Haplomitriopsida
Encyclopedia
Haplomitriopsida is a newly recognized class
of liverworts
comprising fifteen species in three genera
. Recent cladistic
analyses of nuclear
, mitochondrial, and plastid
gene sequences place this monophyletic
group as the basal sister group to all other liverworts. The group thus provides a unique insight into the early evolution
of liverworts in particular and of land plants
in general.
grow as a prostrate leafy thallus
. The bifid leaves extend like wings on either side of the midrib, or may be folded upwards and pressed close together, giving the plants a ruffled appearance. By contrast, Haplomitrium grows as a subterranean rhizome
with erect leafy stems. The thin, rounded leaves are arranged around the upright stems, giving the appearance of a soft moss. The species Haplomitrium ovalifolium of Australia
often has bifid leaves that are asymmetrical
, somewhat like those in Treubia.
Haplomitrium has a number of unique characters that distinguish it from other liverworts, such as lacking rhizoids. The vegetative stems possess a central water
-conducting strand with large perforations derived from plasmodesmata
. This central strand is surrounded by a cylinder of cells that conduct food throughout the plant. Such an arrangement is evocative of the xylem and phloem
found in vascular plant
s. Although some thalloid liverwort species in the Pallaviciniaceae
also possess a central conducting strand, Haplomitrium differs in having a food-conducting layer and in producing no callose
.
Treubia also has features that differ from those found in other bryophytes, such as the differentiation of five identifiable zones in the stem midrib. Unlike other leafy species, the oil bodies
in its cells
are restricted to certain clusters of cells, as they are in the Marchantiopsida. These oil body clusters appear as dark spots in the leaves when the plant is held up to the light.
n distribution with its center of diversity
in Australasia
. Such a distribution implies that the modern genera radiated prior to the beginning of the Cretaceous
when Gondwana broke apart. Schuster proposes that species distributed in the northern hemisphere "rafted" on the India
n subcontinent
to Asia
, then spread across the Bering Strait
into North America
.
Most species in the Haplomitriopsida are found in south of the equator
, though there are northern ones. The genus Treubia
is restricted to the southern hemisphere, while Apotreubia
has one species in New Guinea
and another disjunct between eastern Asia
and British Columbia
. The genus Haplomitrium exhibits a wider distribution, with species in both North
and South America
, northern and central Europe
, the Himalayas
, Japan
, and Australasia
.
, each with one family
. The group as a whole comprises fifteen species
in three genera
. A fourth genus, Gessella
, is known only from Permian
fossil
s. The orders, families, and genera are as follows:
An additional fossil Treubiites kidstonii previously has been compared to the extant genus Treubia
. However, upon re-examination of the material, specimens were determined to be more like Blasia
and not at all to resemble Treubia as previously thought. Accordingly, Treubiites is now assigned to the Blasiales
rather than the Haplomitriopsida.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of liverworts
Marchantiophyta
The Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryophytes, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....
comprising fifteen species in three genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
. Recent cladistic
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
analyses of nuclear
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
, mitochondrial, and plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
gene sequences place this monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
group as the basal sister group to all other liverworts. The group thus provides a unique insight into the early evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of liverworts in particular and of land plants
Embryophyte
The land plants or embryophytes, more formally Embryophyta or Metaphyta, are the most familiar group of plants. They are called 'land plants' because they live primarily in terrestrial habitats, in contrast with the related green algae that are primarily aquatic. The embryophytes include trees,...
in general.
Description
Plants of TreubiaTreubia
Treubia is a genus of liverworts in the family Treubiaceae. There are six species, all of which are restricted to the southern hemisphere. Five of the species occur in Australasia and the other occurs in Chile. All species are dioicous, with separate male and female gametophytes....
grow as a prostrate leafy thallus
Thallus (tissue)
Thallus, from Latinized Greek θαλλός , meaning a green shoot or twig, is the undifferentiated vegetative tissue of some organisms in diverse groups such as algae, fungus, some liverworts, lichens, and the Myxogastria. Many of these organisms were previously known as the thallophytes, a polyphyletic...
. The bifid leaves extend like wings on either side of the midrib, or may be folded upwards and pressed close together, giving the plants a ruffled appearance. By contrast, Haplomitrium grows as a subterranean rhizome
Rhizome
In botany and dendrology, a rhizome is a characteristically horizontal stem of a plant that is usually found underground, often sending out roots and shoots from its nodes...
with erect leafy stems. The thin, rounded leaves are arranged around the upright stems, giving the appearance of a soft moss. The species Haplomitrium ovalifolium of Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
often has bifid leaves that are asymmetrical
Asymmetry
Asymmetry is the absence of, or a violation of, symmetry.-In organisms:Due to how cells divide in organisms, asymmetry in organisms is fairly usual in at least one dimension, with biological symmetry also being common in at least one dimension....
, somewhat like those in Treubia.
Haplomitrium has a number of unique characters that distinguish it from other liverworts, such as lacking rhizoids. The vegetative stems possess a central water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
-conducting strand with large perforations derived from plasmodesmata
Plasmodesmata
Plasmodesmata are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Species that have plasmodesmata include members of the Charophyceae, Charales and Coleochaetales , as well as all embryophytes, better known...
. This central strand is surrounded by a cylinder of cells that conduct food throughout the plant. Such an arrangement is evocative of the xylem and phloem
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
found in vascular plant
Vascular plant
Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...
s. Although some thalloid liverwort species in the Pallaviciniaceae
Pallaviciniaceae
Pallaviciniaceae is a widely-distributed family of liverworts in the order Metzgeriales. All species are thallose, typically organized as a thick central costa , each side with a broad wing of tissue one cell in thickness. All species are dioicous...
also possess a central conducting strand, Haplomitrium differs in having a food-conducting layer and in producing no callose
Callose
Callose is a plant polysaccharide. It is composed of glucose residues linked together through β-1,3-linkages, and is termed a β-glucan. It is thought to be manufactured at the cell wall by callose synthases and is degraded by β-1,3-glucanases. It is laid down at plasmodesmata, at the cell...
.
Treubia also has features that differ from those found in other bryophytes, such as the differentiation of five identifiable zones in the stem midrib. Unlike other leafy species, the oil bodies
Oil body
An oil body is a lipid-containing structure found in plant cells. The term can refer to at least two distinct kinds of structures in different kinds of plants.-Oil bodies in liverworts:...
in its cells
Plant cell
Plant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...
are restricted to certain clusters of cells, as they are in the Marchantiopsida. These oil body clusters appear as dark spots in the leaves when the plant is held up to the light.
Diversity
Living representatives of the group exhibit an essentially GondwanaGondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
n distribution with its center of diversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
in Australasia
Australasia
Australasia is a region of Oceania comprising Australia, New Zealand, the island of New Guinea, and neighbouring islands in the Pacific Ocean. The term was coined by Charles de Brosses in Histoire des navigations aux terres australes...
. Such a distribution implies that the modern genera radiated prior to the beginning of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
when Gondwana broke apart. Schuster proposes that species distributed in the northern hemisphere "rafted" on the India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
n subcontinent
Subcontinent
A subcontinent is a large, relatively self-contained landmass forming a subdivision of a continent. By dictionary entries, the term subcontinent signifies "having a certain geographical or political independence" from the rest of the continent, or "a vast and more or less self-contained subdivision...
to Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
, then spread across the Bering Strait
Bering Strait
The Bering Strait , known to natives as Imakpik, is a sea strait between Cape Dezhnev, Chukotka Autonomous Okrug, Russia, the easternmost point of the Asian continent and Cape Prince of Wales, Alaska, USA, the westernmost point of the North American continent, with latitude of about 65°40'N,...
into North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
.
Most species in the Haplomitriopsida are found in south of the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
, though there are northern ones. The genus Treubia
Treubia
Treubia is a genus of liverworts in the family Treubiaceae. There are six species, all of which are restricted to the southern hemisphere. Five of the species occur in Australasia and the other occurs in Chile. All species are dioicous, with separate male and female gametophytes....
is restricted to the southern hemisphere, while Apotreubia
Apotreubia
Apotreubia is a genus of liverworts in the family Treubiaceae. There are two species: Apotreubia nana, which is found in subalpine New Guinea, and Apotreubia pusilla, which has a disjunct distribution between eastern Asia and British Columbia....
has one species in New Guinea
New Guinea
New Guinea is the world's second largest island, after Greenland, covering a land area of 786,000 km2. Located in the southwest Pacific Ocean, it lies geographically to the east of the Malay Archipelago, with which it is sometimes included as part of a greater Indo-Australian Archipelago...
and another disjunct between eastern Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
and British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
. The genus Haplomitrium exhibits a wider distribution, with species in both North
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, northern and central Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, the Himalayas
Himalayas
The Himalaya Range or Himalaya Mountains Sanskrit: Devanagari: हिमालय, literally "abode of snow"), usually called the Himalayas or Himalaya for short, is a mountain range in Asia, separating the Indian subcontinent from the Tibetan Plateau...
, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, and Australasia
Australasia
Australasia is a region of Oceania comprising Australia, New Zealand, the island of New Guinea, and neighbouring islands in the Pacific Ocean. The term was coined by Charles de Brosses in Histoire des navigations aux terres australes...
.
Classification
Class Haplomitriopsida includes two ordersOrder (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, each with one family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
. The group as a whole comprises fifteen species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
in three genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
. A fourth genus, Gessella
Gessella
Gessella is a fossil genus of liverworts in the family Haplomitriaceae. All known fossils come from Early Permian deposits of Western Sealand....
, is known only from Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s. The orders, families, and genera are as follows:
- Order HaplomitrialesHaplomitrialesHaplomitriales is an order of plants known as liverworts. The order is also called Calobryales in some sources, but the genus Calobryum is a synonym for Haplomitrium....
- Family Haplomitriaceae
- GessellaGessellaGessella is a fossil genus of liverworts in the family Haplomitriaceae. All known fossils come from Early Permian deposits of Western Sealand....
† - Haplomitrium (8 species)
- Gessella
- Family Haplomitriaceae
- Order Treubiales
- Family TreubiaceaeTreubiaceaeTreubiaceae is a family of liverworts in the order Treubiales. Species are large and leafy, and were previously classified among the Metzgeriales....
- ApotreubiaApotreubiaApotreubia is a genus of liverworts in the family Treubiaceae. There are two species: Apotreubia nana, which is found in subalpine New Guinea, and Apotreubia pusilla, which has a disjunct distribution between eastern Asia and British Columbia....
(2 species) - TreubiaTreubiaTreubia is a genus of liverworts in the family Treubiaceae. There are six species, all of which are restricted to the southern hemisphere. Five of the species occur in Australasia and the other occurs in Chile. All species are dioicous, with separate male and female gametophytes....
(6 species)
- Apotreubia
- Family Treubiaceae
An additional fossil Treubiites kidstonii previously has been compared to the extant genus Treubia
Treubia
Treubia is a genus of liverworts in the family Treubiaceae. There are six species, all of which are restricted to the southern hemisphere. Five of the species occur in Australasia and the other occurs in Chile. All species are dioicous, with separate male and female gametophytes....
. However, upon re-examination of the material, specimens were determined to be more like Blasia
Blasia
Blasia pusilla is the only species in the liverwort genus Blasia. It is distinguished from Cavicularia by the presence of a collar around the base of the sporophyte capsule, and a scattered arrangement of sperm-producing antheridia....
and not at all to resemble Treubia as previously thought. Accordingly, Treubiites is now assigned to the Blasiales
Blasiales
Blasiales is an order of liverworts with a single living family and two species. The order has traditionally been classified among the Metzgeriales, but molecular cladistics suggests a placement at the base of the Marchantiopsida.- External links :* *...
rather than the Haplomitriopsida.