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, phase of plant life cycles in which the gametes, i.e., egg and sperm, are produced. The gametophyte is haploid, that is, each cell contains a single complete set of chromosomes, and arises from the germination of a haploid spore.
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an asexual generation in plants which is formed from a zygote and in which alternation of generations is part of the developmental cycle. Sporophytes develop sporangia, which bear spores by means of meiosis. The sporophyte alternates with the gametophyte—the sexual generation—which is formed from a spore and which forms gametes. The merging of gametes produces a zygote. The cells of the sporophyte and those of the zygote have a diploid set of chromosomes, whereas the cells of the gametophyte have a haploid set.
The sporophyte represents different stages in the developmental cycles of different plants. In gymnosperms and angiosperms it plays a dominant role, ensuring the development of greatly reduced gametophytes (male gametophytes are pollen grains; female ones are the primary endosperm and the embryo sac). It also dominates in ferns and laminarians, whose gametophytes (prothallia) exist separately from the sporophytes. The sporophyte occupies a subordinate position in mosses, where it is represented by a sporogonium on the gametophyte, and in a few brown algae, where it exists independently.
In plants with isomorphic cycles of development, such as some green and brown algae, independently existing sporophytes and gametophytes are marked by a morphologically similar development. In a majority of Florideae (red algae) the sporophyte seems to have two phases of development: the first, the carpospo-rophyte, develops on the gametophyte; the second lives independently and develops similarly to the gametophyte. Hetero-morphic cycles of development, with a sporophyte that produces numerous spores, are especially characteristic of all higher plants except mosses. The establishment in plants of such cycles of development is due to the emergence of algal ancestors onto dryland, where reproduction by spores was more advantageous. The transition from isospory to heterospory led to the development of seeds and to a more perfect type of reproduction in plants—seed reproduction.
A. N. SLADKOV