How Mosses Reproduce on Land
When plants came onto the land,
they had to find a new way to reproduce. In water they had just
released their reproductive cells, but these cells dried out
in the air, and had no way to move towards each other. Obviously
some of the pioneer plants must have reproduced asexually, perhaps
by growing new shoots. This could work, but it didn't mix the
chromosomes up to make plants with new characteristics. Innovation
was needed in the new environment.
This is the solution that the
mosses and some other primitive plants developed! It is pretty
Study this diagram. The notes
will help you to understand it. In real life these mosses may be
begin. We start at the place where
from the spores.
You can see the little
sprigs of moss. They have just grown from the spores. Although
they all look pretty much alike, some of these little sprigs
are male and some are female.
If you look at the pictures of the cells
beside the sprigs, you will see that each cell in the picture
has only half the number of chromosomes. This is called the haploid generation. (Hint: haploid = half)
Sperm swims to eggs in female moss plants.
Rain falls on the
moss. (Heavy dew would also work, or the spray from a waterfall.)
The male moss plants have made sperm.
These little sperms have flagella (tails like
little whips). Now that there is a film of water to swim in,
they swim to the eggs that the female moss plants have made.
The eggs are fertilized when the sperms
reach the eggs. The fertilized eggs are called zygotes.
zygotes have chromosomes from both the male and female parents.
They are the diploid generation (Hint: diploid means double) because they have pairs of chromosomes.
You can see the diagram of the cells with the paired chromosomes
in the next phase.
The Zygote grows on top of the moss.
The zygote is vulnerable to being
dried out, but the mosses solved this problem.
The female plant
holds onto her single egg (which has now become a zygote) and
the zygote grows right where it is on top of the sprig of moss.
The zygote gets some of its
nourishment from the green moss that is its mother, and is kept
moist by the moisture that is in the moss plant.
The zygote sends up a thin thread of tissue, and a capsule
grows on the tip of it. The capsule may be shaped like a little
bell. Many tiny spores form inside this capsule. The spores are
haploid, and carry only half of the number of chromosomes
that are in the cells of the
The mature Zygote releases spores.
When the zygote matures, the capsule opens and the little haploid
spores are released into the air.
They are tiny
and float away, seeking out new environments where they can begin to
Now we are back
where we started in this cycle.
Now go back to the diagram at the top of the page and
trace through the cycle of the alternation of generations.
Does it make sense to you?
the way the moss
uses its resources
with the reproductive strategies
of the animals, r-selected
that the most important part, the recombination of the chromosomes
to form the zygote, is in the K-selected pattern, where intensive
care is given to raising a small number of young.
that the haploid spores, which are less expensive to produce,
are more expendable, and produced in greater numbers.
The Evolution of Land Plants
Header by Viau from Jasper
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2000, 2002, 2003, 2005.
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