Fungi and protists, including algae, exhibit this type of life cycle. Plants and some animals are capable of reproducing both asexually and sexually. In asexual reproduction , the offspring are an exact duplicate of the parent. Types of asexual reproduction commonly seen in both plants and animals include parthenogenesis offspring develops from an unfertilized egg , budding offspring develops as a growth on the parent's body , and fragmentation offspring develops from a part or fragment of the parent.
Sexual reproduction involves the uniting of haploid cells cells containing only one set of chromosomes to form a diploid containing two chromosome sets organism. In multicellular animals , the life cycle consists of a single generation. The diploid organism produces haploid sex cells by meiosis. All other cells of the body are diploid and produced by mitosis. A new diploid organism is created by the fusion of male and female sex cells during fertilization.
The organism is diploid and there is no alternation of generations between haploid and diploid phases. In plant multicellular organisms , life cycles vacillate between diploid and haploid generations. In the cycle, the diploid sporophyte phase produces haploid spores via meiosis. As haploid spores grow by mitosis, the multiplied cells form a haploid gametophyte structure. The gametophyte represents the haploid phase of the cycle.
Once mature, the gametophyte produces male and female gametes. When haploid gametes unite, they form a diploid zygote. The zygote grows via mitosis to form a new diploid sporophyte. Thus unlike in animals , plant organisms can alternate between diploid sporophyte and haploid gametophyte phases. Alternation of generations is seen in both vascular and non-vascular plants.
Vascular plants contain a vascular tissue system that transports water and nutrients throughout the plant. Non-vascular plants do not have this type of system and require moist habitats for survival. Non-vascular plants include mosses, liverworts, and hornworts. These plants appear as green mats of vegetation with stalks protruding from them. The primary phase of the plant life cycle for non-vascular plants is the gametophyte generation.
The gametophyte phase consists of green mossy vegetation, while the sporophyte phase consists of elongated stalks with a sporangium tip that encloses the spores. The primary phase of the plant life cycle for vascular plants is the sporophyte generation. In vascular plants that do not produce seeds, such as ferns and horsetails, the sporophyte and gametophyte generations are independent.
In ferns, the leafy fronds represent the mature diploid sporophyte generation. The sporangia on the undersides of the fronds produce the haploid spores, which germinate to form the haploid fern gametophytes prothallia.
These plants thrive in damp environments as water is required for the male sperm to swim toward and fertilize the female egg. Vascular plants that produce seeds are not necessarily dependent upon moist environments to reproduce. The seeds protect the developing embryos. In both flowering plants and nonflowering plants gymnosperms , the gametophyte generation is totally dependent upon the dominant sporophyte generation for survival.
In flowering plants, the reproductive structure is the flower. The flower produces both male microspores and female megaspores. The male microspores are contained within pollen and are produced in the plant stamen. They develop into the male gametes or sperm. The female megaspores are produced in the plant ovary. They develop into female gametes or eggs.
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. 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. 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.
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