Archegonium - Structure & Defination

Archegonium is the female reproductive multi-layered organ of the gametophyte stage in certain plants, particularly mosses and ferns, which produces and carries the female gamete or ovum. The word Archegonia is derived from the word ‘ἀρχή’ i.e., beginning" and ‘γόνος’ which means "offspring". Archegonia are typical of cryptograms and bryophytes (plants without seeds), although they can also be found in some gymnosperms, such as cycads and conifers. The archegonium is made up of a neck and a swelling base that houses the egg. It has a flask-like appearance. The neck-canal cells, which are located above the egg and disappear as the archegonium grows, leave a place for the sperm to enter. The antheridium, the analogous male reproductive organ, produces the sperm. Archegoniate refers to the plants that use archegonia for reproduction. Multiple archegonium can be found on a single plant. Consequently, we use the term "archegonia'' to refer to the collection of archegonium in a plant.

Structure of Archegonia

The gametophyte from which archegonia originates is linked to it by a foot. The haploid gamete-producing state of a plant is referred to as a gametophyte. A bigger, wider region that resembles the bulb of a flask is located over the foot. The venter is the term used to describe this structure. The female gamete or egg will be generated and developed in the venter which extends upwards to become the neck of the archegonium. The archegonium needs to be matured to facilitate the neck to act on its job. The neck is a sturdy structure owing to its multicellular arrangement. The innermost part of the neck is fated to disintegrate after the maturity of the archegonium to facilitate the path to the egg.

Fertilisation takes place inside the archegonium. Until the fertilised egg converts into a sporophyte, it stays inside the archegonium. The plant's spore-producing structure is known as a sporophyte. The sporophyte is released by the archegonium after the complete development.

Placement of Archegonium in Plants

• For archegonia, there is no specific location within a plant. Archegonia are usually found on the thallus surface, however they are deeply lodged inside hornworts.

• Sperm in bryophytes and other cryptogams travel through water films to access the archegonium.

• The sperm is delivered by a pollen tube in Pinophyta and Angiosperms, while the pollen is carried by wind or animal vectors .

• Archegonia are not embedded but rather are found on top of the leafy gametophyte as in moss Physcomitrella patens.

• The archegonium of gymnosperms are severely reduced and incorporated in their megagametophytes.

• The megasporangium (nucellus) of the ovule's diploid megasporangium cells performs the majority of the job of encircling the gamete.

• The archegonium of gymnosperms develops during pollination within female conifer cones.

Role of archegonium in sexual reproduction

• Pteridophytes

Sporangia are carried by the essential plant structure of pteridophytes. The sporangia produce spores through meiosis in the spore parent cells. Prothallus, which are minuscule, multi-layered thalloid gametophytes with antheridia and archegonia, are the result of the spores' development. The presence of water is necessary for the transportation of the male gametes released from the antheridia to the aperture of the archegonium. The egg that is already inside the archegonium combines with the male gamete to form the zygote. The zygote subsequently develops into the primary phase of the pteridophytes, a complex, well-differentiated sporophyte.

• Bryophytes

In the bryophytes, the antherozoids encounter the archegonium after being exposed to water. The egg and an antherozoid combine to form the zygote. Zygotes do not immediately divide into new cells. They produce a group of cells called a sporophyte. The sporophyte's sole food supply is the photosynthetic gametophyte. Some of the cells in the sporophyte undergo reductive mitosis to form haploid spores.

In mosses, at the tips of the leafy branches, sex organs antheridia and archegonia are produced during sexual reproduction. Compared to liverworts, mosses have more complicated sporophytes. The capsule contains spores, which are produced after meiosis.

• Gymnosperms

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 are present. The pollen grain is released by the microsporangium. Megasporophyll-borne ovules establish touch with one another after being carried by wind currents to the aperture of the ovule. Near the archegonia's opening, the pollen tube containing the male gametes emerges from the ovules and moves toward the archegonia. Following implantation, the zygote develops into an embryo, while the ovules develop into seeds.