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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.
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 Marchantiophyta, hornworts have true 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. Classification of the Phylum Anthocerotophyta Stotl. & Crand.-Stotl. 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 elater. The pseudo-elaters are multi-cellular, unlike the elaters of Marchantiophyta. They have Helix thickenings that change shape in response to drying out; they twist and thereby help to disperse the spores. Hornwort spores are relatively large for , measuring between 30 and 80 micrometre 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.
From the protonema grows the adult gametophyte, which is the persistent and independent stage in the life cycle. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter, and several layers of cells in thickness. It is green or yellow-green from the chlorophyll in its cells, or bluish-green when colonies of cyanobacteria grow inside the plant.
When the gametophyte has grown to its adult size, it produces the sex organs of the hornwort. Most plants are Bryophyte, with both sex organs on the same plant, but some plants (even within the same species) are Bryophyte, with separate male and female gametophytes. The female organs are known as archegonium (singular archegonium) and the male organs are known as antheridium (singular antheridium). Both kinds of organs develop just below the surface of the plant and are only later exposed by disintegration of the overlying cells.
The biflagellate spermatozoon must swim from the antheridia, or else be splashed to 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 three basic regions of the sporophyte.
At the bottom of the sporophyte (closest to the interior of the gametophyte), is a foot. This 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 foot), is a meristem that will continue to divide and produce new cells for the third region. This third region is the Sporangia. Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to produce elater and . These are released from the capsule when it splits lengthwise from the tip.
Traditionally, there was a single class of hornworts, called Anthocerotopsida, or older Anthocerotae. More recently, a second class Leiosporocertotopsida has been segregated for the singularly unusual species Leiosporoceros dussii. All other hornworts remain in the class Anthocerotopsida. These two classes are divided further into five orders, each containing a single family.
Among land plants, hornworts are one of the earliest-diverging lineages of the early land plant ancestors; cladistic analysis implies that the group originated prior to the Devonian, around the same time as the mosses and liverworts. There are about 200 species known, but new species are still being discovered. The number and names of genus are 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 chloroplasts and their numbers within cells, the presence of a pyrenoid, the numbers of antheridium within androecia, and the arrangement of jacket cells of the antheridia.
Class Leiosporocerotopsida
Class Anthocerotopsida
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| The current phylogenetics and composition of the Anthocerotophyta. | |
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