Ferns are able to reproduce without using seeds.
We will look at how ferns reproduce as well as the pattern of alternating between diploid and haploid life stages.
Introduction: Fern Reproduction Without Seeds
Ferns are seedless vascular plants. Ferns were the first type of vascular plant and are generally considered to be simpler than vascular plants that make seeds.Ferns can range from very short, unimpressive house plants to larger, full grown trees. Ferns are often used in landscaping and decoration but also occur naturally in a variety of environments.
We already know that many plants go through a life cycle that alternates between diploid and haploid, but let’s review some basic aspects of this before looking at how ferns go through this process.
Review: Alternation of Generations
This alternation of generations is a life cycle that includes both diploid and haploid multicellular stages. Remember that diploid means two sets of chromosomes and is commonly abbreviated as ‘2N’ because the N stands for chromosomes. In diploid cells, one copy of chromosomes comes from each parent. For example, in humans, you get one copy of chromosomes from your dad and one copy of chromosomes from your mom.The same idea is found in plants.
Each diploid cell contains one copy of chromosomes from the male parent and one copy of chromosomes from the female parent. Haploid means one set of chromosomes and is commonly abbreviated as ‘N’ because there is only one copy of the chromosomes.
Let’s look at a diagram of the basic idea of alternation of generations.
We can see in this diagram that the life cycle is broken into N – on the top – and 2N – on the bottom. Remember that the gametophyte contains haploid cells and that the sporophyte contains diploid cells. Previously, we used the garblinx to illustrate the oddity of this. Remember that the diploid, or 2N, organism looks like this.
The fern begins with the haploid stage as a spore.
The spore will undergo mitosis in order to create many identical haploid cells. This group of cells is called the gametophyte. We can see in our diagram that the spore develops into the gametophyte. Most fern gametophytes look like heart-shaped leaves and are smaller than your pinky nail.Interestingly enough, this gametophyte, though unimpressive in size and structure, is capable of photosynthesis.
This means that the gametophyte can make its own energy and grow independently.After development and growth as a gametophyte, the spore is then ready to produce gametes. Gametes are made in a specific area of the gametophyte.
Gametangia are organs in gametophytes that produce gametes.There are two types of gametangia: one produces eggs and the other produces sperm. The archegonia are the female gametangia that produce one egg at a time.
The antheridia are the male gametangia that produce many sperm. You may be able to remember which gametangia are male and which are female because anthers are male structures in flowers that produce sperm. If you know that anthers produce sperm, you can remember that antheridia also produce sperm and therefore are male.Our diagram shows the male and female structures on the same gametophyte.
This means that the fern gametophyte is bisexual because it contains both the male and female structures. The antheridia are located towards the tip of the heart-shaped leaf. The archegonia are located near the top of the gametophyte.
Genetically speaking, it is best for gametes to mix between different gametophytes to start the diploid stage. If the gametes come from the same plant, there will not be much genetic diversity. However, if the egg and sperm come from different plants, this will increase genetic variation, most likely improving the survival chances of the plant.
The Diploid Stage
The diploid – and dominant – stage of life will begin once fertilization occurs.
Remember that fertilization is the fusion of an egg and sperm. Both the egg and sperm are haploid, so when they fuse together, we now have a diploid cell. This new cell will continue to go through mitosis and grow.
Eventually, it will become a mature sporophyte and look like the ferns you know. There are many different structures in the mature sporophyte, but we will focus on those involved in reproduction.Located on the underside of fern leaves, you may find red or brown dots. These dots are called sori and are groups of sporangia.
Now, what are sporangia? Just like gametophytes have gametangia that produce gametes, sporophytes have sporangia to produce spores. Sporangia are organs that produce spores through the process of meiosis.
Within each sporangium are cells that actually go through division to become spores. These sporocytes are diploid cells that undergo meiosis to produce haploid spores. These spores are microscopic, meaning they are very small. While you can see the groups of sporangia on the underside of fern leaves, you cannot see the spores without the aid of a microscope.
Once released, these spores will then start the haploid stage of the life cycle.
Ferns are unique plants because they do not produce seeds. These vascular plants have adapted to a life cycle that uses spores rather than seeds.
This alternation of generations in ferns means that there are multicellular stages that are haploid and diploid.We first looked at the haploid stage, which starts as a spore and develops into a gametophyte. The fern gametophyte looks like a small, heart-shaped leaf and is bisexual, containing both male and female organs that produce gametes. The gametangia that produce sperm are called antheridia, and those that produce eggs are called archegonia.Once the egg is fertilized by the sperm, the fern moves into the diploid stage of the life cycle. This new diploid cell develops into a mature sporophyte which is the structure you think of when you think of ferns.
The sporophyte has unique structures called sporangia located on the underside of some leaves. These structures contain sporocytes which produce spores. These spores start the haploid stage all over again.
At the end of this lesson, you will be able to describe the steps involved in the unique life cycle of ferns.