World Builders™
World Builders™
Session Eight --  Land Plants  
Session Eight  -- Land Plants  

 Alternation of Generations   Alternation of Generations

Reproduction in Early Plants
Reproduction in Early Plants

Mosses and other early plants  use Alternation of Generations for reproduction.

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 clever!

     Study this diagram. The notes will help you to understand it. In real life these mosses may be even smaller.

Let's begin. We start at the place where

Diagram about Alternation of Generations in Moss

Moss grows
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.

      The 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 zygote

                                                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 grow.

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?

Things to think about

Compare the way the moss uses its resources with the reproductive strategies of the animals, r-selected and K-selected.

Notice 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.

Notice that the haploid spores, which are less expensive to produce, are more expendable, and produced in greater numbers.

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© 1996,1997, 1998, 1999, 2000, 2002, 2003, 2005.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .