Marchantiophyta

Liverwort is the common name for any of the small, green, non-vascular land plants of the division Marchantiophyta, characterized by a gametophyte-dominant life cycle and single-celled rhizoids as "roots," and generally a lack of clearly differentiated stems and leaves or the presence of deeply lobed or segmented leaves. They also are known as hepatics from the Latin word for liver; this and the name liverworts can be traced to a superficial appearance of some species to a liver and because it was believed that liverworts cured diseases of the liver.

It is estimated that there are perhaps 10,000 species of liverworts. Some of the more familiar species grow as a prostrate, flattened, leafless, branching structure called a 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 typically small, often less than 20 millimeters, wide with individual plants less than 10 centimeters 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.

Liverworts play important ecological roles, providing food for animals, helping to facilitate decay of dead trees and disintegration of rocks, reducing erosion along stream banks, and aiding in the collection and retention of water in tropical forests.

Overview and description

Liverworts comprise a division of bryophyte plants, which are non-vascular land plants, meaning that they lack water- and food-conducting strands in their roots (xylem and phloem), or that they are poorly developed. They do not have roots, only filamentous rhizoids. Liverworts are one of three main groups of bryophytes, the others being moss (division Bryophyta) and hornworts (division Anthocerotophyta). Originally these three groups were placed together as three separate classes or phyla within the division Bryophyta. However, it was determined that these three groups together form a paraphyletic group, and thus they now are placed in three separate divisions. Together they are still labeled bryophytes because of their similarity as non-vascular, land plants, but the Division Bryophyta now typically refers to the mosses. Algae are also non-vascular, but are not land plants.

Most liverworts are small. They typically range from 2 to 20 millimeters (0.08–0.8 inches) wide and individual plants commonly are less than 10 centimeters (4 inches) long (Schuster 1966). It is estimated that there are 6,000 to 8,000 species of liverworts, though when Neotropical regions are better studied this number may approach 10,000.

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 three or more ranks, the middle rank being conspicuously different from the outer ranks; these are called leafy liverworts or scale liverworts (Kashyap 1929; Schofield 1985).

A thallose liverwort, Lunularia cruciata

Liverworts can most reliably be distinguished from the apparently similar mosses by their single-celled rhizoids (Nehira 1983). Other differences are not universal for all mosses and all liverworts (Schofield 1985); however, 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, all point to the plant being a liverwort (Allison and Child 1975). In addition, ninety percent of liverworts contain oil bodies in at least some of their cells, and these cellular structures are absent from most other bryophytes and from all vascular plants (Bold et al. 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 have a gametophyte-dominant life cycle, with the sporophyte dependent on the gametophyte (Bold et al. 1987). 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 most other 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 (Fosket 1994). Another unusual feature of the liverwort life cycle is that sporophytes (that is, the diploid body) are very short-lived, withering away not long after releasing spores (Hicks 1992). Even in other bryophytes, the sporophyte is persistent and disperses spores over an extended period.

Life cycle

Life cycle of a typical 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 else a flattened thallus (Nehira 1983; Chopra 1988). 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) (Schofield 1985). 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 dioicious liverworts, female and male sex organs are borne on different and separate gametophyte plants. In monoicious liverworts, the two kinds of reproductive structures are borne on different branches of the same plant (Malcolm and Malcolm 2000). In either case, the sperm must swim from the antheridia where they are produced to the archegonium where the eggs are held. The sperm of liverworts is biflagellate, in other words, they have two tail-like flagellae that aid in propulsion (Campbell 1918). Their journey is further assisted either by the splashing of raindrops or the presence of a thin layer of water covering the plants. Without water, the journey from antheridium to archegonium cannot occur.

In the presence of such water, sperm from the antheridia swim to the archegonia and fertilization occurs, leading to the production of a diploid sporophyte. After fertilization, 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 (Campbell 1918). 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.

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 (Schuster 1966). As with most groups of living plants, they are most common (both in numbers and species) in moist tropical areas (Pócs 1982). 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) (Crandall-Stotler and Stotler 2000; Schofield 1985). However, since this grouping makes the Bryophyta paraphyletic, the liverworts are now usually given their own division (Goffinet 2000). The use of the division name Bryophyta sensu latu is still found in the literature, but more frequently the Bryophyta now is used in a restricted sense to include only the mosses.

Two hypotheses on the phylogeny of land plants (embryophyta).

Another reason that liverworts are now classified separately is that they appear to have diverged from all other embryophyte plants near the beginning of their evolution. The strongest line of supporting evidence is that liverworts are the only living group of land plants that do not have stomata on the sporophyte generation (Kenrick and Crane 1997). Among the earliest fossils believed to be liverworts are compression fossils of Pallaviciniites from the Upper Devonian of New York (Taylor and Taylor 1993). These fossils resemble modern species in the Metzgeriales (Oostendorp 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 to the liverworts were announced, Metzgeriothallus sharonae from the Givetian (Middle Devonian) of New York, U.S. (VanAller Hernick et al. 2008).

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 (Crandall-Stotler and Stotler 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 has led to some confusion, partly because it appears to be a taxon-based name derived from the genus Hepatica which is actually a flowering plant of the buttercup family Ranunculaceae. In addition, the name Hepaticophyta is frequently misspelled in textbooks as Hepatophyta, which only adds to the confusion.

The Marchantiophyta is subdivided into three classes (Forrest et al. 2006; Heinrichs et al. 2005; He-Nygrén et al. 2006; Renzaglia et al. 2007)):

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) (Forrest et al. 2006). It also includes the problematic genus Monoclea, which is sometimes placed in its own order Monocleales (Schuster 1992).
A third class, the Haplomitriopsida is newly recognized as a basal sister group to the other liverworts (Renzaglia et al. 2007); it comprises the genera Haplomitrium, Treubia, and Apotreubia.

It is estimated that there are 6000 to 8000 species of liverworts, at least 85 percent of which belong to the leafy group (Crandall-Stotler and Stotler 2000).

Economic importance

In ancient times, it was believed that liverworts cured diseases of the liver, hence the name (Dittmer 1964). In Old English, the word liverwort literally means liver plant (Raven et al. 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 1991).

Liverworts have little direct economic importance today. Their greatest impact is indirect, though 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 aquaria. Their thin, slender branches float on the water's surface and provide habitat for both small invertebrates and the fish that feed on them.

Some species can be a nuisance in shady green-houses or a weed in gardens (Schuster 1992).

Gallery

A small collection of images showing liverwort structure and diversity:

Marchantia polymorpha, with antheridial and archegonial stalks.
The archegonium of Porella.
A sporophyte emerging from its archegonium.
Porella platyphylla clump growing on a tree.
Pellia epiphylla, growing on moist soil.
Plagiochila asplenioides, a leafy liverwort.
Riccia fluitans, an aquatic thallose liverwort.
Conocephalum conicum, a large thallose liverwort.

References
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Allison, K. W., and J. Child. 1975. The Liverworts of New Zealand. Dunedin: University of Otago Press.
Bold, H. C., C. J. Alexopoulos, and T. Delevoryas. 1987. Morphology of Plants and Fungi, 5th edition. New York: Harper-Collins. ISBN 0060408381.
Campbell, D. H. 1918. The Structure and Development of Mosses and Ferns. London: The Macmillan.
Chopra, R. N., and P. K. Kumra. 1988. Biology of Bryophytes. New York: John Wiley. ISBN 0470213590.
Conard, H. S., and P. L. Redfearn. 1979. How to Know the Mosses and Liverworts. Dubuque, IA: William C. Brown. ISBN 0697047687.
Crandall-Stotler, B., and R. E. Stotler. 2000. Morphology and classification of the Marchantiophyta. In A. J. Shaw and B. Goffinet (eds.), Bryophyte Biology. Cambridge: Cambridge University. ISBN 0521660971.
Dittmer, H. J. 1964. Phylogeny and Form in the Plant Kingdom. Toronto: D. Van Nostrand.
Forrest, L. L, C. E. Davis, D. G. Long, B. J. Crandall-Stotler, A. Clark, and M. L. Hollingsworth. 2006. [303%3AUTEHOT2.0.CO%3B2&ct=1 Unraveling the evolutionary history of the liverworts (Marchantiophyta): Multiple taxa, genomes and analyses.] The Bryologist 109(3): 303–334. Retrieved October 20, 2008.
Fosket, D. E. 1994. Plant Growth and Development: A Molecular Approach. San Diego: Academic Press. ISBN 0122624300.
Goffinet, B. 2000. Origin and phylogenetic relationships of bryophytes. In A. J. Shaw and B. Goffinet (eds.), Bryophyte Biology. Cambridge: Cambridge University. ISBN 0521660971.
He-Nygrén, X., A. Juslén, I. Ahonen, D. Glenny, and S. Piippo. 2006. Illuminating the evolutionary history of liverworts (Marchantiophyta): Towards a natural classification. Cladistics 22(1): 1–31. Retrieved October 20, 2008.
Heinrichs, J., S. R. Gradstein, R. Wilson, and H. Schneider. 2005. Towards a natural classification of liverworts (Marchantiophyta) based on the chloroplast gene rbcL. Cryptogamie Bryologie 26(2): 131–150.
Hicks, M. L. 1992. Guide to the Liverworts of North Carolina. Durham: Duke University Press. ISBN 0822311755.
Kashyap, S. R. 1929. Liverworts of the Western Himalayas and the Panjab Plain, volume I. New Delhi: The Chronica Botanica.
Kenrick, P., and P. R. Crane. 1997. The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. ISBN 1560987308.
Malcolm, B., and N. Malcolm. 2000. Mosses and Other Bryophytes: An Illustrated Glossary. New Zealand: Micro-Optics Press. ISBN 0473067307.
Nehira, K. 1983. Spore germination, protonemata development and sporeling development. In R. M. Schuster (ed.), New Manual of Bryology, volume I. Nichinan, Miyazaki, Japan: The Hattori Botanical Laboratory. ISBN 49381633045.
Oostendorp, C. 1987. The Bryophytes of the Palaeozoic and the Mesozoic. Bryophytum Bibliotheca Band 34. ISBN 344362006X.
Pócs, T. 1982. Tropical forest bryophytes. In A. J. E. Smith (ed.), Bryophyte Ecology. London: Chapman and Hall. ISBN 0412223406.
Raven, P. H., R. F. Evert, and S. E. Eichhorn. 2005. Biology of Plants, 7th edition. New York: W. H. Freeman. ISBN 0716710072.
Renzaglia, K. S., S. Schuette, R. J. Duff, R. Ligrone, A. J. Shaw, B. D. Mishler, and J. G. Duckett. 2007. [179%3ABPATMA2.0.CO%3B2 Bryophyte phylogeny: Advancing the molecular and morphological frontiers.] The Bryologist 110(2): 179–213. Retrieved October 20, 2008.
Schofield, W. B. 1985. Introduction to Bryology. New York: Macmillan. ISBN 0029496608.
Schuster, R. M. 1992. The Hepaticae and Anthocerotae of North America, volume VI. Chicago: Field Museum of Natural History. ISBN 0914868217.
Schuster, R. M. 1966. The Hepaticae and Anthocerotae of North America, volume I. New York: Columbia University Press.
Stern, K. R. 1991. Introductory Plant Biology, 5th edition. Dubuque, IA: Wm. C. Brown. ISBN 0697099474.
Stotler, R. E., and B. J. Candall-Stotler. 1977. A checklist of the liverworts and hornworts of North America. The Bryologist 80: 405–428.
Taylor, T. N., and E. L. Taylor. 1993. The Biology and Evolution of Fossil Plants. Englewood Cliffs, NJ: Prentice Hall. ISBN 0136515894.
VanAller Hernick, L., E. Landing, and K. E. Bartowski. 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: 154–162. Retrieved October 20, 2008.

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