Since the earliest of times, ferns have been appreciated for their beauty and have provided shelter and sometimes food. The birth of their young coiled fronds has often been used in artworks such as the Koru. Ferns are popular in our parks and gardens.
and even brought indoors to enhance our living spaces. There are as many as 12,000 species of ferns throughout the world. Some are the size of your fingernail, while others are as tall as a tree.
In the wild ferns are successful at reproducing and spreading throughout the forest. They do this by producing spores. The familiar fern is known as a sporophyte.
If a fern frond is turned over, Small circular areas may be seen. Each of these areas is called a saurus. In some ferns, the saurus is unprotected. In many others, it is covered by a cap, supported by a central stalk.
This protective covering is called an endusium. In some ferns, this covering is cup-shaped. The saurus itself contains numerous sac-like structures.
Each of these is called a sporangium. The outer wall of the sporangium consists of a layer of protective jacket cells. As the sporangium matures, a row of jacket cells enlarge to form a band known as the annulus. The outer wall of each annulus cell is very thin and delicate.
On the opposite side of the sporangium, several delicate lip cells form. Below the jacket, there are two layers of cells called the tapetum, which nourish the fertile tissue within. The fertile tissue consists of sporocyte cells.
Each sporocyte cell is diploid, containing two sets of chromosomes, one from each parent. As each cell matures, its nucleus divides twice by the process of meiosis. Each daughter nucleus now contains one set of chromosomes. The cytoplasm of the sporocyte undergoes cleavage.
This results in a cluster of four adhering cells, called the tetrad. The tapetum now begins to break down, depositing a very tough protective coat of sporopolinone. Surrounded by this thick resistant wall, each cell is now called a spore.
The sporangia are now ready to open. Opening is accomplished by the annulus that encircles the sporangium. The annulus begins to dry out as water evaporates from its surface.
The tension or pull between the remaining water molecules and the wall now increases. The thick inner and side walls resist this, but the thin outer walls are easily pulled inward. Pulling in the outer walls causes the annulus to contract much like an accordion.
Shortening the annulus tears the lip cells apart. As the annulus continues to shorten, the tear enlarges and the spore case opens further. When too much water is lost, the water molecules are no longer able to hold together.
When this happens, the pull on the wall is released and the annulus springs forward, closing the sporangium so fast that the spores are thrown out. This process is repeated in thousands of sporangia on a leaf, so that large quantities of spores are released. When spores land on a moist surface, those which germinate first will form bisexual or hermaphroditic individuals. A rhizoid emerges and attaches the spore to the soil.
This is followed by a sheet of cells, which is the young gametophyte or prothallus. A notch forms on the prothallus that contains dividing cells. This is called the notch meristem.
Continued growth results in a heart-shaped bisexual gametophyte. The lower surface is firmly attached to the soil by numerous rhizoids. Male Gametangia, called Anthuridia, are formed at the posterior end of the gametophyte. The outer wall of Anthuridium consists of ring cells and a cap cell, which surround fertile tissue.
At the anterior end of the prothallus, close to the notch, there are female gametangia called archegonia. An archegonium consists of a neck containing a neck canal cell. At the base of the neck, there is a swollen region called the ventre that contains an egg cell. Gamma Tangier of both sexes may be present at the same time or at different times. The timing of their appearance will determine if there will be self-fertilization or cross-fertilization.
In some ferns, archegonia form first, and when flooded with water, a hormone called antheridiogen is released. This hormone will stimulate adjacent plants to stop growing and form antheridia, but no archegonia. A bisexual gametophyte in a female phase may therefore be surrounded by several male gametophytes, increasing the chances of cross-fertilization. Being close to the soil, the small gametophytes are easily flooded when it rains.
This water plays an important role in fertilization. Water stimulates the cap cell on the antheridium to open, releasing the sperm cells. The flagellated sperm are now able to move within the water in search of an egg. Sperm consists of a spiral cell body bearing numerous flagella that move it forward. Water also stimulates the archegonium to open.
The contents of the neck canal then diffuse into the surrounding water, where they act as a sperm attractant. The attractant stimulates a sperm to swim towards the open archegonium. It then swims into the opening and moves up the neck canal towards the egg cell.
Fertilization is accomplished when the egg and sperm nuclei fuse. This creates a single diploid cell, the zygote. The fertilized egg remains attached to the gametophyte. Inside the archegonium on the lower surface of the gametophyte, the zygote begins to divide, forming the embryo's sporophyte plant.
The uppermost part of the embryo, the foot, absorbs nutrients from the surrounding gametophyte tissue. As the embryo grows, the venter tissue stretches and eventually ruptures. A root now grows into the soil for support and to absorb water. Other parts of the embryo form the first leaf and the future stem, the rhizome.
The leaf grows through the notch in the gametophyte to reach the sunlight. Eventually the rhizome emerges and provides additional leaves and roots. In this way a new plant is formed that matures into another spore producing fern completing the reproductive cycle.
We have seen that the fern reproductive cycle contains two plants, the large familiar spore plant or sporophyte, and a tiny gamete plant or gametophyte. Using spores to travel great distances and gametes which introduce genetic variability, ferns have spread and evolved throughout the world.