The Alternation of generations Consists of the succession of two different generations of the same species. That is, plants alternate between gametophyte and sporophyte generations, haploid and diploid generations. This phenomenon is characteristic of the vegetal kingdom, although it can appear in other kingdoms.
Gametophyte plants reproduce sexually through gametes (that is why they are called"gametophytes") and are haploids, which means that it has cells that have only one endowment of chromosomes.
Sporophyte plants reproduce asexually through spores (hence the name"sporophytes") and are diploids, which means that each chromosome of the cell nucleus is duplicated.
According to Hofmeister (1862, quoted by Haig, David), the first generation, the gametophyte, is intended to create the sexual organs, while the second generation, the sporophyte, has as its object the production of reproductive cells in massive quantities Spores), which will give rise to another generation of gametophytes.
Cycle of alternation
Sporophytes have structures (sporangia) that produce spores through meiosis, a process that refers to cell division that results in four cells that have half the chromosome load of the parent. Therefore, these spores are haploid.
The haploid spores go through a process of mitosis (cell division that produces two cells identical to the stem cell), giving rise to gametophyte organisms. When they reach maturity, they will produce haploid gametes: eggs and sperm.
Gametes are produced in the gametangios through the process of mitosis.
The gametangios represent the reproductive apparatuses of the plants: the masculine one is denominated anteridio whereas the feminine one is called arquegonio.
The union of the gametes produced by the gametophytes will give rise to a diploid zygote, which will become a sporophyte individual. Once it has reached maturity, this plant will produce spores, restarting the cycle.
One of the two generations is dominant while the other is recessive. Dominant individuals tend to live longer and grow taller than others. Recessive individuals are usually very small and, in some cases, live in the dominant organism.
In non-vascular or bryophyte plants, gametophytes are dominant; However, in the vascular plants, sporophytes are dominant.
Alternation of generations in bryophytes
Bryophytes are non-vascular plants, which means they do not have a circulatory system, so they depend on the humidity of the environment to survive. They are usually small in size.
Gametophyte individuals are the dominant generation of this species. An example of these plants is moss.
Alternation of generations in the moss
Mosses are non-vascular plants, which means that the gametophyte generation is the dominant one. The most developed part of the moss is gametophyte and in the upper part of this can be appreciated the sporophyte plant, which is small in size.
Some of these gametophytes have antheridia, which are sac-shaped, while others have archegions, in the shape of a bottle. In the anididios large quantities of anterozoids are produced that can be displaced thanks to a flagellum (filament located in the external wall of the cell, similar to a whip).
On the other hand, in the archegonians, a single oosphere is produced, which lacks flagella and therefore is immobile. The anterozoids are released to the outside and, helped by the humidity of the plant, they move to the oosphere, fertilizing it to create a zygote.
These zygotes possess chromosomes of both progenitors and give rise to the diploid or sporophyte generation. It should be noted that diploid zygotes are extremely vulnerable, which is why the plant carrying the oosphere holds them within themselves and, thus, the sporophyte plant grows on the gametophyte plant.
The sporophyte individual is composed of a filament that culminates in a capsule that resembles a bell. Once the zygote has matured, the capsule opens and releases tiny haploid cells that will grow as gametophyte individuals.
Photos recovered from Word-builders.org
Alternation of generations in tracheophytes
Tracheophytes are plants that have vascular tissues. They are divided into gymnosperms (coniferous plants) and angiosperms (floral plants). Unlike bryophytes, these plants protect their embryos through seeds.
The diploid generation, or sporophyte, is the dominant one. An example of a tracheophyte plant is the fern.
Alternation of generations in ferns
The most visible part of the ferns is a sporophyte. The sporophyte produces leaves called fronds, which produce sera, clusters of tiny spores. When the sera mature, the spores are released and eventually touch the soil.
These spores give rise to the gametophyte, called protalus, which is very small and grows separate from the mother plant.
Your life cycle is reduced to a couple of days. These organisms produce gametes that will then be joined to others. The new plant, the sporophyte, grows on the gametophyte, which deteriorates and eventually dies, initiating a new cycle.
Alternation of generations in angiosperms
As in ferns, the dominant plant of the angiosperms is a sporophyte. On the other hand, the gametophyte is a tiny structure that is in the flower of the plant. In the angiosperms there are two fertilization processes.
In the first, a male cell fuses with a haploid cell, giving rise to a diploid zygote. In the second process, a male cell merges with another diploid, creating endosperm, a reserve tissue from which the seeds of angiosperms are born.
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