archegonium

An archegonium is the female gametangium of land plants. In bryophytes (mosses, liverworts, hornworts) and many ferns, it is a flask-shaped structure that develops on the gametophyte. The archegonium comprises a swollen ventral chamber, called the venter, which houses the egg cell, and a narrow neck that rises to an opening on the surface. At maturity, neck canal cells disintegrate to form a canal through which sperm travel to reach the egg.

During development, archegonia form from archegonial initials on the gametophyte. The egg cell resides inside the venter, while the neck serves as a guiding channel for motile sperm in species with flagellated sperm. After fertilization, the zygote enlarges within the archegonium and develops into a sporophyte, which remains partially or wholly dependent on the gametophyte for nourishment in many groups.

Occurrence and significance vary among plant groups. In bryophytes and many pteridophytes, archegonia are conspicuous female organs on the gametophyte and are central to the sexual life cycle. In gymnosperms, archegonia form within the female gametophyte inside the ovule and produce eggs that are fertilized by pollen-delivered sperm. By contrast, most angiosperms lack a distinct archegonial organ; fertilization occurs within the embryo sac after the pollen tube reaches the egg. The archegonium is a defining feature of the gametophytic generation in several plant lineages and reflects the evolution of diverse strategies for fertilization and early embryo development.