In the fern life cycle there are two well differentiated stages: sporophyte and gametophyte, also called diploid and haploid phases. The diploid phase is asexual and the diploid sexual.
Ferns are land plants that do not produce seeds. These plants are normally described as vascular spore-producing plants and can be easily identified by their leaves, which often look like feathers..
There are many different ferns: one of the most recent counts establishes that there are currently more than 13,000 species, distributed mainly in the tropical forested regions of the planet, where the temperature is relatively warm, there is a lot of humidity and a lot of shade..
Because they reproduce by spores, ferns can grow in a variety of places, including tree stems and branches, rock piles, clumps of dead leaves, ponds, and swamps, among other places..
Ferns are said to be vascular plants because they have a system of "veins" that allow them to transport water and nutrients throughout their bodies, in the same way that occurs in flowering plants (angiosperms) and conifers (gymnosperms), what are vascular plants with seeds.
However, despite this important similarity, ferns differ from "higher" plants such as angiosperms and gymnosperms, in that they need bodies of water to reproduce sexually, which is not true of other plants..
Ferns are plants with beautiful leaves, highly appreciated from an ornamental point of view. The plant that we associate with this name, however, represents only one of the stages of the life cycle, which we know as sporophyte.
The branches of the sporophytes have leaves throughout their structure and are known as fronds. The leaves that surround the branches are called leaflets or pinnae.
The leaflets, like the leaves of higher plants, have two "faces" that we can call higher, the one that "looks" up, "and lower, the one that "looks" down.
Many leaflets have, on the underside, a kind of dark spots (reddish, green, yellowish, brown or blackish) that contain countless spores. The fronds that have leaflets with these characteristics are called fertile fronds.
Ferns, like most plants, have roots or rhizomes that help them to hold on to the substrates on which they grow and through which they absorb water and a large amount of nutrients.
Spores are almost microscopic structures from which a new fern can form.
The dark spots under the leaflets, which are known as soros, correspond to a set of sporangia, which are like "sacks" that carry spores.
These sporangia can be “open” to the air directly or they can be covered by a membrane known as indusian.
Ferns have a life cycle that presents alternation of generations. In other words, its life cycle requires two generations of plants to complete..
The two generations differ from each other regarding several aspects and one of them is the chromosomal load. Thus a fern passes through a generation diploid, where the sporophyte that we have just described participates, and for another generation haploid, It is known as protect it or gametophyte.
The diploid generation consists of a plant with two sets of identical chromosomes in each cell (2n), while the haploid generation has half of these, that is, it has a single set of chromosomes of the species (n).
If we describe the life cycle of a fern starting with the sporophyte, which is the plant that we recognize as a fern, then we can say that it begins with the formation of the spores.
In leaflets, spores are formed by a process of cell division called meiosis, by means of which haploid spores are produced (n).
This process is very similar to the one that takes place in the gonads of animals to produce eggs and sperm, which are sex cells..
When the wind or some animal disperses the spores, they can fall on a suitable substrate and germinate, dividing by mitosis to train the gametophyte or protect it, a haploid plant structure (n) shaped like a kidney or a heart (as described by some books).
Gametophytes are much smaller than sporophytes, so we only associate the word "fern" with the leafy plants that we have previously described..
The gametophyte divides by mitosis and grows in size and number of cells. Its structure is a bit "primitive", as it only has a few rhizoids and little differentiated leaves..
Sooner or later, the gametophyte that arose from the haploid spore produces cells called gametes, which are also haploid cells whose destiny is to fuse with another complementary cell to form a diploid plant, that is, a sporophyte.
Some prothals produce the male and female gametes at the same time, but others only produce one of the two. The specialized structure by which gametophytes produce female gametes (eggs or ovo cells) is known as archegonium and that of the masculine ones like antheridium.
One of the great differences between "higher" vascular plants and ferns is the production of the seeds. The seeds are extremely resistant dispersal structures, which allow the subsistence of a plant species even under adverse conditions..
Ferns, on the other hand, indisputably need water, since fertilization, that is, the process of fusion of a female gamete with a male gamete, occurs in water.
If there is water, then the male gametes, which are flagellated and mobile, can swim to get to the female gametes (still "attached" to the gametophyte) and fertilize them, forming the zygote, which is the diploid cell (2n) that results from the combination of half the chromosomes of two different plants.
It is important that we comment that it may be the case that the same gametophyte produces both types of sex cells, with which a "self-fertilization" could occur.
The zygote, if it has the right conditions around it, divides by mitosis and again forms a sporophyte diploid that will be able to produce haploid spores again, completing the life cycle.
If we had to list the life cycle of a fern we could write something like this:
The life cycle that we have just reviewed contemplates the sexual reproduction of a fern, however, these plants can also reproduce asexually, as is common for many plant organisms..
The apogamy is one of the methods that ferns have to reproduce asexually and has to do with the formation of a sporophyte directly from a gametophyte, without fertilization occurring.
It is usually carried out when conditions are not adequate to reproduce, for example, in times or seasons of drought..
In addition to apogamy, it has also been shown that some ferns can produce a kind of "miniature ferns" at the tips of some fronds, which can detach and fall on a substrate where they can develop roots and form a plant identical to its "parent. ".
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