It is estimated that there are about 9000 species of liverworts.[4] Some of the more familiar species grow as a flattened leaflessthallus, but most species are leafy with a form very much like a flattenedmoss. Leafy species can be distinguished from the apparently similar mosses on the basis of a number of features, including their single-celledrhizoids. Leafy liverworts also differ from most (but not all) mosses in that their leaves never have acosta (present in many mosses) and may bear marginalcilia (very rare in mosses). Other differences are not universal for all mosses and liverworts, but the occurrence of leaves arranged in three ranks, the presence of deep lobes or segmented leaves, or a lack of clearly differentiated stem and leaves all point to the plant being a liverwort. Liverworts are distinguished from mosses in having uniquecomplex oil bodies of high refractive index.
Liverworts are typically small, usually from 2–20 mm (0.079–0.787 in) wide with individual plants less than 10 cm (3.9 in) long, and are therefore often overlooked. However, certain species may cover large patches of ground, rocks, trees or any other reasonably firm substrate on which they occur. They are distributed globally in almost every available habitat, most often in humid locations although there are desert and Arctic species as well. Some species can be a nuisance in shady greenhouses or a weed in gardens.[5]
Most liverworts are small, measuring from 2–20 millimetres (0.08–0.8 in) wide with individual plants less than 10 centimetres (4 in) long,[6] so they are often overlooked. The most familiar liverworts consist of a prostrate, flattened, ribbon-like or branching structure called athallus (plant body); these liverworts are termedthallose liverworts. However, most liverworts produce flattened stems with overlapping scales or leaves in two or more ranks, the middle rank is often conspicuously different from the outer ranks; these are calledleafy liverworts orscale liverworts.[7][8] (See thegallery below for examples.)
Liverworts can most reliably be distinguished from the apparently similarmosses by their single-celledrhizoids.[9] Other differences are not universal for all mosses and all liverworts;[8] but the lack of clearly differentiated stem and leaves in thallose species, or in leafy species the presence of deeply lobed or segmented leaves and the presence of leaves arranged in three ranks,[10][11] as well as frequent dichotomous branching, all point to the plant being a liverwort. With a few exceptions, all liverworts undergo polyplastidic meiosis, in contrast to mosses and hornworts which have monoplastidic meiosis.[12] Unlike any other embryophytes, most liverworts containunique membrane-bound oil bodies containing isoprenoids in at least some of their cells, lipid droplets in the cytoplasm of all other plants being unenclosed.[13] The overall physical similarity of some mosses and leafy liverworts means that confirmation of the identification of some groups can be performed with certainty only with the aid ofmicroscopy or an experiencedbryologist.
Liverworts, like other bryophytes, have agametophyte-dominant life cycle, with thesporophyte dependent on the gametophyte.[13] The sporophyte of many liverworts are non-photosynthetic, but there are also several that are photosynthetic to various degrees.[14] Cells in a typical liverwort plant each contain only a single set of genetic information, so the plant's cells arehaploid for the majority of its life cycle. This contrasts sharply with the pattern exhibited by nearly all animals and by vascular plants. In the more familiarseed plants, the haploid generation is represented only by the tinypollen and theovule, while thediploid generation is the familiar tree or other plant.[15] Another unusual feature of the liverwort life cycle is that sporophytes (i.e. thediploid body) are very short-lived, withering away not long after releasing spores.[16] In mosses, the sporophyte is more persistent and in hornworts, the sporophyte disperses spores over an extended period.[17]
The life of a liverwort starts from the germination of a haploidspore to produce aprotonema, which is either a mass of thread-like filaments or a flattened thallus.[18][19] The protonema is a transitory stage in the life of a liverwort, from which will grow the mature gametophore ("gamete-bearer") plant that produces the sex organs. The male organs are known asantheridia (singular: antheridium) and produce the sperm cells. Clusters of antheridia are enclosed by a protective layer of cells called theperigonium (plural: perigonia). As in other land plants, the female organs are known asarchegonia (singular: archegonium) and are protected by the thin surroundingperichaetum (plural: perichaeta).[8] Each archegonium has a slender hollow tube, the "neck", down which the sperm swim to reach the egg cell.
Liverwort species may be eitherdioicous ormonoicous. In dioicous liverworts, female and male sex organs are borne on different and separate gametophyte plants. In monoicous liverworts, the two kinds of reproductive structures are borne on different branches of the same plant.[20] In either case, the sperm must move from the antheridia where they are produced to the archegonium where the eggs are held. Thesperm of liverworts isbiflagellate, i.e. they have two tail-likeflagellae that enable them to swim short distances,[21] provided that at least a thin film of water is present. Their journey may be assisted by the splashing of raindrops. In 2008, Japanese researchers discovered that some liverworts are able to fire sperm-containing water up to 15 cm in the air, enabling them to fertilize female plants growing more than a metre from the nearest male.[22]
When sperm reach the archegonia,fertilisation occurs, leading to the production of a diploid sporophyte. After fertilisation, the immaturesporophyte within the archegonium develops three distinct regions: (1) afoot, which both anchors the sporophyte in place and receives nutrients from its "mother" plant, (2) a spherical or ellipsoidalcapsule, inside which the spores will be produced for dispersing to new locations, and (3) aseta (stalk) which lies between the other two regions and connects them.[21] The sporophyte lacks anapical meristem, anauxin-sensitive point of divergence with other land plants some time in theLate Silurian/Early Devonian.[23][24] When the sporophyte has developed all three regions, the seta elongates, pushing its way out of the archegonium and rupturing it. While the foot remains anchored within the parent plant, the capsule is forced out by the seta and is extended away from the plant and into the air. Within the capsule, cells divide to produce bothelater cells and spore-producing cells. The elaters are spring-like, and will push open the wall of the capsule to scatter themselves when the capsule bursts. The spore-producing cells will undergomeiosis to form haploidspores to disperse, upon which point the life cycle can start again.
Some liverworts are capable ofasexual reproduction; in bryophytes in general "it would almost be true to say that vegetative reproduction is the rule and not the exception."[25] For example, inRiccia, when the older parts of the forked thalli die, the younger tips become separate individuals.[25]
Some thallose liverworts such asMarchantia polymorpha andLunularia cruciata produce small disc-shapedgemmae in shallow cups.[26]Marchantia gemmae can be dispersed up to 120 cm by rain splashing into the cups.[27] InMetzgeria, gemmae grow at thallus margins.[28]Marchantia polymorpha is a common weed in greenhouses, often covering the entire surface of containers;[29]: 230 gemma dispersal is the "primary mechanism by which liverwort spreads throughout a nursery or greenhouse."[29]: 231
Thalloid liverworts typically harbor symbioticglomeromycete fungi which have arbuscular (cilia-bearing) rootlets resembling those in vascular plants. Species in theAneuraceae, however, associate withbasidiomycete fungi belonging to the genusTulasnella, while leafy liverworts typically harbor symbiotic basidiomycete fungi belonging to the genusSerendipita.[30]
Today, liverworts can be found in many ecosystems across the planet except the sea and excessively dry environments, or those exposed to high levels of direct solar radiation.[31] As with most groups of living plants, they are most common (both in numbers and species) in moist tropical areas.[32] Liverworts are more commonly found in moderate to deep shade, though desert species may tolerate direct sunlight and periods of total desiccation.
Traditionally, the liverworts were grouped together with otherbryophytes (mosses andhornworts) in the Division Bryophyta, within which the liverworts made up the classHepaticae (also called Marchantiopsida).[8][33] Somewhat more recently, the liverworts were given their own division (Marchantiophyta),[34] as bryophytes became considered to beparaphyletic. However, the most recent phylogenetic evidence indicates that liverworts are indeed likely part of a monophyletic clade ("Bryophytasensu lato" or "Bryophyta Schimp.") alongside mosses and hornworts.[35][36][37] Hence, it has been suggested that the liverworts should be de-ranked to a class called Marchantiopsida.[38] In addition, there is strong phylogenetic evidence to suggest that liverworts and mosses form a monophyletic subclade namedSetaphyta.[39][40][41]
Two of the most likely models for bryophyte evolution.[41]
An important conclusion from these phylogenies is that the ancestral stomata appear to have been lost in the liverwort lineage.[39][36] Among the earliestfossils believed to be liverworts arecompression fossils ofPallaviciniites from the UpperDevonian ofNew York.[42] These fossils resemble modern species in theMetzgeriales.[43] Another Devonian fossil calledProtosalvinia also looks like a liverwort, but its relationship to other plants is still uncertain, so it may not belong to the Marchantiophyta. In 2007, the oldest fossils assignable at that time to the liverworts were announced,Metzgeriothallus sharonae from theGivetian (MiddleDevonian) ofNew York, United States.[44] However, in 2010, five different types of fossilized liverwort spores were found in Argentina, dating to the much earlier MiddleOrdovician, around 470 million years ago.[1][45]
Bryologists classify liverworts in the divisionMarchantiophyta. This divisional name is based on the name of the most universally recognized liverwort genusMarchantia.[46] In addition to thistaxon-based name, the liverworts are often calledHepaticophyta. This name is derived from their common Latin name as Latin was the language in which botanists published their descriptions of species. This name is not to be mistakenly associated withflowering plant genusHepatica, of the buttercup familyRanunculaceae. In addition, the name Hepaticophyta is frequently misspelled in textbooks asHepatophyta.
Although there is no consensus among bryologists as to the classification of liverworts above family rank,[47] the Marchantiophyta may be subdivided into three classes:[48][49][50][51]
TheMarchantiopsida includes the three ordersMarchantiales (complex-thallus liverworts), andSphaerocarpales (bottle hepatics), as well as theBlasiales (previously placed among the Metzgeriales).[48][52] It also includes the problematic genusMonoclea, which is sometimes placed in its own order Monocleales.[53]
It is estimated that there are about 9000 species of liverworts, at least 85% of which belong to the leafy group.[3][56] Despite that fact, no liverwort genomes have been sequenced to date and only few genes identified and characterized.[57]
In ancient times, it was assumed that liverworts cured diseases of theliver, hence the name.[59] InOld English, the word liverwort literally meansliver plant.[60] This probably stemmed from the superficial appearance of some thalloid liverworts which resemble a liver in outline, and led to the common name of the group ashepatics, from theLatin wordhēpaticus for "belonging to the liver". An unrelated flowering plant,Hepatica, is sometimes also referred to as liverwort because it was once also used in treating diseases of the liver. This archaic relationship of plant form to function was based in the"Doctrine of Signatures".[61]
Liverworts have little direct economic importance today. Their greatest impact is indirect, through the reduction of erosion along streambanks, their collection and retention of water in tropical forests, and the formation ofsoil crusts in deserts and polar regions. However, a few species are used by humans directly. A few species, such asRiccia fluitans, are aquatic thallose liverworts sold for use inaquariums. Their thin, slender branches float on the water's surface and provide habitat for both small invertebrates and the fish that feed on them.
^abWalker, Matt. "Fossils of earliest land plants discovered in Argentina"[1]. (BBC, Earth News, 2010).
^Stotler, Raymond E.; Barbara J. Candall-Stotler (1977). "A checklist of the liverworts and hornworts of North America".The Bryologist.80 (3). American Bryological and Lichenological Society:405–428.doi:10.2307/3242017.JSTOR3242017.
^abCrandall-Stotler, Barbara; Stotler, Raymond E. (2000). "Morphology and classification of the Marchantiophyta". In A. Jonathan Shaw; Bernard Goffinet (eds.).Bryophyte Biology. Cambridge: Cambridge University Press. p. 21.ISBN0-521-66097-1.
^Schuster, Rudolf M. (1992).The Hepaticae and Anthocerotae of North America. Vol. VI. Chicago: Field Museum of Natural History. p. 19.ISBN0-914868-21-7.
^Schuster, Rudolf M.The Hepaticae and Anthocerotae of North America, vol. I, pp. 243–244. (New York: Columbia University Press, 1966)
^Kashyap, Shiv Ram.Liverworts of the Western Himalayas and the Panjab Plain, vol. I, p. 1. (New Delhi: The Chronica Botanica, 1929)
^abcdSchofield, W. B.Introduction to Bryology, pp. 135–140. (New York: Macmillan, 1985).ISBN0-02-949660-8.
^Nehira, Kunito. "Spore Germination, Protonemata Development and Sporeling Development", p. 347in Rudolf M. Schuster (Ed.),New Manual of Bryology, volume I. (Nichinan, Miyazaki, Japan: The Hattori Botanical Laboratory, 1983).ISBN49381633045.
^Allison, K. W. & John Child.The Liverworts of New Zealand, pp. 13–14. (Dunedin: University of Otago Press, 1975).
^Conard, Henry S. and Paul L. Redfearn, Jr.How to Know the Mosses and Liverworts, revised ed., pp. 12–23. (Dubuque, Iowa: William C. Brown Co., 1979)ISBN0-697-04768-7
^abHarold C. Bold, C. J. Alexopoulos, and T. Delevoryas.Morphology of Plants and Fungi, 5th ed., p. 189. (New York: Harper-Collins, 1987).ISBN0-06-040839-1.
^Nehira, Kunito. "Spore Germination, Protonemata Development and Sporeling Development", pp. 358–374in Rudolf M. Schuster (Ed.),New Manual of Bryology, volume I. (Nichinan, Miyazaki, Japan: The Hattori Botanical Laboratory, 1983).ISBN49381633045.
^Chopra, R. N. & P. K. Kumra.Biology of Bryophytes, pp. 1–38. (New York: John Wiley & Sons, 1988).ISBN0-470-21359-0.
^Malcolm, Bill & Nancy Malcolm.Mosses and Other Bryophytes: An Illustrated Glossary, pp. 6 & 128. (New Zealand: Micro-Optics Press, 2000).ISBN0-473-06730-7.
^Cooke, Todd J; Poli, DorothyBelle; Cohen, Jerry D (2003). "Did auxin play a crucial role in the evolution of novel body plans during the Late Silurian-Early Devonian radiation of land plants?".The Evolution of Plant Physiology.Elsevier. pp. 85–107.doi:10.1016/b978-012339552-8/50006-8.ISBN978-0-12-339552-8.
^Schuster, Rudolf M.The Hepaticae and Anthocerotae of North America, vol. I, pp. 243–249. (New York: Columbia University Press, 1966).
^Pócs, Tamás. "Tropical Forest Bryophytes", p. 59in A. J. E. Smith (Ed.)Bryophyte Ecology. (London: Chapman and Hall, 1982).ISBN0-412-22340-6.
^Crandall-Stotler, Barbara. & Stotler, Raymond E. "Morphology and classification of the Marchantiophyta". pp. 36–38in A. Jonathan Shaw & Bernard Goffinet (Eds.),Bryophyte Biology. (Cambridge: Cambridge University Press: 2000).ISBN0-521-66097-1
^Goffinet, Bernard. "Origin and phylogenetic relationships of bryophytes". pp. 124–149in A. Jonathan Shaw & Bernard Goffinet (Eds.),Bryophyte Biology. (Cambridge: Cambridge University Press:!2000).ISBN0-521-66097-1
^Taylor, Thomas N. & Edith L. Taylor.The Biology and Evolution of Fossil Plants, p. 139. (Englewood Cliffs, NJ: Prentice Hall, 1993).ISBN0-13-651589-4.
^Oostendorp, Cora.The Bryophytes of the Palaeozoic and the Mesozoic, pp. 70–71. (Bryophytum Bibliotheca, Band 34, 1987).ISBN3-443-62006-X.
^VanAller Hernick, L.; Landing, E.; Bartowski, K.E. (2008). "Earth's oldest liverworts –Metzgeriothallus sharonae sp. nov. from the Middle Devonian (Givetian) of eastern New York, USA".Review of Palaeobotany and Palynology.148 (2–4):154–162.Bibcode:2008RPaPa.148..154H.doi:10.1016/j.revpalbo.2007.09.002.
^Crandall-Stotler, Barbara. & Stotler, Raymond E. "Morphology and classification of the Marchantiophyta". p. 63in A. Jonathan Shaw & Bernard Goffinet (Eds.),Bryophyte Biology. (Cambridge: Cambridge University Press:2000).ISBN0-521-66097-1
^Jones, E. W. (2004).Liverwort and Hornwort Flora of West Africa. Scripta Botnica Belgica. Vol. 30. Meise: National Botanic Garden (Belgium). p. 30.ISBN90-72619-61-7.
^abcForrest, Laura L.; Christine E. Davis; David G. Long; Barbara J. Crandall-Stotler; Alexandra Clark; Michelle L. Hollingsworth (2006). "Unraveling the evolutionary history of the liverworts (Marchantiophyta): multiple taxa, genomes and analyses".The Bryologist.109 (3):303–334.doi:10.1639/0007-2745(2006)109[303:UTEHOT]2.0.CO;2.S2CID85912159.
^Heinrichs, Jochen; S. Robbert Gradstein; Rosemary Wilson; Harald Schneider (2005). "Towards a natural classification of liverworts (Marchantiophyta) based on the chloroplast gene rbcL".Cryptogamie Bryologie.26 (2):131–150.
^abRenzaglia, Karen S.; Scott Schuette; R. Joel Duff; Roberto Ligrone; A. Jonathan Shaw; Brent D. Mishler; Jeffrey G. Duckett (2007). "Bryophyte phylogeny: Advancing the molecular and morphological frontiers".The Bryologist.110 (2):179–213.doi:10.1639/0007-2745(2007)110[179:BPATMA]2.0.CO;2.S2CID85788756.
^Forrest, Laura L.; Barbara J. Crandall-Stotler (2004). "A Phylogeny of the Simple Thalloid Liverworts (Jungermanniopsida, Metzgeriidae) as Inferred from Five Chloroplast Genes".Monographs in Systematic Botany. Molecular Systematics of Bryophytes.98. Missouri Botanical Garden Press:119–140.
^Schuster, Rudolf M.The Hepaticae and Anthocerotae of North America, vol. VI, p. 26. (Chicago: Field Museum of Natural History, 1992).ISBN0-914868-21-7.
^Sadava, David; David M. Hillis; H. Craig Heller; May Berenbaum (2009).Life: The Science of Biology (9th ed.). New York: W. H. Freeman. p. 599.ISBN978-1429246446.
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