Liverwort

Template:Short description Template:For Template:Use dmy dates Template:Automatic taxobox Liverworts are a group of non-vascular land plants forming the division Marchantiophyta (Template:IPAc-en). They may also be referred to as hepatics. 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 after his father by French botanist Jean Marchant.

It is estimated that there are about 9000 species of liverwort.<ref>Template:Cite web</ref> 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 rhizoids. 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 Template:Cvt wide with individual plants less than Template:Cvt 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.<ref>Template:Cite book</ref>

Physical characteristics

Description

Most liverworts are small, measuring from Template:Convert wide with individual plants less than Template:Convert long,<ref>Schuster, Rudolf M. The Hepaticae and Anthocerotae of North America, vol. I, pp. 243–244. (New York: Columbia University Press, 1966)</ref> so they are often overlooked. The most familiar liverworts consist of a prostrate, flattened, ribbon-like or branching structure called a thallus (plant body); these liverworts are termed thallose 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 called leafy liverworts or scale liverworts.<ref>Kashyap, Shiv Ram. Liverworts of the Western Himalayas and the Panjab Plain, vol. I, p. 1. (New Delhi: The Chronica Botanica, 1929)</ref><ref name="Schofield">Schofield, W. B. Introduction to Bryology, pp. 135–140. (New York: Macmillan, 1985). Template:ISBN.</ref> (See the gallery below for examples.)

Liverworts can most reliably be distinguished from the apparently similar mosses by their single-celled rhizoids.<ref>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). Template:Listed Invalid ISBN.</ref> Other differences are not universal for all mosses and all liverworts;<ref name="Schofield" /> 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,<ref>Allison, K. W. & John Child. The Liverworts of New Zealand, pp. 13–14. (Dunedin: University of Otago Press, 1975).</ref><ref>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) Template:ISBN</ref> 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.<ref>Template:Cite journal</ref> 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.<ref name="Harold-1987">Harold C. Bold, C. J. Alexopoulos, and T. Delevoryas. Morphology of Plants and Fungi, 5th ed., p. 189. (New York: Harper-Collins, 1987). Template:ISBN.</ref> 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.<ref name="Harold-1987" /> The sporophyte of many liverworts are non-photosynthetic, but there are also several that are photosynthetic to various degrees.<ref>Volume 1, Chapter 11-1: Photosynthesis: The Process</ref> 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 seed plants, the haploid generation is represented only by the tiny pollen and the ovule, while the diploid generation is the familiar tree or other plant.<ref>Fosket, Donald E. Plant Growth and Development: A Molecular Approach, p. 27. (San Diego: Academic Press, 1994). Template:ISBN.</ref> 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.<ref>Hicks, Marie L. Guide to the Liverworts of North Carolina, p. 10. (Durham: Duke University Press, 1992). Template:ISBN.</ref> In mosses, the sporophyte is more persistent and in hornworts, the sporophyte disperses spores over an extended period.<ref>Template:Citation</ref>

Life cycle

The life of a liverwort starts from the germination of a haploid spore to produce a protonema, which is either a mass of thread-like filaments or a flattened thallus.<ref>Nehira, Kunito. "Spore Germination, Protonemata Development and Sporeling Development", pp. 358–374 in Rudolf M. Schuster (Ed.), New Manual of Bryology, volume I. (Nichinan, Miyazaki, Japan: The Hattori Botanical Laboratory, 1983). Template:Listed Invalid ISBN.</ref><ref>Chopra, R. N. & P. K. Kumra. Biology of Bryophytes, pp. 1–38. (New York: John Wiley & Sons, 1988). Template:ISBN.</ref> 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 as antheridia (singular: antheridium) and produce the sperm cells. Clusters of antheridia are enclosed by a protective layer of cells called the perigonium (plural: perigonia). As in other land plants, the female organs are known as archegonia (singular: archegonium) and are protected by the thin surrounding perichaetum (plural: perichaeta).<ref name="Schofield" /> Each archegonium has a slender hollow tube, the "neck", down which the sperm swim to reach the egg cell.

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.<ref>Malcolm, Bill & Nancy Malcolm. Mosses and Other Bryophytes: An Illustrated Glossary, pp. 6 & 128. (New Zealand: Micro-Optics Press, 2000). Template:ISBN.</ref> 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 flagellae that enable them to swim short distances,<ref name="Campbell-1918">Campbell, Douglas H. The Structure and Development of Mosses and Ferns, pp. 73–74. (London: The Macmillan Co., 1918)</ref> 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.<ref>Template:Cite journal</ref>

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.<ref name="Campbell-1918" /> The sporophyte lacks an apical meristem, an auxin-sensitive point of divergence with other land plants some time in the Late Silurian/Early Devonian.<ref name="Cooke-2003">Template:Cite book</ref><ref name="Friedman-2004">Template:Cite journal</ref> 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 spores to disperse, upon which point the life cycle can start again.

Asexual reproduction

Some liverworts are capable of asexual reproduction; in bryophytes in general "it would almost be true to say that vegetative reproduction is the rule and not the exception."<ref name="Lepp-Apr 2008b">Template:Cite web</ref> For example, in Riccia, when the older parts of the forked thalli die, the younger tips become separate individuals.<ref name="Lepp-Apr 2008b"/>

Some thallose liverworts such as Marchantia polymorpha and Lunularia cruciata produce small disc-shaped gemmae in shallow cups.<ref>Smith, AJE (1989) The Liverworts of Britain and Ireland, Cambridge University Press, Cambridge.</ref> Marchantia gemmae can be dispersed up to 120 cm by rain splashing into the cups.<ref name="Equihua-1987">Template:Cite journal</ref> In Metzgeria, gemmae grow at thallus margins.<ref name="Lepp-Feb 2008">Template:Cite web</ref> Marchantia polymorpha is a common weed in greenhouses, often covering the entire surface of containers;<ref name="Wehtje-2006"/>Template:Rp gemma dispersal is the "primary mechanism by which liverwort spreads throughout a nursery or greenhouse."<ref name="Wehtje-2006">Template:Cite journal</ref>Template:Rp

Symbiosis

Thalloid liverworts typically harbor symbiotic glomeromycete fungi which have arbuscular (cilia-bearing) rootlets resembling those in vascular plants. Species in the Aneuraceae, however, associate with basidiomycete fungi belonging to the genus Tulasnella, while leafy liverworts typically harbor symbiotic basidiomycete fungi belonging to the genus Serendipita.<ref>Template:Cite journal</ref>

Ecology

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.<ref>Schuster, Rudolf M. The Hepaticae and Anthocerotae of North America, vol. I, pp. 243–249. (New York: Columbia University Press, 1966).</ref> As with most groups of living plants, they are most common (both in numbers and species) in moist tropical areas.<ref>Pócs, Tamás. "Tropical Forest Bryophytes", p. 59 in A. J. E. Smith (Ed.) Bryophyte Ecology. (London: Chapman and Hall, 1982). Template:ISBN.</ref> Liverworts are more commonly found in moderate to deep shade, though desert species may tolerate direct sunlight and periods of total desiccation.

Classification

Relationship to other plants

Traditionally, the liverworts were grouped together with other bryophytes (mosses and hornworts) in the Division Bryophyta, within which the liverworts made up the class Hepaticae (also called Marchantiopsida).<ref name="Schofield" /><ref>Crandall-Stotler, Barbara. & Stotler, Raymond E. "Morphology and classification of the Marchantiophyta". pp. 36–38 in A. Jonathan Shaw & Bernard Goffinet (Eds.), Bryophyte Biology. (Cambridge: Cambridge University Press: 2000). Template:ISBN</ref> Somewhat more recently, the liverworts were given their own division (Marchantiophyta),<ref>Goffinet, Bernard. "Origin and phylogenetic relationships of bryophytes". pp. 124–149 in A. Jonathan Shaw & Bernard Goffinet (Eds.), Bryophyte Biology. (Cambridge: Cambridge University Press:!2000). Template:ISBN</ref> as bryophytes became considered to be paraphyletic. However, the most recent phylogenetic evidence indicates that liverworts are indeed likely part of a monophyletic clade ("Bryophyta sensu lato" or "Bryophyta Schimp.") alongside mosses and hornworts.<ref name="Leebens-Mack-2019">Template:Cite journal</ref><ref name="Harris-2020">Template:Cite journal</ref><ref name="Su-2021">Template:Cite journal</ref> Hence, it has been suggested that the liverworts should be de-ranked to a class called Marchantiopsida.<ref name="de Sousa-2019">Template:Cite journal</ref> In addition, there is strong phylogenetic evidence to suggest that liverworts and mosses form a monophyletic subclade named Setaphyta.<ref name="Puttick-2018">Template:Cite journal</ref><ref name="Sousa-2020">Template:Cite journal</ref><ref name="Cox-2018">Template:Cite journal</ref>

'Monophyletic bryophytes' model	'Liverworts plus mossesTemplate:En dashbasal' model
Template:Clade	Template:Barlabel
Two of the most likely models for bryophyte evolution.<ref name="Cox-2018"/>

An important conclusion from these phylogenies is that the ancestral stomata appear to have been lost in the liverwort lineage.<ref name="Puttick-2018"/><ref name="Harris-2020"/> Among the earliest fossils believed to be liverworts are compression fossils of Pallaviciniites from the Upper Devonian of New York.<ref>Taylor, Thomas N. & Edith L. Taylor. The Biology and Evolution of Fossil Plants, p. 139. (Englewood Cliffs, NJ: Prentice Hall, 1993). Template:ISBN.</ref> These fossils resemble modern species in the Metzgeriales.<ref>Oostendorp, Cora. The Bryophytes of the Palaeozoic and the Mesozoic, pp. 70–71. (Bryophytum Bibliotheca, Band 34, 1987). Template:ISBN.</ref> 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.<ref name="VanAller Hernick-2008">Template:Cite journal</ref> 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.<ref name="Walker-2010">Template:Cite news</ref><ref>Template:Cite journal</ref>

Internal classification

Bryologists classify liverworts in the division Marchantiophyta. This divisional name is based on the name of the most universally recognized liverwort genus Marchantia.<ref>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). Template:ISBN</ref> 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.

Although there is no consensus among bryologists as to the classification of liverworts above family rank,<ref>Template:Cite book</ref> the Marchantiophyta may be subdivided into three classes:<ref name="Forrest-2006">Template:Cite journal</ref><ref name="Heinrichs-2005">Template:Cite journal</ref><ref name="He-Nygrén-2006">Template:Cite journal</ref><ref name="Renzaglia-2007">Template:Cite journal</ref>

The Jungermanniopsida includes the two orders Metzgeriales (simple thalloids) and Jungermanniales (leafy liverworts).
The Marchantiopsida includes the three orders Marchantiales (complex-thallus liverworts), and Sphaerocarpales (bottle hepatics), as well as the Blasiales (previously placed among the Metzgeriales).<ref name="Forrest-2006" /><ref name="Forrest-2004">Template:Cite journal</ref> It also includes the problematic genus Monoclea, which is sometimes placed in its own order Monocleales.<ref>Schuster, Rudolf M. The Hepaticae and Anthocerotae of North America, vol. VI, p. 26. (Chicago: Field Museum of Natural History, 1992). Template:ISBN.</ref>
A third class, the Haplomitriopsida is newly recognized as the sister group of the other liverworts;<ref name="Renzaglia-2007" /> it comprises the genera Haplomitrium, Treubia, and Apotreubia.

Forrest 2006<ref name="Forrest-2006"/>	Cole, Hilger & Goffinet 2021 <ref>Template:Cite web</ref>
Template:Clade	Template:Clade

An updated classification by Söderström et al. 2016<ref name="Söderström-2016">Template:Cite journal</ref>

Marchantiophyta Stotler & Crandall-Stotler 2000
- Haplomitriopsida Stotler & Crandall-Stotler 1977
  - Calobryales Hamlin 1972
  - Treubiales Schljakov 1972
- Marchantiopsida Cronquist, Takhtajan & Zimmermann 1966
  - Blasiidae He-Nygrén et al. 2006
    - Blasiales Stotler & Crandall-Stotler 2000
  - Marchantiidae Engler 1893 sensu He-Nygrén et al. 2006
    - Lunulariales Long 2006
    - Marchantiales Limpricht 1877 (complex thalloids)
    - Neohodgsoniales Long 2006
    - Sphaerocarpales Cavers 1910 (bottle liverworts)
- Jungermanniopsida Stotler & Crandall-Stotler 1977
  - Jungermanniidae Engler 1893 (leafy liverworts)
    - Jungermanniales von Klinggräff 1858
    - Porellales Schljakov 1972
    - Ptilidiales Schljakov 1972
  - Metzgeriidae Bartholomew-Began 1990
    - Metzgeriales Chalaud 1930
    - Pleuroziales Schljakov 1972
  - Pelliidae He-Nygrén et al. 2006
    - Fossombroniales Schljakov 1972
    - Pallaviciniales Frey & Stech 2005
    - Pelliales He-Nygrén et al. 2006

It is estimated that there are about 9000 species of liverworts, at least 85% of which belong to the leafy group.<ref name="Crandall-Stotler-2000" /><ref name="Sadava-2009">Template:Cite book</ref> Despite that fact, no liverwort genomes have been sequenced to date and only few genes identified and characterized.<ref>Template:Cite journal</ref>

Extinct taxa

There are several known fossil genera from this group that are not assigned to any extant class:<ref>Template:Cite web</ref>

†Discites Harris 1931
†Eohepatica Heard and Jones 1931
†Jungermanniopsis Howe and Hollick 1922
†Jungermannites Göppert 1845
†Schizolepidella Halle 1913
†Thallomia Heard and Jones 1931

Economic importance

In ancient times, it was assumed that liverworts cured diseases of the liver, hence the name.<ref>Dittmer, Howard J. Phylogeny and Form in the Plant Kingdom, p. 286. (Toronto: D. Van Nostrand Co., 1964)</ref> In Old English, the word liverwort literally means liver plant.<ref>Raven, P. H., R. F. Evert, & S. E. Eichhorn. Biology of Plants, 7th ed., p. 351. (New York: W. H. Freeman, 2005). Template:ISBN.</ref> 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".<ref>Stern, Kingsley R. Introductory Plant Biology, 5th ed., p. 338. (Dubuque, Iowa: Wm. C. Brown Publishers, 1991) Template:ISBN.</ref>

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 soil crusts 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 aquariums. Their thin, slender branches float on the water's surface and provide habitat for both small invertebrates and the fish that feed on them.