ARCHEGONIUM: STRUCTURE AND CHARACTERISTICS - GEOGRAFÍA - 2025

The arquegonio is a female reproductive organ of fungi, algae, ferns, mosses or seeds. It is bottle-shaped, that is, it has a neck, with a central channel through which the anterozoids pass and a wide base where the female gamete is formed and retained.

The egg cell is protected by a wall of inert cells that have no participation in reproduction. Archegonia can be located together with antheridia in the same individual (monoecious condition) or be in separate gametophytes (dioecious condition).

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This organ is present in lower plants such as bryophytes, liverworts and anthocerotes and also in pteridophytes and gymnosperms.

Structure

Archegonia differentiate from subepidermal cells of the gametophyte, which begin to differentiate as a result of thallus maturation. The archegonium is the female gametangium.

It is multicellular and bottle-shaped, presenting a long hollow neck that varies in length depending on the group and a wide base where a single egg cell is produced located at its base. In general, the neck is short and hardly distinguishable in the Anthocerophyta and long in liverworts and mosses.

When the archegonium matures, the cells that plug the neck canal rupture and release the chemicals that attract the anterozoids, and large numbers of anterozoids are very common around the cleft of a mature archegonia.

The anterozoid from the male gametangium (antheridium) slides down the neck until it reaches the female gamete using an aquatic medium, which is generally rainwater.

The formed zygote is nourished from the gametophyte, since the basal cells of the archegonium form a kind of foot or haustorium that is attached to the gametophyte tissue. In most archegoniates the outer cells of the archegonium are chlorophyll (photosynthetic), however, the inner ones are not.

characteristics

Archegonia, as well as antheridia, prevent gametes from drying out. Archegonial cells have specializations to facilitate fertilization, retain and nurture the zygote and the embryo resulting from fertilization within the gametangium.

The characteristics and location of the archegonia usually vary depending on the group of arched plants.

Anthocerophyta

In the Anthocerophyta group, (Anthoceros), the archegonia like the antheridia are found on the upper face of the thallus, internalized in chambers that are deep once the archegonia has matured. This does not occur in mosses and liverworts where the archegonia and antheridia are more superficial and exposed.

The cells that constitute the archegonium are poorly differentiated from the thallus. In contrast, the antheridia are exposed when mature and are similar in shape to liverworts with shorter peduncles or pedicels and the wall of the antheridium with less differentiated cells.

Briophyta

In mosses, the archegonia are found at the ends of differentiated areas of the caulidia that are part of the gametophyte, being protected by a group of leaves called perychaetum or periquecial leaves, in contrast the antheridia are protected by the perigonium or perigonial leaves..

Once fertilization occurs, a diploid sporophyte grows. The sporophyte consists of a peduncle and a capsule surrounded by a haploid calyptra, which results from remains of the archegonium neck duct and is expelled once the capsule has matured to spread the spores produced by meiosis.

Marchantiophyta

In complex thalous liverworts (Marchantia) there are gametangiophores that have the appearance of tiny trees and elevate the antheridia and archegonia of the gametophyte thallus by about one centimeter.

The antheridiophores are disk-shaped, the antheridia being located in the upper region. Once they receive dew or rain water, the antheridia expand due to the action of special cells (elaters) and release the sperm that are transported in the drop that falls to the gametophyte.

The archegoniophore, on the other hand, is shaped like an umbrella on the ventral surface of which the archegonia hang. Once the archegonium is mature it opens, and if it is bathed in a drop loaded with sperm, fertilization occurs.

The zygote develops internally in the archegonium which lengthens to form a protective calyptra.

The sporophyte is not very conspicuous and is made up of three areas, including a foot that is immersed in the base of the archegonium to extract nutrients, a very short stem, and a sporangium with multiple spores produced by meiosis. In some cases, liverworts have archegonium submerged in the thallus.

Seedless vascular plants

In this group of plants, the alternation of generations involves gametophytes and sporophytes. The production of oocells and spermatozoa is similar to bryophytes, also having antheridia and archegonia, with the difference that the sporophyte and the gametophyte (short-lived) are independent at maturity and the sporophytes are larger than the gametophyte.

In vascular seedless plants, spore production varies. They can be homosporic as in the case of mosses, in which the spores originate male, female or mixed gametophytes.

On the other hand, they can be heterosporic, generating two types of megaspore spores, in a megasporangium that produce female gametophytes and microspores in a microsporang that produce male gametophytes. They also need an aqueous medium for the movement of sperm into the archegonia.

The young sporophyte grows inside the base of the archegonium developing a foot that joins it to the gametophyte, however, this later separates to constitute an independent plant.

Included here are the phylum members Psilotophyta, Lycophyta, Sphenophyta, and Pteridophyta.

Gymnosperms

Archegonia are one of the most primitive characteristics that gymnosperms share with seedless plants. Archegonia production is characteristic of gymnosperms including conifers, cycads, Ginkgo biloba, and Ephedra.

Archegonia generally form after a megagametophyte has developed into a megagametophyte and it reaches maturity (about a year in pines). Two to five archegonia usually form near the micropyle. Each of these archegonia contains a single egg cell.

In the case of gynmosperms there is no production of antheridia, since in this group there is already production of pollen.

Classic reproduction in Briophyta

Non-vascular plants, as in other plants, have a life cycle with alternating generations. Their main characteristic is that they have a haploid gametophyte (n) that is larger than the diploid sporophyte (2n), which is in contrast to higher vascular plants.

In mosses, a spore (n) germinates and forms a network of horizontal filaments called a protoneme, from which branch-like gametophytes originate. Antheridia (which carry male gametes) and archegonia form in the gametophyte.

Being moisture-dependent plants, the biflagellate or anterozoid sperm are released and swim towards the archegonium that attracts them chemically. If the anterozoids do not have a watery matrix to move the cycle cannot be completed.

The fertilization of the egg cell by the sperm occurs within the archegonium, so that the process is protected. The zygote develops into a sporophyte, which remains attached to the gametophyte and depends on it for nutrition.

The sporophyte consists of a foot, a pedicel, and a single large sporangium (capsule) that contains the stem cells of the spores where it divides and spores are generated.

References

1. Chopra, RN (2005). Biology of bryophytes. New Age International.
2. Curtis, H., & Schnek, A. (2008). Curtis. Biology. Panamerican Medical Ed.
3. Nabors, Murray W. (2004). Introduction to botany. Pearson Education.
4. Sadava, DE, Heller, HC, Purves, WK, Orians, GH, & Hillis, DM (2008). Life: The science of biology. MacMillan.
5. Shaw, AJ, & Goffinet, B. (Eds.). (2000). Bryophyte Biology. Cambridge University Press.