Some species grow in large numbers as tiny weeds in the soil of gardens and cultivated fields.

[3][4] The pyrenoid is a liquid-like organelle which enables a more efficient photosynthesis [5], has evolved independently five to six times in hornworts and is present in half of the roughly 200 species.

[6] It is formed by the fusion of the chloroplast with other organelles and is composed predominantly of RuBisCO, the key enzyme in carbon fixation.

The exceptions are the species Folioceros incurvus, the genus Notothylas and the three closely related genera Megaceros, Nothoceros and Dendroceros, which do not have stomata.

[16] The sporophyte lacks an apical meristem, an auxin-sensitive point of divergence with other land plants some time in the Late Silurian/Early Devonian.

Yellow and brown spores have a thicker wall and contain oils that both protect against desiccation and function as a nutrient storage, allowing them to survive for years.

The species Folioceros fuciformis and the genera Megaceros, Nothoceros and Dendroceros have short-lived spores with thin and colorless walls that appear green due to the presence of a chloroplast.

This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter, and several layers of cells in thickness.

This is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence.

[22] However, the same form of columella is also characteristic of basal moss groups, such as the Sphagnopsida and Andreaeopsida, and has been interpreted as a character common to all early land plants with stomata.

[25] Chromosome-scale genome sequencing of three hornwort species corroborates that stomata evolved only once during land plant evolution.

It also shows that the three groups of bryophytes share a common ancestor that branched off from the other landplants early in evolution, and that liverworts and mosses are more closely related to each other than to hornworts.

[26] Unlike other land plants, the hornwort genome has the low-CO2 inducible B gene (LCIB), which is also found in some species of algae.

Because the diffusion rate of carbon dioxide is 10,000-fold higher in air than in water, aquatic algae require a mechanism to concentrate CO2 in chloroplasts so as to allow the photosynthetic RuBisCo protein to function efficiently.

However, the most recent phylogenetic evidence leans strongly towards bryophyte monophyly,[28] and it has been proposed that hornworts are de-ranked to the original class Anthocerotopsida.

The number and names of genera are a current matter of investigation, and several competing classification schemes have been published since 1988.