The calyptra is the protective cap that covers the archegonia of many bryophytes. It is derived from the remains of the neck of the archegonium, and it helps to protect the developing embryo.

In mosses, the calyptra is a small, membranous structure that covers the archegonium. After fertilization, the calyptra is broken open by the growth of the sporophyte.

In liverworts, the calyptra is larger and more complex. In some cases, it may have a toothed edge or be covered in hairs.

In hornworts, the calyptra is a tube-like structure that encloses the archegonia. It consists of two layers. The inner layer is highly convoluted and is composed of thin-walled cells. The outer layer is smooth and consists of thick-walled cells. The cells on the inner layer become glandular, secreting mucilage into the spaces between the convoluted inner layer and the outer layer. The mucilage may be responsible for the rapid increase in the volume of the calyptra. After the antherozoids are released, the mucilage absorbs water, increasing in volume and lifting the antheridial head well above the thallus. The increased surface area of the mucilage enables water and air exchange needed by the antherozoids. After fertilization, the calyptra continues to grow, enclosing the developing sporophyte.