Liverwort
Liverwort
Liverworts are one of three classes in the plant phylum Bryophyta. The other two classes are mosses and hornworts. Liverworts are small green terrestrial plants. They do not have true roots, stems, or leaves. Instead, they have an above ground leaflike structure, known as a thallus, and an underground structure, known as a rhizoid. Most liverworts are found in moist environments and they tend to be less resistant to desiccation than their relatives, the mosses. Some liverwort species are found in temperate North America, but most species grow in the tropics.
General characteristics
Like mosses and higher plants, liverworts use chlorophyll-a, chlorophyll-b, and carotenoids as photosynthetic pigments and store their food reserves as starch. As in mosses and higher plants, their cell walls are composed of cellulose.
Like mosses and hornworts, liverworts are restricted to moist environments for two principal reasons: First, they lack a vascular system for efficient transport of water and food. Second, their sperm cells must swim through water to reach the egg cells.
The thalli of most liverworts have dorsiventral morphology. In other words, they have distinct front and back sides. In this respect, liverwort thalli are similar to the leaves of higher plants.
The name “liverwort” is centuries old and was given to these plants because their thalli are liver-shaped. In earlier times, people believed in the “doctrine of signatures,” which dictated that a plant part that resembles a bodily organ could be used to treat diseases of that organ. Thus, liverworts were used to treat diseases of the liver. Western science has long since discredited the doctrine of signatures, although it is still advocated by various “New Age” and other pseudoscientific movements.
Life cycle
The general features of the liverwort life cycle are the same as in mosses. Both have a characteristic alternation of generations in which the multicellular diploid sporophyte is dependent on the green and “leafy” haploid gametophyte. As with mosses, the gametophyte of liverworts is the form most commonly seen in nature.
In most species, a haploid liverwort spore germinates and gives rise to a single-celled protonema,
a small filamentous cell. In general, the haploid gametophyte develops from the protonema. In most liverworts, the gametophyte is procumbent, although in some species it is erect. Typically, the gametophyte has a subterranean rhizoid, a specialized single-celled structure that anchors the liverwort to its substrate and takes up nutrients from the soil.
Male and female reproductive organs, the anther-idia and archegonia, grow from the gametophyte. These arise directly from the thallus or are borne on stalks. About 80% of the liverwort species are dioecious (male and female on separate plants) and the other 20% are monoecious (male and female on the same plant). Each archegonium produces a single egg; each antheridium produces many motile sperm cells, each with two flagella. The sperm cells must swim through water to reach the archegonium. Then the sperm fertilizes the egg to form a diploid cell. This eventually develops into a multicellular diploid sporophyte.
The sporophyte of liverworts, like that of mosses, has a terminal capsule borne on a stalk, known as a seta. As the sporophyte develops, haploid spores form inside the capsule. In general, the sporophytes of liverworts are smaller and simpler in morphology than those of mosses. Another difference is that the liverwort seta elongates after capsule maturation, whereas the moss seta elongates before capsule maturation.
Spore Dispersal
Liverworts have a characteristic method of spore dispersal. Inside the sporophyte capsule, spores are attached to specialized cells known as elaters. The elater is tubular in shape and has one or more cell wall thickenings which are helically oriented along the long axis of the cell. These helical thickenings are hydroscopic, in that they readily absorb water.
KEY TERMS
Diploid —Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.
Elater —Specialized tubular cells with helically oriented cell wall thickenings to which liverwort spores are attached.
Gametophyte —The haploid, gamete-producing generation in a plant’s life cycle.
Gemma —Multicellular asexual reproductive structure of mosses and liverworts.
Haploid —Nucleus or cell containing one copy of each chromosome.
Meiosis —Division of the cell’s nucleus in which the number of chromosomes is reduced by half, typically from the diploid to the haploid.
Sporophyte —The diploid, spore-producing generation in a plant’s life cycle.
Thallus —A single plant body lacking distinct stem, leaves, and roots.
As the liverwort capsule dries, it opens. Then the helical cell wall thickenings of the elater dry out and the elater changes its shape. As this happens, the elater releases the bound spores, which are then dispersed by wind.
Asexual Reproduction
Like mosses, many species of liverworts reproduce by making gemmae, small circular or spherical reproductive structures borne inside gemmae cups that form on top of the thallus. Gemmae formation is an important form of asexual reproduction in many species of liverworts and mosses.
Evolution
There are only a few fossils of liverworts and mosses and there are no fossils of hornworts. This is because the soft tissue of these bryophytes does not fossilize well. The oldest known liverwort fossil is from the late Devonian period, about 350 million years ago. Most botanists believe that they originated long before this.
Some botanists have proposed that there are over 10,000 species of liverworts in the world. A more realistic estimate is about 6,000. The number of species may have been overestimated in the past because the morphology of many species is elastic, in that it differs in different environments. This makes identification of liverwort species very difficult, typically more difficult than that of higher plants.
Interestingly, even though liverworts originated several hundred million years before the flowering plants, there are several hundred thousand species of flowering plants but only about 6,000 species of liverworts. The reason for this may be that liverworts rely upon the inefficient mechanism of water-transported sperm for sexual reproduction. Thus, it has been proposed that most species of liverworts rely upon asexual gemmae as a means of reproduction. Asexual reproduction tends to reduce genetic diversity. Since genetic diversity is needed for new species to evolve, the liverworts and other bryophytes may have evolved into a sort of evolutionary dead end.
See also Bryophyte.
Resources
BOOKS
Greenaway, T. Mosses and Liverworts. Austin, TX: Steck-Vaughn, 1992.
Margulis, L., and Schwartz, K.V. Five Kingdoms. New York: W.H. Freeman and Company, 1998.
OTHER
Southern Illinois University Carbondale. “Bryophytes: Mosses, Liverworts, and Hornworts” <http://bryophytes.plant.siu.edu/marchantiophyta.html>(accessed December 2, 2006).
University of Massachusetts, Amherst: Biological Sciences. “Liverworts” <http://www.bio.umass.edu/biology/conn.river/liverwts.html> (accessed December 2, 2006).
Peter A. Ensminger
Liverwort
Liverwort
The liverworts are one of three classes in the plant phylum Bryophyta. The other two classes are mosses and hornworts. Liverworts are small, green, terrestrial plants. They do not have true roots, stems, or leaves. Instead, they have an above ground leaf-like structure, known as a thallus, and an underground structure, known as a rhizoid. Most liverworts are found in moist environments and they tend to be less resistant to desiccation than their relatives, the mosses. Many liverwort species are found in temperate North America , but most species grow in the tropics.
General characteristics
Like mosses and higher plants, liverworts use chlorophyll-a, chlorophyll-b, and carotenoids as photosynthetic pigments and store their food reserves as starch. As in mosses and higher plants, their cell walls are composed of cellulose .
Like mosses and hornworts, liverworts are restricted to moist environments for two principal reasons. First, they lack a vascular system for efficient transport of water and food. Second, their sperm cells must swim through water to reach the egg cells.
The thalli of most liverworts have dorsiventral morphology. In other words, they have distinct front and back sides. In this respect, liverwort thalli are similar to the leaves of higher plants.
The name "liverwort" is centuries old and was given to these plants because their thalli are liver-shaped. In earlier times, people believed in the doctrine of signatures. This dictated that a plant part which resembles a bodily organ can be used to treat diseases of that organ. Thus, liverworts were used to treat diseases of the liver. Western science has long since discredited the "doctrine of signatures," although it is still advocated by various "New Age" and other pseudoscientific movements.
Life cycle
The general features of the liverwort life cycle are the same as in mosses. Both have a characteristic alternation of generations in which the multicellular diploid sporophyte is dependent on the green and "leafy" haploid gametophyte. As with mosses, the gametophyte of liverworts is the form most commonly seen in nature.
In most species, a haploid liverwort spore germinates and gives rise to a single-celled protonema, a small filamentous cell. In general, the haploid gametophyte develops from the protonema. In most liverworts, the gametophyte is procumbent, although in some species it is erect. Typically, the gametophyte has a subterranean rhizoid, a specialized single-celled structure which anchors the liverwort to its substrate and takes up nutrients from the soil .
Male and female reproductive organs, the antheridia and archegonia, grow from the gametophyte. These arise directly from the thallus or are borne on stalks. About 80% of the liverwort species are dioecious (male and female on separate plants) and the other 20% are monoecious (male and female on the same plant). Each archegonium produces a single egg; each antheridium produces many motile sperm cells, each with two flagella . The sperm cells must swim through water to reach the archegonium. Then, the sperm fertilizes the egg to form a diploid cell. This eventually develops into a multicellular diploid sporophyte.
The sporophyte of liverworts, like that of mosses, has a terminal capsule borne on a stalk, known as a seta. As the sporophyte develops, haploid spores form inside the capsule. In general, the sporophytes of liverworts are smaller and simpler in morphology than those of mosses. Another difference is that the liverwort seta elongates after capsule maturation, whereas the moss seta elongates before capsule maturation.
Spore dispersal
Liverworts have a characteristic method of spore dispersal. Inside the sporophyte capsule, spores are attached to specialized cells known as elaters. The elater is tubular in shape and has one or more cell wall thickenings which are helically oriented along the long axis of the cell. These helical thickenings are hydroscopic, in that they readily absorb water.
As the liverwort capsule dries, it opens up. Then the helical cell wall thickenings of the elater dry out and the elater changes its shape. As this happens, the elater releases the bound spores which are then dispersed by wind .
Asexual reproduction
Like mosses, many species of liverworts reproduce by making gemmae. Gemmae are small circular or spherical reproductive structures which are borne inside gemmae cups. The gemmae cups form on top of the thallus. Gemmae formation is an important form of asexual reproduction in many species of liverworts and mosses.
Evolution
There are only a few fossils of liverworts and mosses and there are no fossils of hornworts. This is because the soft tissue of these bryophytes does not fossilize well. The oldest known liverwort fossil is from the late Devonian period, about 350 million years ago. Most botanists believe that they originated long before this.
Some botanists have proposed that there are over 10,000 species of liverworts in the world. A more realistic estimate is about 6,000 species. The number of species may have been overestimated in the past because the morphology of many species is plastic, in that it differs in different environments. This makes identification of liverwort species very difficult, typically more difficult than that of higher plants.
Interestingly, even though liverworts originated several hundred million years before the flowering plants, there are several hundred thousand species of flowering plants but only about 6,000 species of liverworts. The reason for this may be that liverworts rely upon the inefficient mechanism of water-transported sperm for sexual reproduction . Thus, it has been proposed that most species of liverworts rely upon asexual gemmae as a means of reproduction. Asexual reproduction tends to reduce genetic diversity. Since genetic diversity is needed for new species to evolve, the liverworts and other bryophytes may have evolved into a sort of evolutionary dead end.
See also Bryophyte.
Resources
books
Greenaway, T. Mosses and Liverworts. Orlando: Raintree Steck-Vaughn, 1992.
Margulis, L., and K.V. Schwartz. Five Kingdoms. W.H. Free-man and Company, 1998.
Peter A. Ensminger
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Diploid
—Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.
- Elater
—Specialized tubular cells with helically oriented cell wall thickenings to which liverwort spores are attached.
- Gametophyte
—The haploid, gamete-producing generation in a plant's life cycle.
- Gemma
—Multicellular asexual reproductive structure of mosses and liverworts.
- Haploid
—Nucleus or cell containing one copy of each chromosome.
- Meiosis
—Division of the cell's nucleus in which the number of chromosomes is reduced by half, typically from the diploid to the haploid.
- Sporophyte
—The diploid, spore-producing generation in a plant's life cycle.
- Thallus
—A single plant body lacking distinct stem, leaves, and roots.
liverwort
liv·er·wort / ˈlivərˌwərt; -ˌwôrt/ • n. a small flowerless green plant (class Hepaticae) with leaflike stems or lobed leaves, occurring in moist habitats. Liverworts lack true roots and reproduce by means of spores released from capsules.