Fern Reproduction cycles

As already stated in previous posts, ferns exist for millions of years and are as old as the first first plant on land. You should at the same time remember that ferns have sophisticated reproductive system that can adapt to any type of environment. Like many other plants, ferns can reproduce by sexual or asexual methods. Fern has a unique and unusual bi-generational life cycle. Fern reproduction happens through either spores, rhizomes or stems depending on its variety .

Kindly note that ferns have two distinct free living generations

Ferns have two free-living generations:

Sporophyte generation – Diploid in nature

and

Gametophyte generation – Haploid in nature

The basic difference between Diploid & Haploid is Diploid cells have two sets of chromosomes whereas in case of Haploid cells there is only one set of chromosome.

Table of Contents

Structure of ferns:

Ferns are non flowering, seedless and vascular plants. They contain two types of vascular tissues. The primary job of the vascular tissue is to help in movement of water, nutrients and glucose to various parts of the plant. The vascular tissues make a major difference in the plant structure itself. They make the grow taller and taller instead of just remain a creeper along the ground. Due to lack of vascular tissues creeper plants move along the ground depending on osmosis and diffusion for intake of nutrients and stay in close contact with the ground .

There are essentially two types of vascular tissues in all ferns. They are

Xylem – helps in movement of water and nutrients throughout the fern plant. Xylem cells die when they reach maturity. The cell walls remain firm and continue to help in the purpose. The Xylem walls give strength to the plant as it moves taller.
Phloem – Responsible for movement of glucose throughout the plant. Phloem is made of sieve elements. Water also moves through phloem through osmosis and in the process helps in pushing the glucose further up.

Ferns only exhibit primary growth. There is no increase in size of the girth. They only grow upward .

Ferns also contain developed roots and stems and leaves, The total green foliage of fern leaves are called as fronds. This is because when the leaves come out , they are curled and coiled resembling a fiddle.

Cell Structure

Based on the cell structure and phylogeny, ferns can be classified as the following

Since the cells have nuclei , they come under the domain Eukarya.
As ferns have chlorophyll and cell walls for photosynthesis they come under plantae kingdom.
Ferns are classified under Division-Pteridophyta .

Ferns reproduce through 3 methods . They are

Through Gametophyte ( sexual )
By way of Cloning ( asexual)
Reproduction by way of Bulblets
Through Gametophyte – Development of spores

Ferns which we see as plants belong to sporophyte generation. A fully grown fern plant generates and stores the spores in a pouch like structure called sporangia. The sporangia are held on the underside of the fern leaves.

The spores are produced by the sporogenous cells. The sporogenous cells through meiosis form spores. As already explained, spores are haploid, having single chromosome in a cell.
Sporangia are found in clusters called sori on the underside of the fronds. Some ferns have a covering over the sporangia known as an indusium. The ferns release the ferns mostly during summer season.
Spores of most of the ferns perform the same function and similar in size too . As a result , ferns are known as homosporous plants. Ferns release the spores on to the ground as and when they mature.

A.Fern Reproduction – Gametophyte Cycle

Spores, as they land on a favorable place and environment , germinate into a heart shaped gametophyte via mitosis. The gametophyte is only around 5mm long and is bisexual. It is also called as ‘prothallus’.

Gametophyte does not have vascular tissues and anchor itself to the ground using root like structures called Rhizomes.
Prothallus develops sex organs on its underside. The female structure, called an archegonium, contains a single egg. The antheridium, which is the male structure, contains many flagellated sperms. The sperms are released from the antheridium. Sperms swim across a thin film of water to a nearby archegonium to fertilize the egg.
A zygote forms on fertilization of the egg. The Zygote is diploid, having 2 chromosomes in a cell. The Zygote grows into an embryo and is attached to the prothallus.
This embryo is dependent on the parent plant for its survival. Once the embryo grows and develops into a mature plant the prothallus withers away.
With that the gametophyte generation is over. This embryonic plant grows into a mature plant which is commonly called as Fern. This is the sporophyte generation as the fern produces spores as explained above. And the cycle repeats.
Since the antheridium and archegonium are on the same prothallus the fern has several strategies to prevent self-fertilization. These strategies will be discussed later in this paper. Once fertilization of the egg has occurred, a diploid zygote

B. Fern Reproduction – Clonal

All ferns have rhizomes which do the function of a root. In some varieties of ferns , reproduction is done through rhizomes which sprout new sporophyte plants. These individual clonal plants are called ‘ramets’ . They form huge colonies of clonal plants and thus quick fern reproduction is ensured. This helps ferns become a dominant force in its locality where it needs to fight with fellow species for space.

There are also few gametophytes which reproduce asexually by sprouting a bud like structure called ‘gemmae’

C. Bulblets & Walking Ferns

Some fern species sprout small vegetative buds on the fronds which are called Bulblets. The bulblet may take root and grow on its own as and when it falls off the fronds. Then it will grow into a mature fern, for instance. As a result, fern reproduction happens through bulblets too.

Ferns that can reproduce via bulblets are

Anderson’s holly fern of the Northwest coast
Walking fern gets its name from its interesting way of creating new fernlets. It’s simple fronds have long pointed tips. When these tips touch the ground, a new fern develops; thus, the fern “walks” across the landscape.

Fern reproduction mechanism to avoid Self Fertilization

The gametophyte being bisexual, develops both the male sex organ, archegonia and its female counterpart antheridia. This would make you think that high level of inbreeding is sure to happen. But it is not the case , contrary to popular belief.

In case of Intragametophytic fertilization, the sperm and egg belong to the same gametophyte plant. As a result, the resulting sporophyte would become 100% homozygous. This would make it lose all its capability and adaptive nature for further evolution. There is every possibility that the fern species would die out sooner than later .

On the other hand in Intergametophytic fertilization, the sperm and egg belong to different gametophyte . But both the gametophytes have originated from the spore of the same fern. But remember, pteridophyte species like ferns are seldom inbred. As a result they withstand the ravages of evolution.
To avoid the above mentioned condition ferns adopt Intergametophytic crossing. In this mode of reproduction, sperm and egg from different gametophytes whose both parent sporophyte plants are also different.
Moreover, few of the gametophytes produce a pheromone which causes neighboring gametophytes to produce only antheridia. This phenomenon is called antheridiogen. This helps in cross fertilization of the egg. .
Archegonia and antheridia mature at different rates to prevent any type of inbreeding

Polyploidy

Polyploidy is the condition of having more than two complete sets of chromosomes. Polyploids are common among plants, It is a major force in the evolution of wild as well as grown and cultivated plants.

One of the most interesting strategies used by fern reproduction mechanism to prevent evolutionary stagnancy is polyploidy.

As seen from the above , Ferns though bit primitive being non flowering and seedless, they have their own unique ,foolproof way of reproduction which are sexual , asexual and through bulblets too.

Your views and comments are highly solicited .Thanks