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Prothallus of the tree fern Dicksonia antarctica (note new moss plants for scale) Spore-bearing plants, like all plants, go through a life-cycle of alternation of generations. The fully grown sporophyte, what is commonly referred to as the fern, produces genetically unique spores in the sori by meiosis.
At maturity, a gametophyte produces gametes by mitosis, the normal process of cell division in eukaryotes, which maintains the original number of chromosomes. Two haploid gametes (originating from different organisms of the same species or from the same organism) fuse to produce a diploid zygote , which divides repeatedly by mitosis, developing ...
Three of these independent gametophyte cells degenerate and the one that remains is the gametophyte mother cell which normally contains one nucleus. [24] In general, it will then divide by mitosis until it consists of 8 nuclei separated into 1 egg cell, 3 antipodal cells , 2 synergid cells , and a central cell that contains two nuclei.
In ferns and seed plants (including cycads, conifers, flowering plants, etc.) the sporophyte is the dominant generation; the obvious visible plant, whether a small herb or a large tree, is the sporophyte, and the gametophyte is very small. In bryophytes and ferns, the gametophytes are independent, free-living plants, while in seed plants, each ...
The three cells left at the end of the cell near the micropylar become the egg apparatus with an egg cell in the center and two synergids. A cell wall forms around the other set of nuclei and forms the antipodals. The cells in the center develop into the central cell. This entire structure with its eight nuclei is called the embryonic sac.
A protonema (plural: protonemata) is a thread-like chain of cells that forms the earliest stage of development of the gametophyte (the haploid phase) in the life cycle of mosses. When a moss first grows from a spore , it starts as a germ tube , which lengthens and branches into a filamentous complex known as a protonema , from which a leafy ...
The male gametophyte gives rise to sperm cells, which are used for fertilization of an egg cell to form a zygote. Megaspores are structures that are part of the alternation of generations in many seedless vascular cryptogams , all gymnosperms and all angiosperms .
These groups are sometimes called "lower plants", referring to their status as the earliest plant groups to evolve, but the usage is imprecise since both groups are polyphyletic and may be used to include vascular cryptogams, such as the ferns and fern allies that reproduce using spores.