Gametophyte
Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte.
[2] No extant gametophytes have stomata, but they have been found on fossil species like the early Devonian Aglaophyton from the Rhynie chert.
[3] Other fossil gametophytes found in the Rhynie chert shows they were much more developed than present forms, resembling the sporophyte in having a well-developed conducting strand, a cortex, an epidermis and a cuticle with stomata, but were much smaller.
[4] In bryophytes (mosses, liverworts, and hornworts), the gametophyte is the most visible stage of the life cycle.
[6] In some bryophyte groups such as many liverworts of the order Marchantiales, the gametes are produced on specialized structures called gametophores (or gametangiophores).
Exosporic gametophytes can either be bisexual, capable of producing both sperm and eggs in the same thallus (monoicous), or specialized into separate male and female organisms (dioicous).
However, in some groups, notably the clade that includes Ophioglossaceae and Psilotaceae, the gametophytes are subterranean and subsist by forming mycotrophic relationships with fungi.
In the homosporous families Lycopodiaceae and Huperziaceae, spores germinate into bisexual free-living, subterranean and mycotrophic gametophytes that derive nutrients from symbiosis with fungi.
In Isoetes and Selaginella, which are heterosporous, microspores and megaspores are dispersed from sporangia either passively or by active ejection.
At maturity, the megaspore cracks open at the trilete suture to allow the male gametes to access the egg cells in the archegonia inside.
The gametophytes of Isoetes appear to be similar in this respect to those of the extinct Carboniferous arborescent lycophytes Lepidodendron and Lepidostrobus.
[9] The seed plant gametophyte life cycle is even more reduced than in basal taxa (ferns and lycophytes).
Seed plant gametophytes are not independent organisms and depend upon the dominant sporophyte tissue for nutrients and water.
During its development, the water and nutrients that the male gametophyte requires are provided by the sporophyte tissue until they are released for pollination.
However, the female gametophytes of Ginkgo biloba do contain chlorophyll and can produce some of their own energy, though, not enough to support itself without being supplemented by the sporophyte.
[15] The female gametophyte forms from a diploid megaspore that undergoes meiosis and starts being singled celled.
[14] The precursor to the male angiosperm gametophyte is a diploid microspore mother cell located inside the anther.
[19] The development of the three celled male gametophyte prior to dehiscing has evolved multiple times and is present in about a third of angiosperm species allowing for faster fertilization after pollination.
Some botanists consider this endospore as gametophyte tissue with typically 2/3 being female and 1/3 being male, but as the central cell before double fertilization can range from 1n to 8n in special cases, the fertilized central cells range from 2n (50% male/female) to 9n (1/9 male, 8/9th female).
Seed plant microgametophytes consists of several (typically two to five) cells when the pollen grains exit the sporangium.
In seed plants, the microgametophyte (pollen) travels to the vicinity of the egg cell (carried by a physical or animal vector) and produces two sperm by mitosis.
