Alternation of Generations
The cells of most plants and animals have two sets of
chromosomes and only
their reproductive cells (gametes) have just one set.
In Bryophytes the
main plant is the gametophyte stage and its cells have
only one set of
chromosomes. When a Moss sperm fertilises an egg, the
number of chromosomes
is doubled and cells of the sporophyte (capsule and
seta) have two sets.
When spores are formed, the number of chromosomes is
halved again. So
Bryophytes show a clear alternation of gametophyte and
sporophyte
generations that is not obvious in most higher plants,
in which the
gametophyte generation is greatly reduced.
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