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    Alternation of generations

    Alternation of generations is the type of life cycle in plants that alternates between the diploid and haploid stages. Discover in detail different aspects of alternation of generations.

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    Plant Life Cycle and Alternation of Generations

    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

    For more information on the alternation of generations and other related topics, keep visiting BYJU’S website or download BYJU’S app for further reference.

    Related Links:-

    Types of Pollination Difference between pollination and fertilization

    Asexual Reproduction in Plants Seed Germination

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    BIOLOGY Related Links

    Human Anatomy And Physiology Cell Division

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    Plant Life Cycles

    The plant life cycle alternates between haploid and diploid generations. Embryonic development is seen only in the diploid generation. The embryo, however, is produced by the fusion of gametes, which are formed only by the haploid generation. So understanding the relationship between the two generations is important in the study of plant development.


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    Developmental Biology. 6th edition.

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    Plant Life Cycles

    The plant life cycle alternates between haploid and diploid generations. Embryonic development is seen only in the diploid generation. The embryo, however, is produced by the fusion of gametes, which are formed only by the haploid generation. So understanding the relationship between the two generations is important in the study of plant development.

    Unlike animals(see Chapter 2), plants have multicellular haploid and multicellular diploid stages in their life cycle. Gametes develop in the multicellular haploid gametophyte (from the Greek “plant”). Fertilization gives rise to a multicellular diploid sporophyte, which produces haploid spores via meiosis. This type of life cycle is called a haplodiplontic life cycle (Figure 20.1). It differs from our own diplontic life cycle, in which only the gametes are in the haploid state. In haplodiplontic life cycles, gametes are not the direct result of a meiotic division. Diploid sporophyte cells undergo meiosis to produce haploid spores. Each spore goes through mitotic divisions to yield a multicellular, haploid gametophyte. Mitotic divisions within the gametophyte are required to produce the gametes. The diploid sporophyte results from the fusion of two gametes. Among the Plantae, the gametophytes and sporophytes of a species have distinct morphologies (in some algae they look alike). How a single genome can be used to create two unique morphologies is an intriguing puzzle.

    Figure 20.1

    Plants have haplodiplontic life cycles that involve mitotic divisions (resulting in multicellularity) in both the haploid and diploid generations (paths A and D). Most animals are diplontic and undergo mitosis only in the diploid generation (paths B and (more...)

    All plants alternate generations. There is an evolutionary trend from sporophytes that are nutritionally dependent on autotrophic (self-feeding) gametophytes to the opposite‐gametophytes that are dependent on autotrophic sporophytes. This trend is exemplified by comparing the life cycles of a moss, a fern, and an angiosperm (see Figures 20.2– 20.4). (Gymnosperm life cycles bear many similarities to those of angiosperms; the distinctions will be explored in the context of angiosperm development.)

    Figure 20.2

    Life cycle of a moss (genus ). The sporophyte generation is dependent on the photosynthetic gametophyte for nutrition. Cells within the sporangium of the sporophyte undergo meiosis to produce male and female spores, respectively. These spores (more...)

    Figure 20.4

    Life cycle of an angiosperm, represented here by a pea plant (genus ). The sporophyte is the dominant generation, but multicellular male and female gametophytes are produced within the flowers of the sporophyte. Cells of the microsporangium within (more...)

    The “leafy” moss you walk on in the woods is the gametophyte generation of that plant (Figure 20.2). Mosses are heterosporous, which means they make two distinct types of spores; these develop into male and female gametophytes. Male gametophytes develop reproductive structures called antheridia (singular, antheridium) that produce sperm by mitosis. Female gametophytes develop archegonia (singular, archegonium) that produce eggs by mitosis. Sperm travel to a neighboring plant via a water droplet, are chemically attracted to the entrance of the archegonium, and fertilization results.* The embryonic sporophyte develops within the archegonium, and the mature sporophyte stays attached to the gametophyte. The sporophyte is not photosynthetic. Thus both the embryo and the mature sporophyte are nourished by the gametophyte. Meiosis within the capsule of the sporophyte yields haploid spores that are released and eventually germinate to form a male or female gametophyte.

    Ferns follow a pattern of development similar to that of mosses, although most (but not all) ferns are homosporous. That is, the sporophyte produces only one type of spore within a structure called the sporangium (Figure 20.3). One gametophyte can produce both male and female sex organs. The greatest contrast between the mosses and the ferns is that both the gametophyte and the sporophyte of the fern photosynthesize and are thus autotrophic; the shift to a dominant sporophyte generation is taking place.†

    Figure 20.3

    Life cycle of a fern (genus ). The sporophyte generation is photosynthetic and is independent of the gametophyte. The sporangia are protected by a layer of cells called the indusium. This entire structure is called a sorus. Meiosis within the (more...)

    At first glance, angiosperms may appear to have a diplontic life cycle because the gametophyte generation has been reduced to just a few cells (Figure 20.4). However, mitotic division still follows meiosis in the sporophyte, resulting in a multicellular gametophyte, which produces eggs or sperm. All of this takes place in the the organ that characterizes the angiosperms: the flower. Male and female gametophytes have distinct morphologies (i.e., angiosperms are heterosporous), but the gametes they produce no longer rely on water for fertilization. Rather, wind or members of the animal kingdom deliver the male gametophyte—pollen—to the female gametophyte. Another evolutionary innovation is the production of a seed coat, which adds an extra layer of protection around the embryo. The seed coat is also found in the gymnosperms. A further protective layer, the fruit, is unique to the angiosperms and aids in the dispersal of the enclosed embryos by wind or animals.

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    The Life Cycle of Plants: Alternation of Generations

    The Life Cycle of Plants quizzes about important details and events in every section of the book.

    The Life Cycle of Plants

    The Life Cycle of Plants Biology

    Study Guide Summary

    Alternation of Generations

    Summary Alternation of Generations

    All plants undergo a life cycle that takes them through both haploid and diploid generations. The multicellular diploid plant structure is called the sporophyte, which produces spores through meiotic (asexual) division. The multicellular haploid plant structure is called the gametophyte, which is formed from the spore and give rise to the haploid gametes. The fluctuation between these diploid and haploid stages that occurs in plants is called the alternation of generations. The way in which the alternation of generations occurs in plants depends on the type of plant. In bryophytes (mosses and liverworts), the dominant generation is haploid, so that the gametophyte comprises what we think of as the main plant. The opposite is true for tracheophytes (vascular plants), in which the diploid generation is dominant and the sporophyte comprises the main plant.

    Bryophyte Generations

    Bryophytes are nonvascularized plants that are still dependent on a moist environment for survival (see Plant Classification, Bryophytes . Like all plants, the bryophyte life cycle goes through both haploid (gametophyte) and diploid (sporophyte) stages. The gametophyte comprises the main plant (the green moss or liverwort), while the diploid sporophyte is much smaller and is attached to the gametophyte. The haploid stage, in which a multicellular haploid gametophyte develops from a spore and produces haploid gametes, is the dominant stage in the bryophyte life cycle. The mature gametophyte produces both male and female gametes, which join to form a diploid zygote. The zygote develops into the diploid sporophyte, which extends from the gametophyte and produces haploid spores through meiosis. Once the spores germinate, they produce new gametophyte plants and the cycle continues.

    Tracheophyte Generations

    Tracheophytes are plants that contain vascular tissue; two of the major classes of tracheophytes are gymnosperms (conifers) and angiosperms (flowering plants). Tracheophytes, unlike bryophytes, have developed seeds that encase and protect their embryos. The dominant phase in the tracheophyte life cycle is the diploid (sporophyte) stage. The gametophytes are very small and cannot exist independent of the parent plant. The reproductive structures of the sporophyte (cones in gymnosperms and flowers in angiosperms), produce two different kinds of haploid spores: microspores (male) and megaspores (female). This phenomenon of sexually differentiated spores is called heterospory. These spores give rise to similarly sexually differentiated gametophytes, which in turn produce gametes. Fertilization occurs when a male and female gamete join to form a zygote. The resulting embryo, encased in a seed coating, will eventually become a new sporophyte.

    Figure %: Gymnosperm Life Cycle

    Figure %: Angiosperm Life Cycle

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