Hornwort are a group of bryophytes ( a group of non – vascular plants ) constituting the division Anthoceratophyta ( / ˌ n oʊ s r t f t , – oʊ f aɪ t / ) . The common name refers to the elongated horn-like structure, which is the sporophyte . As in mosses and liverworts , the flattened, green plant body of a hornwort is the gametophyte plant.
Hornworts can be found all over the world, although they only grow in places that are moist or humid. Some species grow in large numbers as small weeds in the soil of gardens and cultivated fields. Large tropical and subtropical species of Dendroceros can be found on the bark of trees .
The total number of species is still uncertain. While there are over 300 published species names, the actual number may be as low as 100–150 species.
Stottler & Stottler-Krand., 1977
The plant body of hornwort is a haploid gametophyte stage. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter. There is usually only one chloroplast in each cell of the thallus . In roughly half of 200 hornwort species , this chloroplast is fused with other organs to form a large pyrenoid that enables both more efficient photosynthesis and of food. Pyrenoid mainly contains RuBisCO, key enzyme in carbon fixation. By using an inorganic carbon transporter and carbonic anhydrase, an increase of up to 50-fold in CO levels can be achieved.  This particular feature is very unusual in land plants, unique to hornworts but common among algae.
Many hornworts develop internally phlegm-filled cavities or canals when clusters of cells break up. They will secrete harmonium-inducing factors (HIFs) that stimulate nearby, free-living photosynthetic cyanobacteria, especially species of Nostoc, to invade and colonize these cavities .  Such colonies of bacteria growing inside the thallus give the hornwort a distinctive bluish-green color. There may also be small slime pores on the underside of the thallus. These pores are superficially similar to the stomata of other plants.
The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte. The sporophyte of hornwort is unusual in that it grows from a meristem near its base, and not from its tip in the way that other plants do. Unlike liverworts, most hornworts have true stomata on the sporophyte as do mosses. The exceptions are the genera Notothylus and Megaceros , which do not have stomata. The sporophyte of most hornworts is also photosynthetic, which is not the case with liverworts.  The sporophyte lacks an apical meristem, an auxin sensitive point of divergence in other ground plants with some time in the Late Silurian/Early Devonian.  
When the sporophyte matures, it has a multicellular outer layer, a central rod-like columella running over the center, and a layer of tissue in between that produces spores and pseudo-elators. Pseudo elaters are multi-cellular, unlike elaters liverworts. They have helical thickeners that change shape in response to drying; They are twisted and thus help in spreading the spores. Hornwort spores are relatively large for bryophytes, ranging in diameter between 30 and 80 µm or more. Spores are polar, usually with a distinctive Y-shaped tri-radial ridge on the proximal surface, and with a distal surface ornamented with bumps or spines.
Hornwort’s life begins with a haploid spore. In most species, there is a single cell inside the spore, and a thin extension of this cell, called the germ tube , germinates from the proximal side of the spore.  The end of the germ tube divides to form an octahedron (solid geometry) of cells, and the first rhizoid grows as an extension of the original germ cell. [ clarification needed ] The tip continues to divide to form new cells, which produce a thalloid protonema. In contrast, species in the Dendrocerotaceae family can begin to divide within the spore, becoming multicellular and even photosynthetic before the spore germinates. In any case, the protonema is a transitory stage in a hornwort’s life.
From the protonema grows the adult gametophyte, which is a persistent and independent stage in the life cycle. This stage usually grows in the form of a thin rosette or ribbon-like thallus between several layers of cells between one and five centimeters in diameter and thickness. It is green or yellow-green with chlorophyll in its cells, or blue-green when colonies of cyanobacteria grow inside the plant.
When the gametophyte grows to its adult size, it produces the sex organs of the hornwort. Most plants are monoecious with both sex organs on the same plant, but some plants (even within the same species) are dimorphic, with separate male and female gametophytes. The female organs are known as archegonia (singular archegonium) and the male organs are known as antheridia (singular antheridium). Both types of organs develop just below the surface of the plant and are exposed later only by the disintegration of the cells above them.
The biflagellate spermatozoa must swim from the antheridia, or else disintegrate into the archegonia. When this happens, the sperm and egg cell fuse to form a zygote, the cell from which the sporophyte stage of the life cycle will develop. Unlike all other bryophytes, the first cell division of the zygote is longitudinal. Further divisions produce the three basic regions of the sporophyte.
At the bottom of the sporophyte (closest to the interior of the gametophyte), there is a leg. It is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence. In the middle of the sporophyte (just above the leg), there is a meristem that will continue to divide and produce new cells for the third zone. This is the third field capsule. Both the central and superficial cells of the capsule are sterile, but there is a layer of cells between them that will divide to produce pseudo-elaters and spores. They are released from the capsule when it splits lengthwise from the end.
While the fossil record of the crown group hornworts begins only in the Upper Cretaceous, the lower Devonian may represent a stem group for the Horniophyton clade, as it consists of a sporangium with a central columella attached to the roof.  However, the same form of columella is also characteristic of basal moss groups, such as Sphagnopsida and Andriopsida, and has been interpreted as a character common to all early land plants with stomata.  Even though the split between hornworts and other bryophytes occurred much earlier, it is estimated that the last common ancestor of extant hornworts lived in the middle Permian about 27.5 million years ago. Chromosome-scale genome sequencing of three hornwort species confirms that stomata developed only once during the evolution of land plants. This also suggests that the three groups of bryophytes share a common ancestor that diverged from other landplants early in evolution, and that liverworts and mosses are more closely related to each other than to hornworts. . 
Hornworts are unique in having a genus called LCIB, which is not found in any other known land plants but in some species of algae. This allows them to concentrate carbon dioxide inside their chloroplasts, making sugar production more efficient.
Hornworts were traditionally considered a class within the division Bryophyta (Bryophytes). However, it now appears that this former division is paraphyletic, so hornworts are now given their own division, Anthoceratophyta (sometimes misspelled Anthocerophyta ). The division Bryophyta is now restricted to include only mosses.
Traditionally, hornworts belonged to a single class, called Anthoceratopsida or older Anthocerote . Recently, a class II Leosporocerotopsida has been isolated for the singularly unusual species Liosporoceros dusi . All other hornworts live in the class Anthoceratopsida. These two classes are further divided into five orders, each of which has a single family.
Among land plants, hornworts are one of the earliest-divergent lineages of early land plant ancestors;  Cladistic analysis implies that the group originated before the Devonian, around the same time as mosses and liverworts. About 200 species are known, but new species are still being discovered. The number and name of the genera is a current matter of investigation, and several competing classification schemes have been published since 1988.
Structural features that have been used in the classification of hornworts include: the anatomy of the chloroplasts and their number within cells, the presence of a pyrenoid, the number of antheridia within the androecia, and the arrangement of the jacket cells of the antheridia. 
Recent studies of molecular, infrastructural and morphological data have yielded a new classification of hornworts.
- Leosporoceros (1 species)
- Anthoceros (ca. 83 species)
- Folioceros (17 species)
- Spherosporoceros (2 species)
- Notothylus (21 species)
- Fioceros (ca. 41 species)
- Paraphymatoceros (1-2 species)
- Hettorioceros (1 species)
- Mesoceros (2 species)
order Phymatocerotales order
- Phymatoceros (2 species)
- Dendroceros (43 species)
- Megaceros (8 species)
- Nothoceros (7 species)
- Pheomegaceros (7 species)