In order to continue enjoying our site, we ask that you confirm your identity as a human. Thank you very much for your cooperation. Show “Alternation of generations is a type of life cycle in which subsequent generations of plants alternate between diploid and haploid organisms.” 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 The alternation of generations include the following stages: Following are the two stages of alternation of generations: 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 GenerationThis 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 PlantA flowering plant undergoes the following events during its life cycle:
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:-
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Alternation of generations is a type of life cycle found in terrestrial plants and some algae in which subsequent generations of individuals alternate between haploid and diploid organisms. This can be contrasted to sexual reproduction in animals, in which both haploid and diploid cells are found in every generation. Alternation of generations has several distinct features, and these features can be slightly modified between species. In general, the generations alternate between the sporophytes capable of creating spores and the gametophytes, capable of creating gametes. To form a sporophyte, two haploid gametes come together to form a diploid zygote. Typically, haploid organisms are defined by having an “n” number of chromosomes. When two gametes of the same species come together, they each have n chromosomes. Therefore, the diploid zygote which forms is considered to have 2n worth of genetic material, or exactly twice as much. Not only is there twice as much DNA, but it represents codes for the same proteins in the same organism. The sporophyte is a multicellular organism formed from multiple rounds of mitosis on the zygote. Thus, the sporophyte individual remains a 2n organism. Then, when the sporophyte reaches maturity, a key point in the alternation of generations takes place. The sporophyte develops organs, known as sporangia. These specialized reproductive organs are used to create single-celled, haploid spores. These cells will be released into the air or water and carried away. When they reach a suitable environment, they will begin the process of developing into the gametophyte. This represents the next generation in the alternation of generations, as the haploid spore is created. The spore is technically a new organism, and has only half the DNA as the parent organism. This spore will undergo successive rounds of mitosis to form a new multicellular individual, the gametophyte. Where the sporophyte generation creates spores, the gametophyte generation creates gametes. Gametes are produced by special organs on the gametophyte, the gametangia. These gametes are then broadcast into the environment, or transferred between plants. When they find an opposite gamete, they begin the process of fusing to form another zygote. This zygote will eventually become a sporophyte, and the alternation of generations will keep turning. While this is a simplistic version of the alternation of generations, there are many complexities, which will be discussed below. Because of these complexities, and because all plants undergo some version of alternation of generations, scientists prefer to refer to other aspects of their reproductive cycles to define the species. The simplest form of alternations of generations is found in the fern, as seen below. As seen in the diagram, the gametophyte and sporophyte are clearly different individuals. This is not always the case. There are many additional factors which can complicate this basic theme of alternation of generations. The most notable complication is gender. Both sporophytes and gametophytes can have genders. Further, the sporophyte or gametophyte may not be totally independent of the other generation. Consider flowering plants for example. Some flowers have both male and female gametophytes housed within. In other flower species, the male and female flowers are separated, but may be present on the same plant. In still other flowers, individual sporophytes only give rise to a certain gender of flower. The terms monoicous and dioicous are used to describe the gametophytes. If both genders are housed on the same individual, the species is monoicous. If they are separated, the alternation of generations is dioicous. This applies only to the gametophyte. In the sporophyte, the terms used are monoecious and dioecious. Here, these describe whether the sporophyte will produce both genders, or if the genders are separated into different sporophytes. It is important to note that many plants are monoecious, but dioicous. This means that a single sporophyte can produce both male and female flowers. Many other combinations of these differences in the alternation of generations can be observed in various plant and algae species. During the alternation of generations in some species, there is an unequal distribution in size or dominance of either the sporophyte or the gametophyte. In different species, this can be reversed. In liverworts the gametophyte is the dominant generation. Liverwort is a gametophytic plant. In ferns, the opposite is true and the sporophyte is the larger, more dominant individual. This would be called a sporophytic plant. In other species, the size and shape of the sporophyte and gametophyte is nearly indistinguishable. The only difference is the amount of DNA they carry and whether they will produce spores or gametes. While this may seem like they aren’t really doing anything, alternation of generations still allows sexual reproduction to occur. This greatly mixes the genes and increases the adaptability of the species. Sporophytes and gametophytes from different species also produce different types of spores and gametes. If the gametes are the same, it is called isogamy. Some species of green algae produce only one type of gamete. These are both flagellated and swim in open water until they find each other. This scheme is not particularly useful for terrestrial plants. Most terrestrial plants show anisogamy, or a difference in their gametes. This is most commonly seen as the difference between the mobile male gamete and the larger, immobile female gamete, or sperm and oocytes. Other times, it is seen as a size difference only. Below is a graphic representation of a dioicous, dioecious, anisogametic plant. This means that the plant has separate individuals of both gametophytes and sporophytes, and that the gametes it produces are of different sizes. This would be true of a holly or willow tree, which produces separate male and female flowers, on separate individual sporophytes. Fossil evidence, and the existence of many algae with simplified alternation of generation life cycles, hints that the evolutionary advantage of sexual reproduction through the alternation of generations is what made terrestrial plants adaptable enough to colonize the land. The alternation of generations allows for both the dynamic and volatile act of sexual reproduction and the steady and consistent act of asexual reproduction. When the sporophyte creates spores, the cells undergo meiosis, which allows the gametophyte generation to recombine the genetics present. This allows for great diversity to arise. As plants colonized the land, they were initially isomorphic, or both the gametophytes and sporophytes looked and acted about the same. As time progressed, most plants found it beneficial to reduce one of these life cycles. Most flowering plants now have a much reduced gametophyte life cycle, while liverworts and mosses went the other way, preferring to diminish the sporophyte cycle. In the wide variety of plants alive today there is almost every variation of the alternation of generations conceivable. 1. What is one advantage of the alteration of generations over an asexual species? A is correct. While it is much harder to reproduce, the alternation of generations does allow genes to be recombined and tested in a way few other reproduction systems allow. 2. What is the difference between the alternation of generations seen in plants and the sexual reproduction seen in mammals? B is correct. Mammals are the same every generation, and do not vary between haploid and diploid states. Some animals, like honeybees, do very between haploid and diploid, but this determines their gender within a generation and does not change over generations. In plants, this line becomes blurred as the gametophyte became dependent on the sporophyte, as seen in many flowering plants. 3. Is it better to have a more dominant sporophyte or a more dominant gametophyte? C is correct. The dominance of a particular generation depends entirely on the successes and failures of the past generations of a species. If the sporophyte tends to be weaker, or is predated more, it will become a less prominent feature in the reproductive cycle. The same is true of the gametophyte. Each species has formed a life cycle which has allowed it to survive to the present, so all are essentially equal. References
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The alternation of generations in the life cycle of angiosperms refers to the change between
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