Hornworts are a group of non-vascular Embryophytes (land plants) constituting the division Anthocerotophyta (). The common name refers to the elongated horn-like structure, which is the sporophyte. As in mosses and liverworts, hornworts have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information; the flattened, green plant body of a hornwort is the gametophyte stage of the plant. Hornworts may be found worldwide, though they tend to grow only in places that are damp or humid. Some species grow in large numbers as tiny weeds in the soil of gardens and cultivated fields. Large tropical and sub-tropical species of Dendroceros may be found growing on the bark of trees. The total number of species is still uncertain. While there are more than 300 published species names, the actual number could be as low as 100–150 species.
Description Like all bryophytes, the dominant life phase of a hornwort is the haploid gametophyte. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter. Hornworts have lost two plastid division-associated genes, ARC3 and FtsZ2, and have just a single chloroplast per cell (monoplastidy), with the exception of the genus Megaceros and some species in the genera Nothoceros and Anthoceros, which have more than one chloroplast per cell (polyplastidy). In the polyplastidic species, and also some of the monoplastidic species, a cellular structure called a pyrenoid is absent. The pyrenoid is a liquid-like organelle which enables a more efficient photosynthesis, has evolved independently five to six times in hornworts and is present in half of the roughly 200 species. It is formed by the fusion of the chloroplast with other organelles and is composed predominantly of RuBisCO, the key enzyme in carbon fixation. By using inorganic carbon transporters and carbonic anhydrases, up to a 50-fold increase in CO2 levels can be achieved. This particular feature is very unusual in land plants, unique to hornworts, but is common among algae. They are also the only group of land plants where flavonoids are completely absent. Many hornworts develop internal mucilage-filled cavities or canals when groups of cells break down. These cavities secrete hormogonium-inducing factors (HIF) 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 blue-green color. Symbiotic cyanobacteria have not been reported in Megaceros or Folioceros. There may also be small slime pores on the underside of the thallus. These pores superficially resemble the stomata of other plants. The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte. The growth of the hornwort sporophyte happens from a persistent basal meristem, in contrast to the sporophyte of moss (apical growth) and liverworts (intercalary growth). Unlike liverworts, hornworts have true stomata on their sporophyte as most mosses do. The exceptions are the species Folioceros incurvus, the genus Notothylas and the three closely related genera Megaceros, Nothoceros and Dendroceros, which do not have stomata. Notothylas also differ from other hornworts in having a reduced sporophyte only a few millimeters tall. The sporophyte in hornworts is unique among bryophytes in being long-lived with a persistent photosynthetic capacity. The sporophyte lacks an apical meristem, an auxin-sensitive point of divergence with other land plants some time in the Late Silurian/Early Devonian. When the sporophyte is mature, it has a multicellular outer layer, a central rod-like columella running up the center, and a layer of tissue in between that produces spores and pseudo-elaters. The pseudo-elaters are multi-cellular, unlike the elaters of liverworts. They have helical thickenings that change shape in response to drying out; they twist and thereby help to disperse the spores. Hornwort spores are relatively large for bryophytes, measuring between 30 and 80 μm in diameter or more. The spores are polar, usually with a distinctive Y-shaped tri-radiate ridge on the proximal surface, and with a distal surface ornamented with bumps or spines.
Life cycle The life of a hornwort starts from a haploid spore. The spores can be yellow, brown or green. Yellow and brown spores have a thicker wall and contain oils that both protect against desiccation and function as a nutrient storage, allowing them to survive for years. The species Folioceros fuciformis and the genera Megaceros, Nothoceros and Dendroceros have short-lived spores with thin and colorless walls that appear green due to the presence of a chloroplast. In most species, there is a single cell inside the spore, and a slender extension of this cell called the germ tube germinates from the proximal side of the spore. The tip of the germ tube divides to form an octant (solid geometry) of cells, and the first rhizoid grows as an extension of the original germ cell. The tip continues to divide new cells, which produces a thalloid protonema. By contrast, species of the family Dendrocerotaceae may begin dividing within the spore, becoming multicellular and even photosynthetic before the spore germinates. In either case, the protonema is a transitory stage in the life of a hornwort.
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