Hornwort
Overview
Hornworts are a group of bryophyte
Bryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...

s, or non-vascular plant
Non-vascular plant
Non-vascular plants is a general term for those plants without a vascular system . Although non-vascular plants lack these particular tissues, a number of non-vascular plants possess tissues specialized for internal transport of water....

s, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte
Sporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...

. The flattened, green plant body of a hornwort is the gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

 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.
Encyclopedia
Hornworts are a group of bryophyte
Bryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...

s, or non-vascular plant
Non-vascular plant
Non-vascular plants is a general term for those plants without a vascular system . Although non-vascular plants lack these particular tissues, a number of non-vascular plants possess tissues specialized for internal transport of water....

s, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte
Sporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...

. The flattened, green plant body of a hornwort is the gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

 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.

Description

The plant body of a hornwort is a haploid gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

 stage. This stage usually grows as a thin rosette
Rosette (botany)
In botany, a rosette is a circular arrangement of leaves, with all the leaves at a single height.Though rosettes usually sit near the soil, their structure is an example of a modified stem.-Function:...

 or ribbon-like thallus between one and five centimeters in diameter. Each cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 of the thallus usually contains just one chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

 per cell. In most species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

, this chloroplast is fused with other organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

s to form a large pyrenoid
Pyrenoid
In cell biology, pyrenoids are organelles, centers of carbon dioxide fixation within the chloroplasts of algae and hornworts. Pyrenoids are not membrane-bound, but specialized areas of the plastid that contain high levels of ribulose-1,5-bisphosphate carboxylase/oxygenase .RubisCO fixes carbon...

 that both manufactures and stores food. This particular feature is very unusual in land plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s, but is common among algae.

Many hornworts develop internal mucilage
Mucilage
Mucilage is a thick, gluey substance produced by most plants and some microorganisms. It is a polar glycoprotein and an exopolysaccharide.It occurs in various parts of nearly all classes of plant, usually in relatively small percentages, and is frequently associated with other substances, such as...

-filled cavities when groups of cells break down. These cavities are invaded by photosynthetic cyanobacteria, especially species of Nostoc
Nostoc
Nostoc is a genus of cyanobacteria found in a variety of environmental niches that forms colonies composed of filaments of moniliform cells in a gelatinous sheath.The name "Nostoc" was invented by Paracelsus...

. Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue-green color. 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
Sporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...

 grows from an archegonium
Archegonium
An archegonium , from the ancient Greek ἀρχή and γόνος , is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female gamete. The archegonium has a long neck canal and a swollen base...

 embedded deep in the gametophyte. The sporophyte of a hornwort is unusual in that it grows from a meristem
Meristem
A meristem is the tissue in most plants consisting of undifferentiated cells , found in zones of the plant where growth can take place....

 near its base, instead of from its tip the way other plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s do. Unlike liverworts
Marchantiophyta
The Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryophytes, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....

, most hornworts have true stoma
Stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...

ta on their sporophyte as mosses do. The exceptions are the genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Notothylas
Notothylas
Notothylas is a genus of hornworts in the family Notothyladaceae. The genus is found globally, but is usually overlooked. It is the smallest of all the hornworts, with a yellow-green gametophyte thallus that is seldom more than a centimeter in diameter, and usually much smaller.The genus...

and Megaceros
Megaceros
Megaceros is a genus of hornworts in the family Dendrocerotaceae. The genus is found in the Old World tropics of east Asia and Australia. Its name means 'big horn', and refers both to the exceptionally large size of the gametophyte thallus and to the large, horn-shaped sporophyte that the plants...

, which do not have stomata.

When the sporophyte is mature, it has a multicellular outer layer, a central rod-like columella
Columella (botany)
Columel′la , an axis of sterile tissue which passes through the center of the spore-case of mosses....

 running up the center, and a layer of tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

 in between that produces spores and pseudo-elaters
Elater
An elater is a cell that is hygroscopic, and therefore will change shape in response to changes in moisture in the environment. Elaters come in a variety of forms, but are always associated with plant spores...

. The pseudo-elaters are multi-cellular, unlike the elaters of liverwort
Marchantiophyta
The Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryophytes, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....

s. 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 bryophyte
Bryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...

s, measuring between 30 and 80 µm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is 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. 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
Octant
An octant is one of eight divisions.-Octant in the plane :Traditionally wind direction is given as one of the 8 octants because that is more accurate than merely giving one of the 4 quadrants, and the wind vane typically does not have enough accuracy to bother with more precise indication.-Octant...

 of cells, and the first rhizoid
Rhizoid
Rhizoids are thread-like growths from the base or bottom of a plant, found mainly in lower groups such as algae, fungi, bryophytes and pteridophytes, that function like roots of higher plants ....

 grows as an extension of the original germ cell. The tip continues to divide new cells, which produces a thalloid protonema
Protonema
A protonema is a thread-like chain of cells that forms the earliest stage of a bryophyte life cycle...

. By contrast, species of the family Dendrocerotaceae
Dendrocerotaceae
The Dendrocerotaceae is the only family of hornworts in the order Dendrocerotales....

 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.

From the protonema grows the adult gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

, which is the persistent and independent stage in the life cycle. This stage usually grows as a thin rosette
Rosette (botany)
In botany, a rosette is a circular arrangement of leaves, with all the leaves at a single height.Though rosettes usually sit near the soil, their structure is an example of a modified stem.-Function:...

 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
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...

 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 monoicous
Bryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...

, with both sex organs on the same plant, but some plants (even within the same species) are dioicous
Bryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...

, with separate male and female gametophytes. The female organs are known as archegonia
Archegonium
An archegonium , from the ancient Greek ἀρχή and γόνος , is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female gamete. The archegonium has a long neck canal and a swollen base...

 (singular archegonium) and the male organs are known as antheridia
Antheridium
An antheridium or antherida is a haploid structure or organ producing and containing male gametes . It is present in the gametophyte phase of lower plants like mosses and ferns, and also in the primitive vascular psilotophytes...

 (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 sperm
Spermatozoon
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...

 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
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...

, 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
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

. Further divisions produce three basic regions of the sporophyte.

At the bottom of the sporophyte
Sporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...

 (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
Meristem
A meristem is the tissue in most plants consisting of undifferentiated cells , found in zones of the plant where growth can take place....

 that will continue to divide and produce new cells for the third region. This third region is the capsule. Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to produce pseudo-elaters
Elater
An elater is a cell that is hygroscopic, and therefore will change shape in response to changes in moisture in the environment. Elaters come in a variety of forms, but are always associated with plant spores...

 and spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...

s. These are released from the capsule when it splits lengthwise from the tip.

Evolutionary history

While the fossil record of crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...

 hornworts only begins in the upper Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

, the lower Devonian Horneophyton
Horneophyton
Horneophyton, a member of the Horneophytopsida, was an early plant which may form a "missing link" between the hornworts and the Rhyniopsida. It is among the most abundant organisms found in the Rhynie chert.-Description:...

may represent a stem group to the clade, as it possesses a sporangium
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...

 with central columella not attached at the roof. However, the same form of columella is also characteristic of basal moss groups, such as the Sphagnopsida
Sphagnopsida
The Sphagnopsida includes a single subclass Sphagnidae, with two orders. The order Sphagnales contains four living genera: Sphagnum, with all but three of the species, and Ambuchanania, Eosphagnum, and Flatbergium, each with a single species. The Protosphagnales contains a single fossil...

 and Andreaeopsida, and has been interpreted as a character common to all early land plants with stoma
Stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...

ta.

Classification

Hornworts were traditionally considered a class within the division Bryophyta (bryophyte
Bryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...

s). However, it now appears that this former division is paraphyletic, so the hornworts are now given their own division, Anthocerotophyta. The division Bryophyta
Bryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...

 is now restricted to include only moss
Moss
Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...

es.

Traditionally, there is 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
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

, each containing a single family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

.

Among land plants, hornworts appear to be one of the oldest surviving lineages; cladistic analysis implies that the group originated prior to the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

, around the same time as the mosses and liverworts. There are only about 100 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 known, but new species are still being discovered. The number and names of genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 are a current matter of investigation, and several competing classification schemes have been published since 1988.

Phylogeny

Recent studies of molecular, ultrastructural, and morphological data have yielded a new classification of hornworts.
order Leiosporocerotales
Leiosporocerotaceae
  • Leiosporoceros
    Leiosporoceros
    Leiosporoceros dussii is the only species in the hornwort genus Leiosporoceros. The species is placed in a separate family, order, and class for being "genetically and morphologically distinct from all other hornwort lineages." Cladistic analysis of genetic data supports a position at the very...

    (1 species)


order Anthocerotales
Anthocerotaceae
Anthocerotaceae
The Anthocerotaceae is the only family of hornworts in the order Anthocerotales....

  • Anthoceros
    Anthoceros
    Anthoceros is a genus of hornworts in the family Anthocerotaceae. The genus is global in its distribution. Its name means 'flower horn', and refers to the characteristic horn-shaped sporophytes that all hornworts produce...

    (ca. 83 species)
  • Folioceros
    Folioceros
    Folioceros is a genus of hornworts in the family Anthocerotaceae. The genus is common locally in the tropical and subtropical regions of Asia, growing on moist rocks, in fallow fields, and near waterfalls. It has a yellow-green gametophyte thallus that is crispy and translucent, with short...

    (17 species)
  • Sphaerosporoceros
    Sphaerosporoceros
    Sphaerosporoceros is a genus in the hornwort family Anthocerotaceae. It includes only two species, both originally published as species of Anthoceros but since transferred to the new genus....

    (2 species)


order Notothyladales
Notothyladaceae
Notothyladaceae
The Notothyladaceae is the only family of hornworts in the order Notothyladales....

  • Notothylas
    Notothylas
    Notothylas is a genus of hornworts in the family Notothyladaceae. The genus is found globally, but is usually overlooked. It is the smallest of all the hornworts, with a yellow-green gametophyte thallus that is seldom more than a centimeter in diameter, and usually much smaller.The genus...

    (21 species)
  • Phaeoceros
    Phaeoceros
    Phaeoceros is a genus of hornworts in the family Notothyladaceae. The genus is global in its distribution. Its name means 'yellow horn', and refers to the characteristic yellow spores that the plants produce in the horn-shaped sporophyte. The genus Phaeoceros was first recognized in 1951 by...

    (ca. 41 species)
  • Paraphymatoceros (1-2 species)
  • Hattorioceros (1 species)
  • Mesoceros (2 species)


order Phymatocerotales
Phymatocerotaceae
  • Phymatoceros
    Phymatoceros
    Phymatoceros is the only genus in the hornwort family Phymatocerotaceae and order Phymatocerotales. It includes only two species....

    (2 species)


order Dendrocerotales
Dendrocerotaceae
Dendrocerotaceae
The Dendrocerotaceae is the only family of hornworts in the order Dendrocerotales....

  • Dendroceros
    Dendroceros
    Dendroceros is a genus of hornworts in the family Dendrocerotaceae. The genus contains about 51 species native to tropical and sub-tropical regions of the world.- Description :...

    (43 species)
  • Megaceros
    Megaceros
    Megaceros is a genus of hornworts in the family Dendrocerotaceae. The genus is found in the Old World tropics of east Asia and Australia. Its name means 'big horn', and refers both to the exceptionally large size of the gametophyte thallus and to the large, horn-shaped sporophyte that the plants...

    (8 species)
  • Nothoceros
    Nothoceros
    Nothoceros is a genus of hornworts in the family Dendrocerotaceae. The genus is found in New Zealand, South America, and neotropical and eastern North America....

    (7 species)
  • Phaeomegaceros
    Phaeomegaceros
    Phaeomegaceros is a genus of hornworts in the family Dendrocerotaceae. It includes seven species....

    (7 species)
{clade| style=font-size:100%;line-height:100% 1=
}}
The current phylogeny
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...

and composition of the Anthocerotophyta.

External links

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