Archegonium: Structure and Sexual Reproduction

Archegonium is the female reproductive system of ferns and mosses. Some gymnosperms, including cycads and conifers, also have archegonium. It is a flask-shaped structure made up of an egg-containing venter and a neck with one or more layers of cells. 

• As the archegonium develops, the neck-canal cells, which are positioned above the egg, vanish, creating a space for the sperm to enter. 
• The antheridium, the analogous male reproductive organ, produces sperm.
• Living beings can reproduce sexually, asexually, or through a combination of both mechanisms. 
• Non-flowering plants such as mosses and ferns do not generate pollen for sexual reproduction.

Read More: Megasporangia

Key Terms: Archegonia, Sexual Reproduction, Bryophytes, Pteridophytes, Nephrolepis, Gymnosperms.

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What is Archegonia?

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It is a multicellular organ that produces and contains the female gamete or ovum during the gametophyte phase of certain plants. The antheridium is the name of the male counterpart. The base of the archegonium is enlarged, and it has a long neck canal or venter. Although they are embedded in hornworts, archegonia are frequently found on the thallus surface of plants.

• Archegoniate refers to the plants that use archegonia for reproduction. 
• Multiple archegoniums can be found on a single plant. 
• Consequently, we use the term "archegonia" to describe the group of archegonium in a plant.
• For archegonia, there is no specific location within a plant. 
• Archegonia are usually seen on the thallus surface of plants, however they are deeply entrenched within hornworts.

Read More: Difference between sporophyte and gametophyte

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Structure of Archegonium

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The base of the gametophyte, from which archegonia emerge, serves as their anchor. The female gamete or egg will develop and mature inside the archegonium's core, which is its enlarged portion. Although they are often seen in some gymnosperms as well, archegonia are a hallmark of bryophytes and cryptograms.

• It has a flask-like form.
• It has a neck and a venter, which is its inflated base.
• The egg is inside the venter.
• The neck is visible above the gametophyte's surface.
• It is made up of four tiers of cells that together comprise the neck canal.
• Sperm can enter through a route formed by the neck canal cells.
• When mature, the neck canal cells may lyse to provide the sperm an easier passage.
• The neck has a strong structure since it is initially composed of many cells. 
• When the archegonium achieves maturity, the cells inside the neck will dissolve, opening a path to the egg.
• Within the archegonium itself, fertilisation occurs. 
• The fertilised egg remains in the archegonium until it develops into a sporophyte, at which point it leaves.

Read More: Sexual Reproduction in Flowering Plants

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Sexual Reproduction in Bryophytes

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All bryophytes have archegonia bearing eggs and antheridia that generate motile sperm. These organs have more complex structures than those seen in other plant families and are generally homogeneous among the three bryophyte groups. The antheridia are small, almost spherical to ovoid sacs on short stalks that contain sperm mother cells first and then sperm in a wall that is one cell thick. 

• Although the sperms themselves can only swim a very short distance and have two flagella, in certain species the sperm mother cells are first spread passively in water. 
• The base of the cylindrical archegonia is expanded. 
• The egg is encased in a jacket of sterile cells in the protruded basal portion and around 10 canal cells in the neck above, which degenerate with maturation. 
• Typically, archegonia have short stalks. 
• The size of these sex organs varies depending on the species, from 0.1 mm in the majority of species to 1.5 mm in a few archegonia. 
• The sex organs are dispersed along the stems of most liverworts and certain mosses, but in others, they are grouped in 'inflorescences' at the branch tips. 
• These inflorescences may be encircled by a cup-shaped cluster of leaves, making them noticeable. 
• Clusters of either sex organ may be accompanied by sterile hairs, which aid in water retention and maybe in dispersal. 
• Some thallose liverworts and hornworts have pitted antheridia, which cause explosive sperm release. 
• Additionally, hornworts feature tiny, depressed archegonia. 
• The archegonial stem in some thallose liverworts is the most prominent sexual reproductive feature. 
• Archegonia are created on the underside of a similar but smaller structure that is about 2 cm long and resembles a little parasol. 
• Antheridia are produced on the upper side of this structure. 
• There are several capsules in certain monoecious bryophytes, which have male and female reproductive organs. 
• Sporophyte production is often infrequent in dioecious bryophytes, where each plant has just one of the male or female organs.

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Sexual Reproduction in Liverworts and Mosses

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The typical liverwort sporophyte is composed of three parts: a foot that is immersed in the gametophyte; a colorless stalk that may reach a length of 2 cm; and a capsule that is 0.5-2 mm in diameter and is typically glossy black at the tip. It often lasts only a few days and is transient. 

• Throughout the sporophyte's existence, the foot receives nourishment from the gametophyte where it is implanted. 
• The capsule wall breaks into four valves as the spores grow. 
• Elaters are sterile hairs that are combined with spores and help spread spores because they have spiral thickening and react to drying out by jerky contracting motions. 
• They lengthen and produce fewer spores when it is humid. 
• The sporophyte's morphology can be utilised as a reliable, if tiny, trait to distinguish between different groupings of liverworts and different species. 
• Some liverworts have a smaller sporophyte, and the tiny thallose liverwort Riccia only has a spore sac that stays lodged in the gametophyte until the gametophyte itself decomposes around it. 
• In this liverwort, the sporophyte generation has almost entirely vanished.

The fundamental difference between the three types of mosses—typical mosses, bog mosses, and rock mosses—is the structure of the sporophyte. The sporophyte of most mosses lasts for a number of weeks. A strong stalk that survives and elongates early in the sporophyte's life, an anchoring foot implanted in the gametophyte, and a capsule are present.

 

• A component of the gametophyte known as the calyptra, which is generated from the archegonium, is frequently connected to the capsule as the sporophyte develops. 
• This safeguards the growing sporophyte and won't fall until the capsule is completely developed. 
• The capsule typically includes stomata, a central column of sterile tissue, and is photosynthetic. 
• When the capsule is fully developed, the cover at the tip is thrown off. 
• Most of them contain one or two layers of teeth within, along with the peristome, whose arrangement and quantity identify distinct species. 
• In response to humidity, they either twist or actively toss out the spores via a combination of the outer and inner teeth adhering together, which opens the capsule for spore dispersion in dry weather and closes it in wet weather. 
• One peculiar family of common mosses, the Splachnaceae, feeds mostly on dung and uses flies to spread its spores. 
• Their remarkable sporophytes include lengthy stalks that can reach a length of 10 cm, a swelling base of the capsule that can measure up to 2 cm wide, and vividly coloured leaves that resemble toadstools. 
• They give forth an odour that draws flies, who then transport the gummy spores to another dung heap.

Read More: Asexual reproduction

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Sexual Reproduction in Pteridophytes

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Sporangia are carried by the main plant body of pteridophytes. The sporangia produce spores by meiosis in the spore parent cells. The spores grow into prothallus, which are thalloid gametophytes that are minuscule, distinct, and multicellular.

• Archegonia and antheridia are present in the gametophytes. 
• The presence of water is necessary for the transfer of the male gametes released from the antheridia to the aperture of the archegonium.
• The male gamete unites with the egg that is already present inside the archegonium to form the zygote. 
• The zygote subsequently develops into a multicellular, well-differentiated sporophyte, which is the primary phase of the pteridophytes.

Read More: Bryophytes and Pteridophytes

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Nephrolepis

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The pteridophyte category includes the plant Nephrolepis. One of the prime example will be Ferns.

• Nephrolepis may reproduce both sexually and asexually.
• Buds are created during asexual reproduction, and these buds develop into new plants.
• In the sporangium, spores are created during sexual reproduction.
• These spores germinate and produce a prothallus under ideal circumstances.
• Antheridia and archegonia, which are sexual organs, are generated on the ventral side of this prothallus when it reaches sexual maturity.
• Archegonia initially emerges, then antheridia.
• Antherozoids are created in antheridium.
• A cell of an egg is created in the archegonium.
• The medium of water is necessary for fertilisation.
• The zygote transforms into a diploid sporophyte following fertilization.

Read More: Double Fertilization in Angiosperms

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Sexual Reproduction in Gymnosperms

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There are no separate male and female gametophytes in gymnosperms. The sporangia that are already on the sporophytes still contain them whole. 

• In the cycas ovule, archegonia is present. The pollen grain is released by the microsporangium. 
• After being carried by air currents, the ovules carried by megasporophylls make contact with the opening. 
• The pollen tube containing the male gametes grows in the ovules as they move towards the archegonia and bursts open just before the archegonia opens. 
• Following implantation, the zygote develops into an embryo, while the ovules develop into seeds.

Read More: Pollination and Fertilization

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Things to Remember

• Archegonium is the female reproductive system of ferns and mosses.
• It is a flask-shaped structure made up of an egg-containing venter and a neck with one or more layers of cells. 
• The base of the archegonium is enlarged, and it has a long neck canal or venter.
• The female gamete or egg will develop and mature inside the archegonium's core, which is its enlarged portion.
• All bryophytes have archegonia bearing eggs and antheridia that generate motile sperm.
• The typical liverwort sporophyte is composed of three parts: a foot that is immersed in the gametophyte; a colorless stalk that may reach a length of 2 cm; and a capsule that is 0.5-2 mm in diameter and is typically glossy black at the tip.
• Sporangia are carried by the main plant body of pteridophytes. 
• The pteridophyte category includes the plant Nephrolepis.
• There are no separate male and female gametophytes in gymnosperms.