A haploid moss spore germinates and gives rise to a protonema, a green multicellular tissue that superficially resembles a filamentous green alga. Under appropriate conditions, other moss cells can also be induced to form protonema. The protonema is typically subterranean and is rarely seen.
Buds form on the protonema producing the familiar "leafy" moss plant. Male and female reproductive organs typically grow near the tips of the "leafy" gametophyte. These are termed antheridia and archegonia, respectively. Most species are monoecious, in that antheridia and archegonia are on the same individual. Other species are dioecious in that antheridia and archegonia are on different individuals.
The archegonium is multicellular and produces a single immobile egg. The antheridium is also multicellular but it makes many motile sperm cells each with two flagella. A sperm cell swims through water to reach the archegonium. Then it travels down a tube in the archegonium to fertilize the egg and form a diploid zygote.
The zygote undergoes repeated cell division and elongates to become a multicellular sporophyte while still attached to the 'leafy' gametophyte. The thin stalk of the sporophyte is called the seta and the enlarged tip is called the capsule. The moss sporophyte is photosynthetic early in development, but later depends on the gametophyte for nutrition. Late in development, the sporophyte dries out and turns brown. Then the operculum (lid) comes off the capsule and haploid spores are released to the environment. The subclass of true mosses (Bryideae) are unique in having special peristome teeth inside the cap, which regulate spore dispersal.
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