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 "Hepaticae" from Ernst Haeckel's Kunstformen der Natur, 1904 | ||||
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Jungermanniopsida
Marchantiopsida
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Liverworts are a division of plants commonly called hepatics, Marchantiophyta or liverworts. They are typically small plants that are often overlooked. They frequently have the appearance of small irregular leaf-like plaques, often covering large areas of the ground but they may also occur on rocks, trees or any other reasonably firm substrate. They can also take on a form very much like flattened mosses. They most often occur in damp locations and are typically found in moderate to deep shade. Some species can be a nuisance in shady green-houses. They do not have flowers or seeds.
Overview
Botanically, liverworts are bryophytes, or non-vascular plants. They 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, 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.
Aside from lacking a vascular system, liverworts have a gametophyte-dominant life cycle, i.e. the plant's cells are haploid for most of its life cycle. Sporophytes (i.e. the diploid body) are short-lived and dependent on the gametophyte. This is in contrast to the pattern exhibited by vascular plants and by most animals. In vascular plants, for example, the haploid generation is represented by the pollen and the ovule, whilst the diploid generation is the familiar flowering plant.
Originally, the Marchantiophyta were grouped as class Hepaticae alongside the mosses in the Division Bryophyta, but the liverworts are now usually given their own division with two classes: Jungermanniopsida (simple thalloids and leafy liverworts) and the Marchantiopsida (complex-thallus liverworts and bottle hepatics). It is estimated that there are 6000 to 8000 species of liverworts, at least 85% of which belong to the leafy group.
Today, liverworts can be found in many ecosystems across the planet except the sea and 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.
Description
The most familiar liverworts consist of a prostrate, flattened, 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 conspicously different from the outer ranks. These are called leafy liverworts or scale liverworts.
They can be distinguished from the apparently similar mosses by their single celled rhizoids. Other differences are not universal for all mosses and all liverworts, but the lack of clearly differentiated stem and leaves, 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. Confirmation of the identifiaction of a moss or a leafy liverwort can only be performed with certainty by micr oscopical investigation.
Aside from lacking a vascular system, liverworts have a gametophyte-dominant life cycle, i.e. the plant's cells are haploid for most of its life cycle. Sporophytes (i.e. the diploid body) are short-lived and dependent on the gametophyte. This is in contrast to the pattern exhibited by most higher plants and animals. In higher plants, for example, the haploid generation is represented by the pollen and the ovule while the diplod generation is the familiar flowering plant.
Life cycle
The life of a liverwort starts from a haploid spore, which germinates to produce a protonema, which is either a mass of filaments or thalloid (flat and thallus-like). This is a transitory stage in the life of a liverwort. From the protonema grows the gametophore ("gamete-bearer") that produces the sex organs of the liverworts. The female organs are known as archegonia (singular archegonium) and are protected by the perichaetum (plural perichaeta). The archegonia have necks called venters which the male sperm swim down. The male organs are known as antheridia (singular antheridium) and are enclosed by the perigonium (plural perigonia).
Liverworts can be either dioicous or monoicous. In dioecious liverworts, female and male sex organs are borne on different plants. In monoecious liverworts, they are borne on the same plant. In the presence of water, sperm from the antheridia swim to the archegonia and fertilisation occurs, leading to the production of a diploid sporophyte. The sperm of liverworts is biflagellate, i.e. they have two flagellae that aid in propulsion. Without water, fertilisation cannot occur. After fertilisation, the immature sporophyte elongates, pushing its way out of the archegonial venter. The sporophyte body comprises a long stalk, called a seta, and a spherical or ellipsoidal capsule. Within the capsule, cells divide to produce elater cells and spore-producing cells that will undergo meiosis to form haploid spores, upon which the cycle can start again.
Classification
In ancient times, it was believed that liverworts cured diseases of the liver, hence the name. In Old English, the word liverwort literally means liver plant. This probably stemed 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 for 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".
Bryologists classify liverworts in the division Marchantiophyta. This divisional name is based on the name of the type species Marchantia polymorpha. 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.
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). However, since this grouping makes the Bryophyta paraphyletic, the liverworts are now usually given their own division. 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.
Another reason that liverworts are now classified separately is that liverworts 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. The earliest fossils believed to be liverworts are compression fossils of Pallaviciniites from the Upper Devonian of New York. 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.
The Marchantiophyta is subdivided into two classes. The Jungermanniopsida includes primarily the two orders Metzgeriales (simple thalloids) and Jungermanniales (leafy liverworts), as well as a smaller order Haplomitriales. The Marchantiopsida includes primarily the orders Marchantiales (complex-thallus liverworts) and Sphaerocarpales (bottle hepatics), as well as the problematic genus Monoclea, which is sometimes placed in its own order Monocleales.
See also
References
- Bold, Alexopoulos, and Delevoryas (1987). Morphology of Plants and Fungi. New York: Harper-Collins.
- Crandall-Stotler, Barbara. & Stotler, Raymond E. (2000). Morphology and classification of the Marchantiophyta. In A. Jonathan Shaw & Bernard Goffinet (Eds.), Bryophyte Biology, pp. 21-70. Cambridge: Cambridge University Press. ISBN 0-521-66097-1.
- Kenrick, Paul & Crane, Peter R. (1997). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. ISBN 1-56098-730-8.
- Schofield, W. B. (1985). Introduction to Bryology. New York: Macmillan. ISBN 0-02-949660-8.
- Taylor, Thomas N. & Taylor, Edith L. (1993). The Biology and Evolution of Fossil Plants. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-651589-4.