Raffi at 06:45, 7 April 2007

2007-04-07T06:45:18Z

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:''This is an article about the non-vascular plants. The name Hornwort is also often applied to an aquatic plant ''Ceratophyllum demersum'' in the family [[Ceratophyllaceae]]''

{{Taxobox
| color = lightgreen
| name = Hornwort
| image = Anthoceros levis.jpg
| image_width = 240px
| image_caption = Phaeoceros laevis ([[Carolus Linnaeus|L.]]) [[Johannes Max Proskauer |Prosk.]]
| regnum = [[Plant]]ae
| divisio = '''Anthocerotophyta'''
| classis = '''Anthocerotopsida'''
| ordo = '''Anthocerotales'''
| subdivision_ranks = Families & Genera
| subdivision =
[[Anthocerotaceae]]
* ''[[Anthoceros]]''
* ''[[Folioceros]]''
* ''[[Leiosporoceros]]''
* ''[[Phaeoceros]]''
* ''[[Sphaerosporoceros]]''
[[Dendrocerotaceae]]
* ''[[Dendroceros]]''
* ''[[Megaceros (botany) |Megaceros]]''
* ''[[Notoceros]]''
[[Notothyladaceae]]
* ''[[Notothylas]]''
}}

'''Hornworts''' are a group of [[bryophyte]]s, or [[non-vascular plant]]s, comprising the division '''Anthocerotophyta'''. The common name refers to the elongated horn-like structure, which is the [[sporophyte]]. The flattened, green plant body of a hornwort is the [[gametophyte]] plant.

Hornworts may be found world-wide, 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]] stage. This stage usually grows as a thin [[rosette (botany) |rosette]] or ribbon-like [[thallus (tissue) |thallus]] between one and five [[centimeter]]s in diameter. Each [[cell (biology) |cell]] of the thallus usually contains just one [[chloroplast]] per cell. In most [[species]], this chloroplast is fused with other [[organelle]]s to form a large [[pyrenoid]] that both manufactures and stores food. This particular feature is very unusual in land [[plant]]s, but is common among [[alga]]e.

Many hornworts develop internal [[mucilage]]-filled cavities when groups of cells break down. These cavities are invaded by [[photosynthetic]] [[cyanobacteria]], especially species of ''[[Nostoc]]''. 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.

[[Image:Phaeoceros_spores.jpg|thumb|160px|left|Spores of ''Phaeoceros sp.'' showing the Y-shaped triradiate ridge.]]
The horn-shaped [[sporophyte]] grows from an [[archegonium]] embedded deep in the gametophyte. Hornworts sporophytes are unusual in that the sporophyte grows from a [[meristem]] near its base, instead of from its tip the way other [[plant]]s do. Unlike [[Marchantiophyta|liverwort]]s, most hornworts have true [[stoma]]ta on the sporophyte as mosses do. The exceptions are the [[genus |genera]] ''[[Notothylas]]'' and ''[[Megaceros (botany) |Megaceros]]'', which do not have stomata.

When the sporophyte is mature, it has a multicellular outer layer, a central rod-like [[columella (botany)|columella]] running up the center, and a layer of [[tissue (biology)|tissue]] in between that produces spores and [[elater|pseudo-elaters]]. The pseudo-elaters are multi-cellular, unlike the elaters of [[Marchantiophyta|liverwort]]s. They have [[helical]] thickenings that change shape in response to drying out, and thereby twist in and thereby help to disperse the spores. Hornwort spores are relatively large for [[bryophyte]]s, measuring between 30 and 80 [[micrometre|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]] 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.

[[Image:Hornwort structures.jpg|thumb|300px|left|Life cycle of a typical hornwort ''Phaeoceros''. Click on the image to enlarge.]]

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 (botany) |rosette]] or ribbon-like [[thallus (tissue) |thallus]] between one and five [[centimeter]]s 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|monoicous]], with both sex organs on the same plant, but some plants (even within the same species) are [[Bryophyte|dioicous]], with separate male and female gametophytes. The female organs are known as [[archegonium|archegonia]] (singular archegonium) and the male organs are known as [[antheridium|antheridia]] (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|sperm]] 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|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 [[elater|pseudo-elaters]] and [[spore]]s. These are released from the capsule when it splits lengthwise from the tip.

== Classification of Hornworts ==

Hornworts were traditionally considered a class within the Division Bryophyta ([[bryophyte]]s). However, it now appears that this group is [[paraphyletic]], so the hornworts tend to be given their own division, called '''Anthocerotophyta'''. The [[Bryophyta]] is now restricted to include only [[moss]]es.

There is a single class of hornworts, called '''Anthocerotopsida''', or traditionally '''Anthocerotae'''. This class includes a single order of hornworts ('''Anthocerotales''') in this [[scientific classification |classification scheme]]. In some other classification schemes, a second order '''Notothyladales''' (containing only the [[genus]] ''[[Notothylas]]'') is recognized because of the unique and unusual features present in that group.

Among land plants, hornworts appear to be one of the oldest surviving groups. There are only about 100 [[species]] known, but new species are still being discovered. The number and names of [[genus |genera]] are a current matter of investigation, and several competing classification schemes have been published since 1988.

[[Image:Dendroceros.jpg|thumb|200px|The hornwort ''[[Dendroceros]] crispus'' growing on the bark of a tree.]]
==== Families and genera ====
'''[[Anthocerotaceae]]'''
* ''[[Anthoceros]]''
* ''[[Folioceros]]''
* ''[[Leiosporoceros]]''
* ''[[Phaeoceros]]''
* ''[[Sphaerosporoceros]]''
'''[[Dendrocerotaceae]]'''
* ''[[Dendroceros]]''
* ''[[Megaceros (botany) |Megaceros]]''
* ''[[Notoceros]]''
'''[[Notothyladaceae]]'''
* ''[[Notothylas]]''

== See also ==
* [[Embryophyte]]
* [[Bryophyte]]

== References ==

* Chopra, R. N. & Kumra, P. K. (1988). ''Biology of Bryophytes''. New York: John Wiley & Sons. ISBN 0-470-21359-0.
* Grolle, Riclef (1983). "Nomina generica Hepaticarum; references, types and synonymies". ''Acta Botanica Fennica'' '''121''', 1-62.
* Hasegawa, J. (1994). "New classification of Anthocerotae". ''J. Hattori Bot. Lab'' '''76''': 21-34.
* Renzaglia, Karen S. (1978). "A comparative morphology and developmental anatomy of the Anthocerotophyta". ''J. Hattori Bot. Lab'' '''44''': 31-90.
* Renzaglia, Karen S. & Vaughn, Kevin C. (2000). Anatomy, development, and classification of hornworts. In A. Jonathan Shaw & Bernard Goffinet (Eds.), ''Bryophyte Biology'', pp. 1-20. Cambridge: [[Cambridge University Press]]. ISBN 0-521-66097-1.
* Schofield, W. B. (1985). ''Introduction to Bryology''. New York: Macmillan.
* Schuster, Rudolf M. (1992). ''The Hepaticae and Anthocerotae of North America, East of the Hundredth Meridian, Volume VI''. Chicago: [[Field Museum of Natural History]].
* Smith, Gilbert M. (1938). ''Cryptogamic Botany, Volume II: Bryophytes and Pteridophytes''. New York: McGraw-Hill Book Company.
* Watson, E. V. (1971). ''The Structure and Life of Bryophytes'' (3rd ed.). London: Hutchinson University Library. ISBN 0-09-109301-5.

== External links ==
* [http://www3.uakron.edu/biology/hornworts/hornworts.html Hornwort Web Portal]
* [http://koning.ecsu.ctstateu.edu/Plant_Biology/hornwort.html Hornwort biology information]
* [http://www.ucmp.berkeley.edu/plants/anthocerotophyta.html Anthocerotophyta description and fossil history at UCMP]
* [http://www.natureserve.org/explorer/speciesIndex/Class_Anthocerotopsida_106589_1.htm Hornwort species in the United States and Canada]
* [http://www.peripatus.gen.nz/Taxa/Bryophyta/NZAnthocerotae.html New Zealand Anthocerotae]

[[Category: Bryophytes]]

Hornwort - Revision history

Raffi at 06:45, 7 April 2007