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What we see and know as moss is the
gametophyte form of the moss plant.
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Understanding the alternation of generations
The way that almost all land plants reproduce is by means of two
distinct, alternating life forms, a sexual
phase that produces and releases gametes or sex cells and allows fertilisation,
and a dispersal phase – both of
which are adaptations to an essentially waterless environment. The sexual phase is known as the GAMETOPHYTE or haploid (n) generation and the dispersal phase is the SPOROPHYTE or diploid (2n) generation.
Mature gametophyte plants produce
haploid sex cells (egg and sperm) in sex organs (the male antheridia and female
archegonia). These sex cells (also called gametes) fuse during fertilisation to
form a diploid (2n) zygote which grows, by means of mitosis (that
results in two daughter cells each having the same number and kind of
chromosomes as the parent cell), into a new sporophyte
plant.
The diploid sporophyte produces haploid (n) spores (i.e. each spore has
a single set of chromosomes) by means of the process of cell division called
meiosis. Meiosis results in four daughter cells each with half the number of chromosomes
of the parent cell. The spores are dispersed and eventually germinate and grow into
haploid gametophyte plants – and so the cycle continues
Coping out of water
Bryophytes, which
include moss, are primitive plants that give us some idea of how the first
plants that ventured onto land coped with their new waterless environment. They
share many features with other plants, but differ in some ways – such as the
lack of an effective vascular system (specialised tissue for transporting water
and nutrients – xylem and phloem) which distinguishes them from ferns, conifers
and flowering plants. They usually form low-growing, dense cushions on rocks,
the bark of trees, and other surfaces including buildings. The plants absorb
and lose water depending on their surroundings and they need a film of water in
which to reproduce as the sperm need to swim from the male reproductive organs
to the eggs in the female organs.
Bryophyte life cycle
The diagram above shows the
life cycle of a typical moss. What you see growing on rocks at Kirstenbosch, or
on damp walls on your house, is the gametophyte form of the moss plant. Sexual
cells or gametes are formed by male and female gametophyte plants – and the
male gamete or sperm needs water in which to swim towards the female eggs in
the archegonia of the female plants (see photo below).
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In the tufted moss Polytrichum the male sex organs (antheridia) occur within a whorl of leaves and discharge sperm into the water-filled, reddish splash caps. From there raindrops splash the sperm cells onto nearby female plants. Photo: Chris Vynbos, iSpot
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Once this occurs, and
fusion takes place, a diploid zygote forms with two sets of chromosomes. The
zygote develops into the sporophyte which is a stalk growing out from the
gametophyte plant that supports a sporangium or capsule in which spores are
produced (see photo below).
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In bryophytes the sporophyte plant is a stalk growing from the gametophyte plant. The stalk supports a sporangium that produces spores which give rise to new gametophyte plants. Photo: Sally Adam.
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The spores are dispersed and grow into the
new gametophyte generation. So, although the gametophyte and sporophyte are two
different plants, in mosses the sporophyte is always attached to the gametophyte.
This is different to the life cycle of ferns, conifers and flowering plants
which will be covered in subsequent factsheets.
Download the Factsheet on The Classification of Plants (above) here.
Download the Factsheet on The Classification of Life here.
Download these articles that are relevant to the study of the classification of moss:
Cocks, Martin 1996. Surviving at the edge of life: The tiny plants that eke out an existence on the frozen continent. Veld & Flora 82(2), p. 46–48.
Online textbook - Bryophytes (Prentice Hall).
Josh Hall Educreations UTube video on Moss and Liverwort lifecycles.
