Everything about Marchantiophyta totally explained
Mid-
Devonian emend. 2000
| subdivision_ranks = Classes and Orders
| subdivision =
Haplomitriopsida Stotler & Stotl.-Crand.
The
Marchantiophyta are a division of
bryophyte plants commonly referred to as
hepatics or
liverworts. Like other bryophytes, they've a
gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information.
It is estimated that there are 6000 to 8000 species of liverworts, though when
Neotropical regions are better studied this number may approach 10,000. 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 typically small, usually from 2-20 mm wide with individual plants less than 10 cm 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 green-houses or a weed in gardens.
Physical characteristics
Description
Most liverworts are small, usually from 2-20 mm wide with individual plants less than 10 cm long, so they're 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 three or more ranks, the middle rank being conspicuously different from the outer ranks; these are called
leafy liverworts or
scale liverworts. (
See the gallery below for examples.)
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; In addition, 90% 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. 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. Another unusual feature of the liverwort life cycle is that sporophytes (for example the
diploid body) are very short-lived, withering away not long after releasing spores. Even in other
bryophytes, the sporophyte is persistent and disperses spores over an extended period.
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 else 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 swim from the antheridia where they're produced to the archegonium where the eggs are held. The
sperm of liverworts is
biflagellate, for example they've two tail-like
flagellae that aid in propulsion. 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 can't occur.
In the presence of such water, sperm from the antheridia swim to the archegonia and
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. As with most groups of living plants, they're 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
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). 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 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 don't have
stomata on the
sporophyte generation. Among 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. In 2007, the oldest fossils assignable to the liverworts were announced,
Metzgeriothallus sharonae from the
Givetian (Middle
Devonian) of
New York,
USA.
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 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:
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).
A third class, the Haplomitriopsida is newly recognized as a basal sister group to the other liverworts; In Old English, the word liverwort literally means liver plant. 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".
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.
Gallery
A small collection of images showing liverwort structure and diversity:
Image:Marchantia.jpg|Marchantia polymorpha, with antheridial and archegonial stalks.
Image:Archegonium.jpg|The archegonium of Porella.
Image:Porella SPT.jpg|A sporophyte emerging from its archegonium.
Image:Porella platyphylla.jpg|Porella platyphylla clump growing on a tree.
Image:Pellia epiphylla5 ies.jpg|Pellia epiphylla, growing on moist soil.
Image:Plagiochila aspleniodes0.jpg|Plagiochila asplenioides, a leafy liverwort.
Image:RicciaFluitans1.jpg|Riccia fluitans, an aquatic thallose liverwort.
Image:Liverwort.jpg|Conocephalum conicum, a large thallose liverwort.
Further Information
Get more info on 'Marchantiophyta'.
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