Perigynium

[1][2] The perigynium is a modified prophyll, also known as a glume, which is tissue of leaf origin that encloses the dry, one-seeded achene.

[3] In liverworts, "perigynium" refers to a tube-shaped structure which encases the archegonium and the developing sporophyte.

[4] The location, size, shape, hairiness, color, and other aspects of the perigynium are important structures for distinguishing Carex species.

[7] Seed dispersal by animals such as ants (myrmechory) has been recorded, as some species of sedges have developed elaiosomes at the base of the perigynia.

Ants carry the perigynium back to the nest, use the elaiosome for food, and the seed germinates away from the parent plant.