Riella
Encyclopedia
Riella is the only genus in the liverwort
family Riellaceae, and includes about eighteen species. Plants in the genus are small and grow submerged in shallow temporary pools. Although the genus is widely distributed in the Northern Hemisphere, locating populations is often difficult. Its occurrence is sporadic and local, and the tiny plants are ephemeral. The ornamented spore
s remain viable for several years, allowing the plants to survive annual drying of their habitat. The plants are easily grown in laboratory cultures.
. The plant consists of an erect central axis ("stem") that is commonly forked, but only sparingly, and the plants are bright green.
The stem bears a thin dorsal lamina or "wing", which being on only one side gives the plant an asymmetrical appearance. This lamina is ruffled or undulate. Early descriptions of the species Riella helicophylla overemphasized the spiralled form of the lamina, which does not occur in other species. One species, the Algerian Riella bialata bears two wings along its stems instead of the usual single lamina. The lamina has a thickness of only a single layer of cells, with all the cells thin-walled
and chlorophyllose.
In addition to the lamina, the central stem bears delicate, small, leaf-like scales in three series along its lateral and ventral aspects. The leaf scales are dimorphic and contain scattered oil cells, with a single oil body
per cell. Riella is thus the only member of its order with oil cells, although other genera in the class Marchantiopsida commonly have them.
Prior to 1900, Riella was thought to occur only in regions around the Mediterranean. The first collections from outside this region were published in 1902 when Porsild
reported collecting R. paulsenii in Kazakhstan
in 1898. Three further specimens from the United States
were reported in 1903, although one of these had been collected much earlier in 1855. Since that time, additional populations have been reported from South Africa
, southern Argentina
, and southeastern Australia
.
Riella species primarily grow as submerged aquatic plant
s, rarely floating or emergent. Although it is unique among hepatics as a submerged aquatic, it is not the only aquatic liverwort. The plants grow in fresh or brackish water of temporary bodies of water, and rarely in permanent ones.
. The group was given its own family Riellaceae 40 years later by Adolf Engler
. Modern bryologists classify the Riellaceae in the order Sphaerocarpales, along with the family Sphaerocarpaceae
. Studies comparing genetic sequences of Riella plants support this relationship.
Porsild
recognized a subgenus Trabutiella distinguished by the presence of longitudinal wings on the involucre. The only other subgenus (Riella subg. Riella) bears smooth involucres that lack these wings of tissue.
Species of Riella are distinguished using morphology of the spores or archegonial involucres, since the gametophytes of the genus possess few distinguishing non-reproductive characters. There are 19, 18, or 17 species in the genus.
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....
family Riellaceae, and includes about eighteen species. Plants in the genus are small and grow submerged in shallow temporary pools. Although the genus is widely distributed in the Northern Hemisphere, locating populations is often difficult. Its occurrence is sporadic and local, and the tiny plants are ephemeral. The ornamented spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s remain viable for several years, allowing the plants to survive annual drying of their habitat. The plants are easily grown in laboratory cultures.
Description
Riella plants are small, usually 2 inches (5.1 cm) or less, and thalloid, with an appearance like an immature algaAlgae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
. The plant consists of an erect central axis ("stem") that is commonly forked, but only sparingly, and the plants are bright green.
The stem bears a thin dorsal lamina or "wing", which being on only one side gives the plant an asymmetrical appearance. This lamina is ruffled or undulate. Early descriptions of the species Riella helicophylla overemphasized the spiralled form of the lamina, which does not occur in other species. One species, the Algerian Riella bialata bears two wings along its stems instead of the usual single lamina. The lamina has a thickness of only a single layer of cells, with all the cells thin-walled
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
and chlorophyllose.
In addition to the lamina, the central stem bears delicate, small, leaf-like scales in three series along its lateral and ventral aspects. The leaf scales are dimorphic and contain scattered oil cells, with a single oil body
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:...
per cell. Riella is thus the only member of its order with oil cells, although other genera in the class Marchantiopsida commonly have them.
Distribution
Although found nearly worldwide, Riella populations are sporadic and limited to only a few known localities. The plants are never common, and are found most often in places where the summer is dry and the winter is wet and mild.Prior to 1900, Riella was thought to occur only in regions around the Mediterranean. The first collections from outside this region were published in 1902 when Porsild
Morten Pedersen Porsild
right|thumb|200px|M. P. Porsild on his 80th birthday, September 1, 1952. Photo: Albert Schou jr.Morten Pedersen Porsild was a Danish botanist who lived and worked most of his adult life in Greenland. He participated in expeditions to Greenland in 1898 and 1902, together with the physiologist...
reported collecting R. paulsenii in Kazakhstan
Kazakhstan
Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...
in 1898. Three further specimens from the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
were reported in 1903, although one of these had been collected much earlier in 1855. Since that time, additional populations have been reported from South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, southern Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
, and southeastern 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...
.
Riella species primarily grow as submerged aquatic plant
Aquatic plant
Aquatic plants are plants that have adapted to living in aquatic environments. They are also referred to as hydrophytes or aquatic macrophytes. These plants require special adaptations for living submerged in water, or at the water's surface. Aquatic plants can only grow in water or in soil that is...
s, rarely floating or emergent. Although it is unique among hepatics as a submerged aquatic, it is not the only aquatic liverwort. The plants grow in fresh or brackish water of temporary bodies of water, and rarely in permanent ones.
Classification
The genus Riella was first published in 1852 by Camille MontagneCamille Montagne
Jean Pierre François Camille Montagne was a French military physician and botanist who specialized in the fields of bryology and mycology. He was born in the commune of Vaudoy in the department of Seine-et-Marne....
. The group was given its own family Riellaceae 40 years later by Adolf Engler
Adolf Engler
Heinrich Gustav Adolf Engler was a German botanist. He is notable for his work on plant taxonomy and phytogeography, like Die Natürlichen Pflanzenfamilien , edited with Karl A. E...
. Modern bryologists classify the Riellaceae in the order Sphaerocarpales, along with the family Sphaerocarpaceae
Sphaerocarpaceae
Sphaerocarpaceae is a family of liverworts known as bottle liverworts. Approximately ten species are included in this family, most of them in the genus Sphaerocarpos, but one additional species in the genus Geothallus....
. Studies comparing genetic sequences of Riella plants support this relationship.
Porsild
Morten Pedersen Porsild
right|thumb|200px|M. P. Porsild on his 80th birthday, September 1, 1952. Photo: Albert Schou jr.Morten Pedersen Porsild was a Danish botanist who lived and worked most of his adult life in Greenland. He participated in expeditions to Greenland in 1898 and 1902, together with the physiologist...
recognized a subgenus Trabutiella distinguished by the presence of longitudinal wings on the involucre. The only other subgenus (Riella subg. Riella) bears smooth involucres that lack these wings of tissue.
Species of Riella are distinguished using morphology of the spores or archegonial involucres, since the gametophytes of the genus possess few distinguishing non-reproductive characters. There are 19, 18, or 17 species in the genus.