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They have exosporic gametophytes — that is, the gametophyte is free-living and develops outside of the spore wall. Exosporic gametophytes can either be bisexual, capable of producing both sperm and eggs in the same thallus , or specialized into separate male and female organisms (dioicous).
It is the biological process of gametogenesis during which cells that are haploid or diploid divide to create other cells. It can take place either through mitotic or meiotic division of diploid gametocytes into different cells depending on an organism's biological life cycle. For instance, gametophytes in plants undergo mitosis to produce gametes.
However, the parent sporophyte may be monoecious, producing both male and female gametophytes or dioecious, producing gametophytes of one gender only. Seed plant gametophytes are extremely reduced in size; the archegonium consists only of a small number of cells, and the entire male gametophyte may be represented by only two cells. [27]
These spores then germinate and divide by mitosis to form a haploid multicellular phase, the gametophyte, which produces gametes directly by mitosis. This type of life cycle, involving alternation between two multicellular phases, the sexual haploid gametophyte and asexual diploid sporophyte, is known as alternation of generations .
The resulting meiospores develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they only have one set of chromosomes. The mature gametophyte produces male or female gametes (or both) by mitosis. The fusion of male and female gametes produces a diploid zygote which develops into a new sporophyte.
The gametophytes produce both motile sperm in the antheridia and egg cells in separate archegonia. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the antheridia where they fertilize the egg.
The sporophyte creates spores via meiosis which also then divide mitotically producing haploid individuals called gametophytes. The gametophytes produce gametes via mitosis. In some plants the gametophyte is not only small-sized but also short-lived; in other plants and many algae, the gametophyte is the "dominant" stage of the life cycle. [19]
The tops of the gametophytes (2) can be discerned as well. Inset shows the surrounding, black poplars growing on sandy loam on the bank of a kolk , with the detail area marked. In biology , a spore is a unit of sexual (in fungi) or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in ...