Liverworts are non-vascular land plants in the division Marchantiophyta (), also known as hepatics. Liverworts, mosses, and hornworts are collectively known as bryophytes. Like mosses and hornworts, liverworts have a gametophyte-dominant life cycle in which the sporophyte is dependent on the gametophyte.
Liverworts occur as either thallose or leafy forms. They are distinguished from mosses by features including their single-celled rhizoids, the absence of a costa (midrib) in the leaves, and the presence of unique membrane-bound complex oil bodies.
There are an estimated 9,000 species of liverworts, most of them leafy species. They occur in many terrestrial ecosystems worldwide, especially in moist tropical regions, although some species are adapted to deserts and other dry environments.
Physical characteristics Description Most liverworts are small, measuring from wide with individual plants less than long,Schuster, Rudolf M. The Hepaticae and Anthocerotae of North America, vol. I, pp. 243–244. (New York: Columbia University Press, 1966) so they are 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 two or more ranks, the middle rank is often conspicuously different from the outer ranks; these are called leafy liverworts or scale liverworts.Kashyap, Shiv Ram. Liverworts of the Western Himalayas and the Panjab Plain, vol. I, p. 1. (New Delhi: The Chronica Botanica, 1929)Schofield, W. B. Introduction to Bryology, pp. 135–140. (New York: Macmillan, 1985). . (See the gallery below for examples.)
Liverworts can most reliably be distinguished from the apparently similar mosses by their single-celled rhizoids.Nehira, Kunito. "Spore Germination, Protonemata Development and Sporeling Development", p. 347 in Rudolf M. Schuster (Ed.), New Manual of Bryology, volume I. (Nichinan, Miyazaki, Japan: The Hattori Botanical Laboratory, 1983). . Other differences are not universal for all mosses and all liverworts; but the lack of clearly differentiated stem and leaves in thallose species, or in leafy species the presence of deeply lobed or segmented leaves and the presence of leaves arranged in three ranks,Allison, K. W. & John Child. The Liverworts of New Zealand, pp. 13–14. (Dunedin: University of Otago Press, 1975).Conard, Henry S. and Paul L. Redfearn, Jr. How to Know the Mosses and Liverworts, revised ed., pp. 12–23. (Dubuque, Iowa: William C. Brown Co., 1979) as well as frequent dichotomous branching, all point to the plant being a liverwort. With a few exceptions, all liverworts undergo polyplastidic meiosis, in contrast to mosses and hornworts which have monoplastidic meiosis. Unlike any other embryophytes, most liverworts contain unique membrane-bound oil bodies containing isoprenoids in at least some of their cells, lipid droplets in the cytoplasm of all other plants being unenclosed.Bold, H.C.; Alexopoulos, C.J. & Delevoryas, T. (1987) Morphology of Plants and Fungi, 5th ed., p. 189. (New York: Harper-Collins). . 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, like other bryophytes, have a gametophyte-dominant life cycle, with the sporophyte dependent on the gametophyte. The sporophyte of many liverworts are non-photosynthetic, but there are also several that are photosynthetic to various degrees.Glime, J. M. 2017. Photosynthesis: The Process. Chapt. 11-1. In: Glime, J. M. Bryophyte Ecology. Volume 1. Physiological 11-1-1 Ecology. Ebook sponsored by Michigan Technological University and the International Association of Bryologists. Last updated 18 July 2020. Available at <http://digitalcommons.mtu.edu/bryophyte-ecology/>. Cells in a typical liverwort plant each contain only a single set of genetic information, so the plant's cells are haploid for the majority of its life cycle. This contrasts sharply with the pattern exhibited by nearly all animals and by vascular plants. In the more familiar seed plants, the haploid generation is represented only by the tiny pollen and the ovule, while the diploid generation is the familiar tree or other plant.Fosket, Donald E. Plant Growth and Development: A Molecular Approach, p. 27. (San Diego: Academic Press, 1994). . Another unusual feature of the liverwort life cycle is that sporophytes (i.e. the diploid body) are very short-lived, withering away not long after releasing spores.Hicks, Marie L. Guide to the Liverworts of North Carolina, p. 10. (Durham: Duke University Press, 1992). . In mosses, the sporophyte is more persistent and in hornworts, the sporophyte 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 a flattened thallus.Nehira, Kunito. "Spore Germination, Protonemata Development and Sporeling Development", pp. 358–374 in Rudolf M. Schuster (Ed.), New Manual of Bryology, volume I. (Nichinan, Miyazaki, Japan: The Hattori Botanical Laboratory, 1983). .Chopra, R. N. & P. K. Kumra. Biology of Bryophytes, pp. 1–38. (New York: John Wiley & Sons, 1988). . 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). Each archegonium has a slender hollow tube, the "neck", down which the sperm swim to reach the egg cell.
Liverwort species may be either dioicous or monoicous. In dioicous liverworts, female and male sex organs are borne on different and separate gametophyte plants. In monoicous liverworts, the two kinds of reproductive structures are borne on different branches of the same plant.Malcolm, Bill & Nancy Malcolm. Mosses and Other Bryophytes: An Illustrated Glossary, pp. 6 & 128. (New Zealand: Micro-Optics Press, 2000). . In either case, the sperm must move from the antheridia where they are produced to the archegonium where the eggs are held. The sperm of liverworts is biflagellate, i.e. they have two tail-like flagellae that enable them to swim short distances,Campbell, Douglas H. The Structure and Development of Mosses and Ferns, pp. 73–74. (London: The Macmillan Co., 1918) provided that at least a thin film of water is present. Their journey may be assisted by the splashing of raindrops. As well, researchers have observed liverworts "firing" sperm-containing water up to 15 cm through the air, enabling fertilization of female plants growing more than a metre away.
When sperm reach the archegonia, 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. The sporophyte lacks an apical meristem, an auxin-sensitive point of divergence with other land plants some time in the Late Silurian/Early Devonian.
When the sporophyte has developed all three regions, the seta elongates, pushing its way out of the archegonium and rupturing it. While the foot remains anchored within the parent plant, the capsule is forced out by the seta and is extended away from the plant and into the air. Within the capsule, cells divide to produce both elater cells and spore-producing cells. The elaters are spring-like, and will push open the wall of the capsule to scatter themselves when the capsule bursts. The spore-producing cells will undergo meiosis to form haploid spores to disperse, upon which point the life cycle can start again.
Asexual reproduction Some liverworts are capable of asexual reproduction; in bryophytes in general "it would almost be true to say that vegetative reproduction is the rule and not the exception." For example, in Riccia, when the older parts of the forked thalli die, the younger tips become separate individuals.
Some thallose liverworts such as Marchantia polymorpha and Lunularia cruciata produce small disc-shaped gemmae in shallow cups.Smith, AJE (1989) The Liverworts of Britain and Ireland, Cambridge University Press, Cambridge. Marchantia gemmae can be dispersed up to 120 cm by rain splashing into the cups. In Metzgeria, gemmae grow at thallus margins. Marchantia polymorpha is a common weed in greenhouses, often covering the entire surface of containers; gemma dispersal is the "primary mechanism by which liverwort spreads throughout a nursery or greenhouse."
Symbiosis Thalloid liverworts typically harbor symbiotic glomeromycete fungi which have arbuscular (cilia-bearing) rootlets resembling those in vascular plants. Species in the Aneuraceae, however, associate with basidiomycete fungi belonging to the genus Tulasnella, while leafy liverworts typically harbor symbiotic basidiomycete fungi belonging to the genus Serendipita.
Ecology Today, liverworts can be found in many ecosystems across the planet except the sea and excessively dry environments, or those exposed to high levels of direct solar radiation.Schuster, Rudolf M. The Hepaticae and Anthocerotae of North America, vol. I, pp. 243–249. (New York: Columbia University Press, 1966). As with most groups of living plants, they are most common (both in numbers and species) in moist tropical areas.Pócs, Tamás. "Tropical Forest Bryophytes", p. 59 in A. J. E. Smith (Ed.) Bryophyte Ecology. (London: Chapman and Hall, 1982). . Liverworts are more commonly found in moderate to deep shade, though desert species may tolerate direct sunlight and periods of total desiccation.
Classification Evolution Epiphytic thalloid liverworts evolved during the Triassic to the Cretaceous.
Relationship to other plants Liverworts and mosses have many similar properties but can be distinguished with some work. A key difference is that the rhizoid of a liverwort is unicellular while for mosses the structure will be multicellular. Liverworts frequently have a thallus which is never present for mosses. Conversely, moss leaves may have costa, or a midrib, which do not appear in liverwort leaves. 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 also distinguished from mosses in having unique complex oil bodies of high refractive index.
Leafy species can be distinguished from the apparently similar mosses on the basis of a number of features, including the thalloid structure, their single-celled rhizoids and the presence of a costa (midrib) in the leaves. Liverworts are also distinguished from mosses in having unique complex oil bodies of high refractive index.