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The Marchantiophyta () are a division of non-vascular embryophyte commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information. The division name was derived from the genus name Marchantia, named by French botanist Jean Marchant after his father.
It is estimated that there are about 9000 species of liverworts. Some of the more familiar species grow as a flattened leafless thallus, but most species are leafy with a form very much like a flattened moss. Leafy species can be distinguished from the apparently similar mosses on the basis of a number of features, including their single-celled . Leafy liverworts also differ from most (but not all) mosses in that their leaves never have a costa (present in many mosses) and may bear marginal cilia (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 unique complex oil bodies of high refractive index.
Liverworts are typically small, usually from wide with individual plants less than 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.
Liverworts can most reliably be distinguished from the apparently similar by their single-celled .Nehira, Kunito. "Spore Germination, Protonemata Development and Sporeling Development", p. 347 in Rudolf M. Schuster (Ed.), New Manual of Bryology, volume I. (Nichinan, Miyazaki, Japan: The Hattori Botanical Laboratory, 1983). . Other differences are not universal for all mosses and all liverworts; 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,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) 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. Sporogenesis in Physcomitrium patens: Intergenerational collaboration and the development of the spore wall and aperture Unlike any other embryophytes, most liverworts contain unique membrane-bound oil bodies containing isoprenoids in at least some of their cells, lipid droplets in the cytoplasm of all other plants being unenclosed.Harold C. Bold, C. J. Alexopoulos, and T. Delevoryas. Morphology of Plants and Fungi, 5th ed., p. 189. (New York: Harper-Collins, 1987). . 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 of microscopy or an experienced bryologist.
Liverworts, like other bryophytes, have a gametophyte-dominant life cycle, with the sporophyte dependent on the gametophyte. The sporophyte of many liverworts are non-photosynthetic, but there are also several that are photosynthetic to various degrees. Volume 1, Chapter 11-1: Photosynthesis: The Process Cells in a typical liverwort plant each contain only a single set of genetic information, so the plant's cells are haploid 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 familiar , the haploid generation is represented only by the tiny pollen and the ovule, while the diploid generation is the familiar tree or other plant.Fosket, Donald E. Plant Growth and Development: A Molecular Approach, p. 27. (San Diego: Academic Press, 1994). . Another unusual feature of the liverwort life cycle is that sporophytes (i.e. the diploid body) are very short-lived, withering away not long after releasing spores.Hicks, Marie L. Guide to the Liverworts of North Carolina, p. 10. (Durham: Duke University Press, 1992). . In mosses, the sporophyte is more persistent and in hornworts, the sporophyte disperses spores over an extended period.
Liverwort species may be either dioicous or monoicous. 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.Malcolm, Bill & Nancy Malcolm. Mosses and Other Bryophytes: An Illustrated Glossary, pp. 6 & 128. (New Zealand: Micro-Optics Press, 2000). . In either case, the sperm must move from the antheridia where they are produced to the archegonium where the eggs are held. The sperm of liverworts is biflagellate, i.e. they have two tail-like flagellum that enable them to swim short distances, Campbell, Douglas H. The Structure and Development of Mosses and Ferns, pp. 73–74. (London: The Macmillan Co., 1918) 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.
When sperm reach the archegonia, fertilisation occurs, leading to the production of a diploid sporophyte. After fertilisation, the immature sporophyte within the archegonium develops three distinct regions: (1) a foot, which both anchors the sporophyte in place and receives nutrients from its "mother" plant, (2) a spherical or ellipsoidal capsule, inside which the spores will be produced for dispersing to new locations, and (3) a seta (stalk) which lies between the other two regions and connects them. 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 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 both elater 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 undergo meiosis to form haploid to disperse, upon which point the life cycle can start again.
Some thallose liverworts such as Marchantia polymorpha and Lunularia produce small disc-shaped gemmae in shallow cups.Smith, AJE (1989) The Liverworts of Britain and Ireland, Cambridge University Press, Cambridge. Marchantia gemmae can be dispersed up to 120 cm by rain splashing into the cups. In Metzgeria, gemmae grow at thallus margins. Marchantia polymorpha is a common weed in greenhouses, often covering the entire surface of containers; gemma dispersal is the "primary mechanism by which liverwort spreads throughout a nursery or greenhouse."
| Two of the most likely models for bryophyte evolution. | |
An important conclusion from these phylogenies is that the ancestral stomata appear to have been lost in the liverwort lineage. Among the earliest believed to be liverworts are compression fossils of Pallaviciniites from the Upper Devonian of New York.Taylor, Thomas N. & Edith L. Taylor. The Biology and Evolution of Fossil Plants, p. 139. (Englewood Cliffs, NJ: Prentice Hall, 1993). . These fossils resemble modern species in the Metzgeriales.Oostendorp, Cora. The Bryophytes of the Palaeozoic and the Mesozoic, pp. 70–71. ( Bryophytum Bibliotheca, Band 34, 1987). . Another Devonian fossil called Protosalvinia 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 the Givetian (Middle Devonian) of New York, United States. However, in 2010, five different types of fossilized liverwort spores were found in Argentina, dating to the much earlier Middle Ordovician, around 470 million years ago.Walker, Matt. "Fossils of earliest land plants discovered in Argentina" [4]. (BBC, Earth News, 2010).
Although there is no consensus among bryologists as to the classification of liverworts above family rank, the Marchantiophyta may be subdivided into three classes:
An updated classification by Söderström et al. 2016
It is estimated that there are about 9000 species of liverworts, at least 85% of which belong to the leafy group. Despite that fact, no liverwort genomes have been sequenced to date and only few genes identified and characterized.
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 of in deserts and polar regions. However, a few species are used by humans directly. A few species, such as Riccia fluitans, are aquatic thallose liverworts sold for use in . Their thin, slender branches float on the water's surface and provide habitat for both small invertebrates and the fish that feed on them.
Internal classification
Bryology classify liverworts in the division Marchantiophyta. This divisional name is based on the name of the most universally recognized liverwort genus Marchantia.Crandall-Stotler, Barbara. & Stotler, Raymond E. "Morphology and classification of the Marchantiophyta". p. 63 in A. Jonathan Shaw & Bernard Goffinet (Eds.), Bryophyte Biology. (Cambridge: Cambridge University Press:2000). In addition to this taxon-based name, the liverworts are often called Hepaticophyta. 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 with flowering plant genus Hepatica, of the buttercup family Ranunculaceae. In addition, the name Hepaticophyta is frequently misspelled in textbooks as Hepatophyta.
Extinct taxa
There are several known Fossil taxa from this group that are not assigned to any extant class:
Economic importance
In ancient times, it was assumed that liverworts cured diseases of the liver, hence the name.Dittmer, Howard J. Phylogeny and Form in the Plant Kingdom, p. 286. (Toronto: D. Van Nostrand Co., 1964) In Old English, the word liverwort literally means liver plant.Raven, P. H., R. F. Evert, & S. E. Eichhorn. Biology of Plants, 7th ed., p. 351. (New York: W. H. Freeman, 2005). . 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 as hepatics, from the Latin word hēpaticus 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".Stern, Kingsley R. Introductory Plant Biology, 5th ed., p. 338. (Dubuque, Iowa: Wm. C. Brown Publishers, 1991) .
Gallery
A small collection of images showing liverwort structure and diversity:
See also
External links
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