Hornworts, like liverworts and mosses, are land plants that do not have a vascular system. Their lack of veinlike tubes to conduct moisture and nutrients throughout the plant limits them to a small size. Like ferns, mosses, and liverworts, they produce spores instead of seeds. Their form of reproduction usually requires them to be in wet or moist places.
Like mosses and liverworts, hornworts lack true stems, leaves, and roots. The main vegetative part of the hornwort is a flattened, leaflike or ribbonlike body called a thallus. Out of the thallus, spore-bearing structures (sporophytes) grow as upright, often elongated, stalks that resemble horns, hence the name. These stalks start out looking like little prongs or bullets and, in many cases, continue to grow taller; eventually they split lengthwise to release tiny spores. See Life Cycle below to learn more about the vegetative body and reproductive structures of these plants.
Hornworts often have a distinct bluish-green hue and greasy look, which might help you notice them at a glance.
Specialists identify the various species of hornworts using microscopes to examine the cell structure, spores, and other tiny details.
Four species of hornworts have been recorded for Missouri.
- The Carolina phaeoceros, Phaeoceros carolinianus (syn. P. mohrii), has been reported from at least 27 counties in Missouri and appears to be the most widespread and commonly encountered hornwort in the state. Phaeoceros means “yellow horn,” referring to the yellow spores shed by the sporophyte; as the spores mature, the tips of the horns usually appear yellowish as a result.
- The Oregon phaeoceros, Phaeoceros oreganus, has been recorded only from Howell County. Like the Carolina phaeoceros, it sheds yellow spores, and it has sometimes been considered the same species as the Carolina phaeoceros.
- The field hornwort, Anthoceros agrestis, is dark green and rather frilly and grows in a rosette shape up to about ½ inch in diameter. The spores are black, so the sporophyte “horns” turn black as they mature. Apparently this species is scattered statewide; it occurs throughout the Northern Hemisphere.
- The round notothylas, Notothylas orbicularis, has been recorded in Missouri only from Johnson and Mississippi counties. Like others in its genus, it is a tiny flat rosette, usually much less than ½ inch in diameter. The sporophyte is shaped like a bullet or tiny finger, grows outward (sideways) instead of upward, and rarely exceeds an eighth of an inch in length. Use a hand lens!
Similar species:
- There are about 112 species of liverworts in our state, and the liverworts with a thalloid or thallose growth pattern — looking like green ribbons or scales growing flat against a surface — could easily be confused with hornworts. Liverworts, however, are in a completely different division within the plant kingdom. They do not have the horn, thread, or bullet-shaped sporophytes that are characteristic of hornworts. Also, most liverworts lack the distinctive bluish-green hue and greasy look more typical of hornworts.
- Certain lichens have flattened forms similar to hornwort and liverworts, but most have a drier texture and lack the anatomy of these small plants. One lichen, the common powderhorn (Cladonia coniocraea), has fruiting structures that look like upright horns, but the dry, powdery, scaly texture, pale greenish-gray color, and lack of a leafy, liverwort-like thallus, easily identify it.
- Finally, there is a very common, submerged or floating aquatic vascular plant called hornwort (also coontail or coon’s tail, Ceratophyllum demersum). It is similar in name only. Its needlelike leaves are grouped in whorls around the stem, and the plant branches prolifically to form large, tangled masses that float freely in the water. It looks nothing like the hornworts that are the focus of this page.
Diameter: A patch of hornworts usually only covers an area a few inches wide. An individual thallus (leaflike vegetative body) may be less than ½ inch in diameter, depending on species.
Statewide. Different species can have different habitats and distributions. The distribution of Missouri’s hornworts is poorly known, because certain parts of the state have been more thoroughly studied than others.
Habitat and Conservation
Hornworts, like the very similar-looking liverworts, usually occur in moist or wet, cool, shady places such as along stream banks, on the terraces above streams, cool valleys on the north sides of slopes, and so on. Certain species are associated with particular substrates and habitats. You might find hornworts in surprisingly ordinary places such as farm fields and roadsides. They are annuals (living for only one year) and are most often found taking advantage of bare, disturbed soils.
Missouri’s hornworts are species that generally occur in most of eastern North America. Missouri represents the westernmost reaches of some of these species.
Status
Of Missouri’s four species of hornworts, three are listed as species of conservation concern, meaning that they are rare or are to some degree imperiled within our state. The three are Anthoceros agrestis, Notothylas orbicularis, and Phaeoceros oreganus .
Taxonomy: hornworts (division Anthocerotophyta) are an oddball group of plants, and they are not well understood by scientists. For a long time, they were grouped together with liverworts and mosses into a single division, or main branch, of the plant kingdom called Bryophyta. Now, however, the three groups are each in separate divisions. Only the mosses remain in division Bryophyta. Still, hornworts, liverworts, and mosses are all informally called bryophytes — with a little b — because of their many similarities. Although they share many characteristics, and all have ancient lineages and are considered “primitive” (ancestral) plants, hornworts, liverworts, and mosses do not represent a single branch of a taxonomic tree, so grouping them together is informal.
As they are doing with many other groups of organisms, scientists are using molecular DNA studies to understand the relationships among hornworts and their relatives.
Life Cycle
Similar to the life cycle of liverworts, mosses, and ferns, the two-parted life cycle of hornworts involves what are technically two different plants. The green, flattened, leafy or low rosette structure, called a thallus, is the gametophyte generation. In separate organs, the gametophyte produces gametes (sex cells, that is, sperm and eggs). Under damp conditions, the sperm can swim to the eggs; when they unite, they form a sporophyte plant that grows out of the egg-bearing organ of the gametophyte. The sporophyte, then, is the horn, thread, or bullet-shaped structure that grows out of the thallus. The sporophyte generation produces spores. When the spores mature, are shed, and find suitable growing conditions, they may germinate and grow into a new gametophyte.
Human Connections
Scientists are intrigued by hornworts, whose relationship with other plants is unique. For example, some believe hornworts may represent a separate lineage from all other land plants. Among their unusual features are their form, growth pattern, certain cell structures, the way the cells divide, an unusual symbiotic relationship with cyanobacteria (blue green algae), and the suite of chemicals they produce. For one example, it's remarkable that hornwort sporophyte stalks grow taller by adding cells at the base of the stalk instead of at the tip. (In nearly all other plants, stems and roots grow longer by adding cells at the growing tip.)
Hornworts and liverworts are understudied globally, mainly because they’re so small and identifying them can be tedious. Estimates of worldwide numbers range from between 100 to more than 300 species. Hornworts are most diverse in tropical regions. Many botanists are focusing their efforts in the tropics, where thousands of plant species remain undescribed.
Learning to identify these tiny plants takes time and practice, but it can be a fun challenge. Some people find great joy in “geeking out” over plant identification. The Internet provides a host of resources for anyone wishing to start learning more about these unusual plants.
Ecosystem Connections
Many types of hornworts have colonies of cyanobacteria (blue green algae; especially those in genus Nostoc) living in small internal chambers of the gametophyte. As they do in the root nodules of legumes, these cyanobacteria can transform atmospheric nitrogen into a form the plant can use as a nutrient. These colonies of algae can give hornworts a greasy, blue-green hue.
Hornworts, liverworts, and mosses can be the first colonizers on eroded streambanks, flooded areas, or other barren, disturbed soils, preventing further erosion and gradually changing the environment for other, larger plants to take root and survive.
The bryophyte flora of an area, including hornworts, liverworts, and mosses, is an important micro habitat for springtails, insects, centipedes, sowbugs, and other tiny animals. The presence of these invertebrates makes bryophyte-rich areas a prime hunting ground for shrews, salamanders, frogs, tiny snakes, and other insectivores.
Mosses, liverworts, hornworts, and lichens seem rather similar, but these organisms are in very different groups. Mosses, liverworts, and hornworts are small, low plants usually found in damp habitats. Unlike more familiar plants, they lack veinlike structures and do not produce flowers or seeds — instead, they produce spores. Meanwhile, lichens are not plants at all: they are a collection of different fungi that have photosynthetic algae living within their tissues.