Any member of four to six genera of creeping annual or perennial plants of the class Anthocerotopsida.
Hornworts usually grow on damp soils or on rocks in tropical and warm temperate regions. The gametophyte typically is a flattened structure covered with small irregular lobes; the sporophyte forms a tapered cylinder (see alternation of generations ). Rhizoids (rootlike structures) on the undersurface anchor the plant. Cavities in the gametophyte sometimes contain colonies of the cyanobacterium Nostoc (see cyanobacteria , nostoc ). Hornworts have a region of continuous growth at the base of the sporophyte, and a large, irregular foot. The stalk that attaches the foot to the spore-bearing capsule in liverworts is absent in hornworts.