The life cycles of higher plants are the alternation of two generations of organisms: the haploid multicellular gametophyte and the diploid sporophyte. Gametophytes develop the genitals of gametangia, in which gametes are formed by mitosis.
While merging, the latter gives rise to the diploid generation of organisms. At the sporophyte stage, haploid spores are formed by meiosis, from which the gametophyte is formed again. At the same time, the haploid stage clearly dominates in the life cycle of the most simply organized higher-living plants – mosses.
Complex animal life cycles are the alternation of generations of diploid organisms. Unlike plants and fungi, in which the haploid phase is usually represented by a separate generation of organisms, in animals, it is reduced to the stage of gametes. In this case, germ cells are formed by meiosis, which is called gamete.
Therefore, the complex life cycles of animals can be associated only with the alternation of generations of diploid organisms, which differ from each other only in the way of reproduction. It should be borne in mind that most animal species have a simple life cycle that begins with the zygote and ends with sexual reproduction. Only the primitive and often parasitic representatives of this kingdom have preserved complex life cycles.
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