Domain Eukarya
Kingdom Plantae
Division Bryophyta
Phylum Anthocerophyta
Images below courtesy The University of Wisconsin, Madison
Diagram courtesy The New York Botanical Garden
 A hornwort diagram |
 A hornwort with sporangia |
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A hornwort diagram |
A hornwort with sporangia |
Phylum Anthocerophyta comprises the hornworts. Gametophytes are small, usually only one to two centimeters across, with no significant height. The gametophyte is made up of a thallus, a simple plant body resembling a mound of ruffled, dark-green leaflets.
The long, slender sporophyte generations grow upward from the thallus and are typically 0.5 to 12 centimeters in height. They are found across the world, especially in tropical rainforests. Hornworts grow on exposed, moist, shaded soil, and so they can be seen in places such as around the edges of lakes and rivers, as well as by the sides of roads.
Like other bryophytes, hornworts are anchored to the ground by thin rhizoids. Hornworts are closely related to marine algae, and were some of the first plants to colonize land. Though hornworts are terrestrial plants, they can be grown experimentally on top of water, and they survive quite well.
Anthocerophyta, the hornworts, named for the tall narrow sporophyte at the top of the plant, are divided into two groups by the most recent classification by Hasegawa in 1994: Notothyladacae and the larger Anthoceratacae. Due to a sparse fossil record and their unique morphology, there has been much debate among taxonomists and others concerning their classification.