Bryophyte

The hornworts are the smallest of the three groups of bryophytes with only about 100 species in six genera. Hornworts are especially diverse in the tropics, although Anthoceros occurs in temperate regions.

The gametophyte of hornworts is saucer-shaped, with upturned edges, and only 0.4-0.8 in (1-2 cm) in diameter. These interesting little plants are more similar to algae than are any other plants, especially because they have only one, large chloroplast in association with a pyrenoid in each photosynthetic cell. Yet, the hornworts are more advanced in some ways than liverworts, for example, they possess stomates, which exchange gases between the plant and the air. They are also unique among plants in having stomates on their gametophytes. Unlike the liverworts in which internal spaces between cells are filled with air, in hornworts the cavities are filled with mucilage, a water-absorbing material within which the cyanobacterium Nostoc can be found. This symbiotic relationship greatly benefits the hornwort, because cyanobacteria are among the few organisms that can fix molecular nitrogen (as N₂) into nitrogen compounds that are in a form useable to plants as nutrients; no plants can produce these essential compounds on their own.

Hornworts derive their name from their sporophyte, which has the appearance of a tapered horn. The sporophyte has a mass of undifferentiated tissue called a meristem at its base. The meristem can actively grow, so that the sporophyte can continue to increase in height, especially if damaged at the top, and can reach a height of 0.4-1.6 in (1-4 cm). The sporophyte of hornworts possesses stomate-like openings and remains photosynthetic for several months. It is, therefore, only semi-dependent on the gametophyte to which it remains attached. The sporophyte of hornworts represents a transitional stage to more highly evolved plants such as ferns, in which the sporophyte is for the most part independent of the gametophyte.