Ferns have multicellular stages in both the haploid and diploid generations of the sexual life cycle. This is characteristic of most plants and is called "alternation of generations." Contrast this to animals in which there is a mulficellular diploid but only a single cell haploid stage (the gametes) and many fungi which have a multicellular haploid stage but only a single cell diploid (the zygote) that soon undergoes meiosis. Different groups of plants vary in the size and duration of the two generations. Ferns often have significant stages at both the haploid and diploid part of the life cycle. The clover leaf fern Marsilea is especially good for seeing the gametes and the gametophyte generation.
Culturing
Marsilea
The sporophyte of Marsilea has fronds
that are very un-fern-like. The fern looks like little four leaf clovers.
These "clover leaf ferns can be found in bogs and swamps in the south eastern
United States, and can be purchased from biological supply houses. They
will grow in a wet terrarium. At the base of a group of fronds the sporophyte
produces its spores and packages them in a little case called a sporocarp.
The sporocarp looks like a little bean or rice grain and can sometimes
form on terrarium grown plants.
An easier way to get sporocarps is to order
them from a biological supply house. They come in a little envelope and
can be stored at room temperature for months. The method for germination
of the spores can be found in the section on observing Marsilea.
Observing Marsilea
SPOROCARP
To germinate the haploid spores in the
sporocarp, you need only bring the spores in contact with water. The coat
on the sporocarp is very resistant to water, so you have to breach that
defense. One way is to scrape away the coat with a piece of fine sand paper.
Grasp the sporocarp between the thumb and first finger so that one end
is sticking out. Put a square of sand paper on the table and then rub the
sporocarp on it. The dark brown outer coat will be worn away. Keep sanding
until a light cream-colored line appears where the sanding has been happening.
Once the coat is worn away, put the sporocarp in a petri dish of water.
It may float at first, so push it down under the water so it sinks. Then
wait for about an hour, observing the sporocarp for about an hour. You
should see a gelatinous "ring" (it's actually linear, not circular) burst
out of the sporocarp along with two kinds of spores in little sporangia.
The gelatinous ring takes up considerable water, causing it to swell and
burst the sporocarp coat. Look at the sporangia under a dissecting
microscope.
You will see two kinds of spores -- big
megaspores and little microspores. Both will have begun to germinate
by the time they are out of the sporocarp. These are single cells
at this stage --spores. Both will begin to divide to form the gametophyte
generation, and then the gametophytes will form gametes. Are the
gametes themselves formed by mitosis or meiosis in the gametophyte generation?
(Answer)
When does meiosis occur? (Answer)
The megaspore grows into a megagametophyte
that resembles an olive with a little pimento sticking out of it. The "pimento"
is the egg and will be covered on the exposed end with a gelatinous layer.
The olive is haploid tissue that supports and nourishes the egg.
The gelatinous material surrounds the archegonium and egg and attracts
the sperm.
I
Coiled
and Uncoiled sperm
It will take at least 7-8 hours for the sperm gametes to appear. The sperm are motile for about an hour. The microspores divide many times to develop into microgametophytes, a ball of cells. Some of the cells will become sperm. In later stages of development you can see that each microspore coat that originally contained 1 cell now contains many smaller cells. If you're lucky, you will be able to see the developing sperm rolling around inside the spore coat. These sperm are bizarre multiflagellated affairs. A bit later the sperm will break out and swim around the dish, eventually arriving at the jelly layer of the egg. One of them will then fertilize the egg.The nucleus is coiled like a spring with a set of long mitochondria coiled in parallel with it. When it gets near the egg, the sperm is trapped in a gelatinous layer and can be observed more easily because they are stopped from their usual darting motion. Often the nucleus-mitochondrion will uncoil near the egg, and on rare occasions one can seven see a nucleus enter the egg.
Keep the petri dish in a lighted area for a week, and you will see the first frond of the new sporophyte emerge from the megagametophyte. It will also grow its first little rhizoids. After a week or so, when there is a definite sporophyte, you can try to grow them in little pots of peat moss. Constant attention to keeping them wet is necessary, and the success rate is not always high, but it's worth a try if you get a new clover leaf fern as a result.
The Marsilea life cycle is a little different from the standard fern life cycle. Obtain a diagram of a typical fern life cycle and compare it to Marsilea. Draw a Marsilea life cycle diagram.
Reprinted from Life Cycles from the Woodrow Wilson 1997 Biology Institute and amended with "Plants are Invertebrates Too - Sperm development and Fertilization in Marsilea, the Clover Fern" NABT Convention 1999. Author Doanld Cronkite, Hope College, Holland, MI 49423
Supply Source
Carolina Biological Supply
Company
Catalog # K 3-15-6932
Marsilea Sporocarps
They also sell live marsilea
ferns.
Answers:
Are the gametes
themselves formed by mitosis or meiosis in the gametophyte generation?
Gametes are formed by mitosis because the gametophytes are already made
of haploid tissue.
When does meiosis
occur? Meiosis occured during the formation of the spores.