 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. Because of this reason, the life cycle of plants is termed as an 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 plant. The dominant generation is haploid and the gametophyte comprises the main plant in Bryophytes. In tracheophytes, the generation that dominates is diploid and the sporophyte has the main plant. 

The plants’ life cycle in one of the two generations is dominant over the other. The plants grow larger and live longer in the dominant generation whereas the plants in the non-dominant generations are small and hardly visible. On the other hand, 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

The alternation of generations include the following stages:

• The diploid sporophyte has a structure called a 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

When two haploid gametes fuse together they 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.

• Gametophyte Generation

This is the next generation in the alternation of generations. In this, 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 put 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 the 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 takes place. This is called fertilization.

• Dispersal: The seeds are scattered by the wind and animals. Some of these seeds emerge into a new plant.

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