Hornworts, liverworts and mosses used to be classified as classes of a single phylum, Bryophyta. Modern texts, however, now assign each to its own phylum. This reflects the current taxonomic wisdom that the hornworts and liverworts are more primitive and only distantly related to mosses and other plants. Hornworts are all grouped into a single class, Anthocerotae, containing a single order, Anthocerotales.
Hornworts are very similar to liverworts but differ in the shape of the sporophyte generation. Instead of generating spores in a capsule on top of a stalk, the hornwort generates spores inside a green horn-like stalk. When the spores mature the stalk splits, releasing the spores. Under the microscope, hornwort cells look quite distinct as well: they have a single, large chloroplast in each cell. Other plants typically have many small chloroplasts per cell. This structure imparts a particular quality of colour and translucency to the body (thallus) of the plant.