Liverwort - WikiHQ

Liverworts are a group of non-vascular land plants forming the division Marchantiophyta (). 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. 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 the thalloid structure, their single-celled rhizoids and the presence of a costa (midrib) in the leaves.

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.

Physical characteristics

Description

Most liverworts are small, measuring from wide with individual plants less than long, 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. (See the gallery below for examples.)

left|thumb|A [[Marchantiales|thallose liverwort, Lunularia cruciata]]

Liverworts can most reliably be distinguished from the apparently similar mosses by their single-celled rhizoids. Other differences are not universal for all mosses and all liverworts; 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. 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. 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. 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. 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. In mosses, the sporophyte is more persistent and in hornworts, the sporophyte disperses spores over an extended period.

Life cycle

thumb|300px|Sexual life cycle of a Marchantia-like liverwort

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. 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). 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, provided that at least a thin film of water is present. Their journey may be assisted by the splashing of raindrops. As well, researchers have observed liverworts "firing" sperm-containing water up to 15 cm through the air, enabling fertilization of female plants growing more than a metre away.

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.

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." For example, in Riccia, when the older parts of the forked thalli die, the younger tips become separate individuals. 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;

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.

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. As with most groups of living plants, they are most common (both in numbers and species) in moist tropical areas. Liverworts are more commonly found in moderate to deep shade, though desert species may tolerate direct sunlight and periods of total desiccation.

Classification

Evolution

Epiphytic thalloid liverworts evolved during the Triassic to the Cretaceous.

Relationship to other plants

Liverworts and mosses have many similar properties but can be distinguished with some work. A key difference is that the rhizoid of a liverwort is unicellular while for mosses the structure will be multicellular. Liverworts frequently have a thallus which is never present for mosses. Conversely, moss leaves may have costa, or a midrib, which do no appear in liverwort leaves.

Leafy species can be distinguished from the apparently similar mosses on the basis of a number of features, including the thalloid structure, their single-celled rhizoids and the presence of a costa (midrib) in the leaves. Somewhat more recently, the liverworts were given their own division (Marchantiophyta), 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. Hence, it has been suggested that the liverworts should be de-ranked to a class called Marchantiopsida. In addition, there is strong phylogenetic evidence to suggest that liverworts and mosses form a monophyletic subclade named Setaphyta.

{| cellpadding=5 style="border:1px solid #BBB" cellpadding=10;

! 'Monophyletic bryophytes' model

! 'Liverworts plus mossesbasal' model

| colspan="2"|Two of the most likely models for bryophyte evolution. These fossils resemble modern species in the Metzgeriales. 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.

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. 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, as of 2004, there is no consensus among bryologists as to the classification of liverworts above family rank, the Marchantiophyta may be subdivided into three classes:

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). It also includes the problematic genus Monoclea, which is sometimes placed in its own order Monocleales.
A third class, the Haplomitriopsida is newly recognized as the sister group of the other liverworts;

| style="vertical-align:top|

An updated classification by Söderström et al. 2016

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. Despite that fact, no liverwort genomes have been sequenced to date and only few genes identified and characterized.

Extinct taxa

There are several known fossil genera from this group that are not assigned to any extant class:

†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. In Old English, the word liverwort literally means liver plant. This  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.) The archaic relationship of plant form to function is based in a concept termed the "Doctrine of Signatures".

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.

Gallery

A small collection of images showing liverwort structure and diversity:

File:Marchantia.jpg|Marchantia polymorpha, with antheridial and archegonial stalks

File:Archegonium.jpg|The archegonium of Porella

File:Porella SPT.jpg|A sporophyte of Porella emerging from its archegonium

File:Porella platyphylla.jpg|Porella platyphylla clump growing on a tree

File:Pellia epiphylla5 ies.jpg|Pellia epiphylla, growing on moist soil

File:Plagiochila aspleniodes0.jpg|Plagiochila asplenioides, a leafy liverwort

File:RicciaFluitans1.jpg|Riccia fluitans, an aquatic thallose liverwort

File:Liverwort.jpg|Conocephalum conicum, a large thallose liverwort

</gallery>

References

External links

Liverwort structure in pictures
LiToL: Assembling the Liverwort Tree of Life (note: for 500,000 million years ago read "480 million years ago".)
Inter-relationships of Mosses, Liverworts, and Hornworts
Additional information on Liverworts
Liverworts