Hornwort
| Hornwort Temporal range: Upper Cretaceous (but see text) to present
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|---|---|
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| Phaeoceros laevis (L.) Prosk. | |
Scientific classification 
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| Kingdom: | Plantae |
| Clade: | Embryophytes |
| Division: | Anthocerotophyta Stotler & Stotl.-Crand., 1977[1] |
| Classes and orders | |
see Classification. | |
| Synonyms | |
|
Anthocerotae | |
Hornworts are a group of
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
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.[2]
Description
Like all bryophytes, the dominant life phase of a hornwort is the
Many hornworts develop internal
The horn-shaped
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
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 monoecious, with both sex organs on the same plant, but some plants (even within the same species) are dioecious, 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 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 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
Evolutionary history
![[icon]](/File:Wiki_letter_w_cropped.svg){kind=small} | This section needs expansion. You can help by adding to it. (June 2008) |
While the fossil record of
Classification
Hornworts were traditionally considered a class within the division Bryophyta (bryophytes). Later on, the bryophytes were considered paraphyletic, and hence the hornworts were given their own division, Anthocerotophyta (sometimes misspelled Anthocerophyta). However, the most recent phylogenetic evidence leans strongly towards bryophyte monophyly,[26][27][28][29][30][31][24][32][33][excessive citations] and it has been proposed that hornworts are de-ranked to the original class Anthocerotopsida.[28]
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 .
Among land plants, hornworts are one of the earliest-diverging lineages of the early land plant ancestors;[24] 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 genera 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 antheridia within androecia, and the arrangement of jacket cells of the antheridia.[34]
Phylogeny
Recent studies of molecular, ultrastructural, and morphological data have yielded a new classification of hornworts.[35][36]
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Class Leiosporocerotopsida
Class Anthocerotopsida
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| The current phylogeny and composition of the Anthocerotophyta.[35][37][38][39] | ||||||||||||||||||||||||||||||||||||||||||||||
See also
References
- JSTOR 3242017.
- ^ Lepp, Heino (12 September 2012). "What is a hornwort?". Australian Bryophytes. Australian National Botanic Gardens.
- ^ Loss of plastid developmental genes coincides with a reversion to monoplastidy in hornworts - bioRxiv
- ^ Hornworts: An Overlooked Window into Carbon-Concentrating Mechanisms - Villarreal Lab
- ^ Hornwort pyrenoids, carbon-concentrating structures, evolved and were lost at least five times during the last 100 million years
- PMID 27194106.
- ^ Hornwort pyrenoids, carbon-concentrating structures, evolved and were lost at least five times during the last 100 million years - PNAS
- ^ BTI researchers unlocking hornworts' secrets | EurekAlert! Science News
- JSTOR 1313353.
- ^ Stress Biology of Cyanobacteria: Molecular Mechanisms to Cellular Responses
- ^ How was apical growth regulated in the ancestral land plant? Insights from the development of non-seed plants
- ^ Hornwort Stomata: Architecture and Fate Shared with 400-Million-Year-Old Fossil Plants without Leaves
- ^ Classification of the Phylum Anthocerotophyta Stotl. & Crand.-Stotl.
- ^ The deepest divergences in land plants inferred from phylogenomic evidence - PNAS
- ISBN 978-0-12-339552-8.
- PMID 21652320.
- ^ Bryophyte Biology
- ^ NEW CLASSIFICATION OF ANTHOCEROTAE - J-Stage
- ^ ISBN 0-470-21359-0.
- PMID 17030812.
- ISBN 1-56098-730-8.
- PMID 36175544.
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- ^ PMID 32170292.
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- ^ S2CID 85582943.
- ^ Cole, Theodor C. H.; Hilger, Hartmut H.; Goffinet, Bernard. "Bryophyte phylogeny poster: systematics and Characteristics of Nonvascular Land Plants (Mosses, Liverworts, Hornworts)". 2021. Retrieved 6 December 2022.
- .
- S2CID 209591279.
- ^ Brinda, John C.; Atwood, John J. "The Bryophyte Nomenclator". 7 December 2022. Retrieved 7 December 2022.
- Grolle, Riclef (1983). "Nomina generica Hepaticarum; references, types and synonymies". Acta Botanica Fennica. 121: 1–62.
- Hasegawa, J. (1994). "New classification of Anthocerotae". Journal of the Hattori Botanical Laboratory. 76: 21–34.
- Renzaglia, Karen S. (1978). "A comparative morphology and developmental anatomy of the Anthocerotophyta". Journal of the Hattori Botanical Laboratory. 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: 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. Vol. 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.