Plant reproduction can occur sexually or asexually, depending on the plant group or species. Reproduction in plant organisms is of utmost importance, not only for their propagation (increase in the number of individuals) but also for their dispersal, since it must be remembered that they are generally immobile or sessile beings settled on a substrate that supports nutritionally and structurally.
Plants can be classified into two large divisions or phyla known as Bryophyta and Tracheophyta. Mosses, hornworts, and liverworts belong to the Bryophyta division, while all vascular plants with and without seeds belong to the Tracheophyta division.
Plants that reproduce without seeds belong to the groups Psilopsida, Lycopsida, Sphenopsida and Pteropsida (which includes ferns), while plants with seeds are Angiosperms and Gymnosperms (flowering plants and plants without flower and with bare seeds , respectively).
With few exceptions, no matter the plant group in question, many plants can exhibit both sexual and asexual reproduction at some point in their life cycle, which depends on different endogenous and exogenous factors.
Asexual reproduction in plants
Asexual reproduction in all living beings consists of the formation of new organisms without the participation of two genetically different individuals or without the production of specialized cells with half the genetic load. This type of reproduction occurs mainly by mitosis.
It is said that it is a “conservative” type of reproduction, since it does not promote gene variation, since in each reproductive cycle, clonal (genetically identical) individuals are formed from a “mother” individual. It is one of the most “ancestral” types of reproduction and is highly exploited by plants.
Many authors consider that the different asexual reproduction mechanisms are perfectly suited to stable or constant environments, as they seek to ensure the continuity of an individual’s life when conditions are favorable or advantageous.
There are different forms of asexual reproduction in plants and these can be differentiated according to the structures used for this purpose.
– Asexual reproduction by fragmentation
Most plant cells have the ability to “de-differentiate” or lose their identity and form a new individual if they are isolated from the plant that gave rise to them. This property allows them to reproduce asexually through fragments of their own bodies or even from individual cells.
Thus, asexual reproduction by fragmentation consists, simply, in the detachment of organs or “fragments” from the body of a plant, which later give rise to a new plant, genetically identical to the parent.
– Asexual reproduction by means of specialized structures
Some plants that reproduce sexually also do so through an asexual pathway similar to fragmentation, but which usually occurs through specialized structures such as stolons, rhizomes, tubers, corms, bulbs, and others.
Certain plants use their own leaves as asexual propagation and reproduction structures, especially succulents with fleshy leaves, on whose margins (in certain species) “primordia” of new plants or leaves are formed that can be detached from them and transplanted to a suitable substrate for its growth.
These asexual reproduction routes are widely exploited in horticulture and landscaping, as they ensure the “regeneration” or formation of a large number of identical plants in a considerably short time.
Some non-flowering plants, including mosses, liverworts, hornbills, and ferns, produce spores as a means of asexual reproduction.
Although these structures are the product of meiotic divisions, the spores are recognized as specialized “asexual” structures in these organisms, since when they germinate they can produce new individuals directly, without the fusion between several of them.
In this group of plants there are also “modified organs” for asexual reproduction, such as specialized buds, gemmules, etc.
Sexual reproduction in plants
Sexual reproduction in plants, as well as in the rest of living beings, involves the fusion of two cells known as “sex cells” or “gametes” (with half the chromosomal load of the individual that gave rise to them), the formation of a zygote, the subsequent development of an embryo and, finally, the development of a new plant with genetic characteristics different from those of its parents.
Gametes are generally different cells (heterogamy). The female gamete is usually the largest, is immobile and is known as an “ovocell” or “egg cell”; while the male gamete is considerably smaller, mobile and is known as “sperm”.
However, according to the morphology of the gametes, three types of sexual reproduction are distinguished: isogamine, anisogamy and oogamy.
Isogamy and anisogamy are typical of unicellular organisms composed of plant cells, while oogamy (a form of heterogamy) is typical of plants with sexual reproduction and is characterized by the presence of an immobile or fixed female gamete and a male gamete small and mobile.
Gametes are produced in very particular structures known as gametophytes, which in turn function as temporary “containers” for them.
In some plants, the female gametophytes are called archegonia and the male antheridia. A plant can have female individuals and male individuals, but plants can also be bisexual when they contain both types of gametophytes on the same foot.
The life cycle of most of the vascular plants that we know begins with the fusion of sex cells and the development of the embryo from a zygote. From this embryo a diploid structure is formed (with half the chromosomal load of one parent and half of the other) that is known as a sporophyte.
The sporophyte is often the dominant form in the life cycle and is from which a plant can reproduce asexually or sexually. Sexual reproduction from the sporophyte occurs thanks to the formation of the gametophyte that will give rise to sexual cells.
The size and dependence of the gametophytes with respect to the sporophyte depends on the group or plant species, with flowering plants being those where the gametophyte is more reduced and is completely dependent on the sporophyte.
According to the type of reproduction, the plants have been separated into seed plants and seedless plants. Plants with seeds are differentiated into Angiosperms or flowering plants, and Gymnosperms or plants without flower and with bare seeds.
In these plants, sexual reproduction is aimed at the production of propagation structures known as seeds, within which the embryo resulting from gametic fusion lies.
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