Prothallus

Prothallus refers to the free-living gametophytic stage of ferns (pteridophytes) that develops from a germinating spore. In the fern life cycle, the sporophyte grows from a fertilized egg on the prothallus, which is typically a small, photosynthetic structure. The prothallus is usually short-lived, persisting until the young sporophyte becomes capable of independent growth.

Morphology and development: Prothalli are often small and thalloid, frequently heart-shaped or flat, and they may range from a few millimeters to a few centimeters in size. They possess rhizoids on the underside to anchor the plant and absorb water and minerals. The surface often bears a midrib or network of veins, and their appearance can vary by species. The prothallus produces gametangia: antheridia, which generate sperm, and archegonia, which house eggs. Depending on the species, a single prothallus may bear both types of organs (monoecious) or different prothalli may bear one type (dioecious).

Reproduction and lifecycle: Fertilization requires external water for the motile sperm to swim to the archegonia. When a sperm fertilizes an egg, a diploid zygote forms and grows into the sporophyte, the typical fern plant, marking the transition to the dominant phase of the life cycle. The prothallus thus represents the haploid, gametophyte generation that precedes the familiar fern sporophyte.

Ecology and variation: Prothalli arise in moist, shaded habitats where spore germination and fertilization are feasible. Their development and longevity are influenced by environmental conditions, and some species produce particularly small or inconspicuous prothalli.