hornworts

Hornworts are a division of non-vascular land plants known as Anthocerotophyta. They are one of the three bryophyte groups, along with mosses and liverworts, and occur worldwide in damp, shaded habitats from forests to wetlands. Most hornworts are small and thallose, lacking true roots, leaves, or stems; the body (the gametophyte) is a flat, green or olive thallus that may be embedded in soil or grow on damp surfaces. The sporophyte that arises from the archegonium is horn-shaped, slender, and often long relative to the gametophyte; it remains attached to the gametophyte and is photosynthetic for much of its development.

Symbiosis with cyanobacteria: Hornworts commonly host Nostoc colonies within specialized cavities in the thallus. The Nostoc fix atmospheric nitrogen that benefits the hornwort, while receiving carbon from the plant.

Cells of hornworts characteristically contain a single large chloroplast in each photosynthetic cell, often with a pyrenoid; this is a distinguishing feature among land plants.

Life cycle and reproduction: The dominant generation is the haploid gametophyte. In moist conditions, hornworts produce antheridia and archegonia; fertilization requires water for the swimming sperm. The resulting zygote grows into a diploid sporophyte, which produces spores at its tip; spores are released as the capsule dehisces. Asexual reproduction can occur by fragmentation of the thallus.

Ecology and evolution: Hornworts are among the earliest-diverging lineages of land plants and provide insights into the origin of land plant traits such as stomata on the sporophyte. They contribute to nutrient cycling in some ecosystems through their Nostoc symbiosis and colonize a range of moist microhabitats, including soil, rock, and tree bases.