Archegonia

Archegonium is the female reproductive organ of the gametophyte in several groups of land plants, most notably bryophytes (mosses, liverworts, hornworts) and many ferns. It is a multicellular, flask-shaped structure that produces and houses the egg cell. A typical archegonium has a swollen ventral part called the ventor where the egg develops, and a narrow neck that forms a canal to the exterior. The neck canal, often lined with specialized cells, guides sperm to the egg during fertilization.

Archegonia form on the gametophyte and fertilization generally requires the movement of sperm through water or a liquid medium to reach the egg. After fertilization, the zygote develops within the archegonium and grows into a sporophyte that is usually dependent on the parent gametophyte for nutrients.

In bryophytes, archegonia are produced on the still-multicellular haploid gametophyte tissue, and each fertilized archegonium produces a stalked sporophyte that remains attached. In most ferns, archegonia arise on the free-living gametophyte (the prothallus) and function similarly.

In seed plants, the term archegonium is applied to the structures inside the ovule in gymnosperms, which contain the egg and may be fertilized by a pollen tube; in angiosperms the familiar female gametophyte (embryo sac) is not organized as a separate archegonial organ, so archegonia are not present as discrete structures.

Archegonia are a defining feature of the gametophytic generation in many ancient plant lineages and illustrate the diversity of reproductive strategies in the plant kingdom.