Liverworts
Categories: Medicine and health; Liverworts Hornworts and Mosses Plasma Membrane" href="/plasma-membrane/the-nontracheophytes-liverworts-hornworts-and-mosses.html">nonvascular plants; paleobotany; Plantae; taxonomic groups
Liverworts (phylum Hepatophyta) are one of three ancient lines ofbryophytes (liverworts, hornworts, and mosses): low-growing land plants that depend on free water (rain) for fertilization.
Liverworts, with about six thousand species, generally prefer somewhat cooler, moister, shadier, and more acidic habitats than mosses. Like any bryophyte, a liverwort has a dominant (conspicuous) green gametophyte and a small, attached sporophyte, which is a single-stalked sporangium that developed from a fertilized egg. As in hornworts, liverwort gametophytes are typically dorsi-ventrally symmetrical (flattened). A unique feature of liverworts is the presence, in the gametophyte, of oil bodies, cellular organelles that produce aromatic terpenoids.
Many freshly collected liverworts have a pleasing aroma, which quickly disappears as oil bodies disintegrate. Possibly defending liverworts from herbivores, terpenoids (chemically diverse and found in 90 percent of liverworts) have potential medicinal value. Liverwort sporophytes mature while completely enclosed in the gametophyte. Thereby shielded from natural selection, they are far more uniform than moss sporophytes. A typical liverwort sporophyte comprises a foot, a fleshy stalk (seta), and a round to cylindrical capsule that splits open to release spores and elaters. The seta is green when young but is short-lived and grows only by cell elongation (not by meristematic cells as in other bryophytes). Elaters, unique to liverworts, are cells with spirally thickened walls. Their jerky, hygroscopic movements help disperse spores from the capsule.
Leafy Liverworts
Liverwort gametophytes are distinctive. They are either leafy (about two-thirds of the species) or thalloid (straplike), whereas all mosses are leafy. Liverwort leaves are often round and lobed, unlike the pointed leaves of mosses. Liverwort gametophytes are anchored by unicellular rhizoids (hairs), whereas the rhizoids of mosses are multicellular. Leafy liverworts are placed in the class Junger-manniopsida, with most species in the order Junger-manniales. The leaves are only one cell thick and lack midribs. The rounded cells have numerous chloroplasts and variable numbers of oil bodies; these resemble clusters of grapes in some species.
Stems are creeping or ascending and usually bear three rows of leaves: two rows of dorsal leaves and (in most species) one row of ventral leaves or underleaves. Leaves generally overlap and are attached to the stem at a slanted angle (a transverse angle is less common). The arrangement of the leaves in leafy liverworts can be referred to as being either succubous or incubous, based on the way the leaves overlap. In succubous species the leaves overlap, as do the shingles of a roof; the upper part of a leaf is covered by the next leaf above it (toward the apex). In incubous species, leaves overlap in the opposite way (away from the apex). Leaves of many species are divided into lobes and filaments, giving the gametophyte a delicate appearance. For example, Frullania has two rows of dorsal leaves, one row of bifid underleaves, and two rows of helmet-shaped ventral leaf lobes or "water sacs" (in which "wheel animals," or rotifers, may live). Trichocolea has leaves divided into filaments that resemble wool. The external complexity of leafy liverworts makes them well suited for capillary conduction and storage of rainwater. However, like most bryo-phytes, leafy liverworts have a thin cuticle (or lack one); after a rain, they soon dry out and become inactive. Upon remoistening, they quickly revive.
The archegonia (egg sacs) of leafy liverworts develop at the tips of stems and branches, whereas the Sexual Reproduction 1 Flowering Plants" href="/flowering-plants/sexual-reproduction-1.html">antheridia (sperm sacs) are produced behind the apex. These gametangia are protected from drying out by slime hairs (which secrete mucilage) and bracts (specialized leaves). Arche-gonia may be concealed within an envelope of fused leaves (the perianth). After fertilization, the base of the archegonium swells into a calyptra that protects the embryo. The embryo may also be enclosed by a sheath of stem tissue (the perigynium). After sporo-phyte maturation (often in spring or fall) the seta elongates and forces the capsule through the protective layers. The capsule splits open, its four valves resembling a small flower, and releases the spores and elaters. A few days later, the delicate seta collapses and dies.
If a spore lands in a favorable site, it may germinate and develop into a new leafy gameto-phyte, which grows by means of an apical cell (initial) at the stem tip. Liverworts of moist habitats, such as Frullania, often show precocious spore germination, and juvenile gametophytes (rather than spores) are shed from the capsule. Juvenile gametophytes are often globular, rather than threadlike, as in mosses. Liverwort spores that are unusually tolerant of cold, dry conditions may be dispersed over great distances by wind. Many leafy liverworts also produce abundant spe-
cialized asexual propagules (gemmae), which may be dispersed by rain, wind, or the feet of animals. Asexual reproduction helps compensate for infrequent sexual reproduction; most leafy liverworts (like most bryophytes) are unisexual, and sometimes male and female plants live far apart.
Leafy liverworts flourish in humid, shaded habitats and are often pioneers on rocks, tree trunks, decaying logs, stumps, and soil by streams, ponds, footpaths, and roads. Habitats range from sunny ridges to deeply shaded gorges. A few species are aquatic, such as Scapania undulata, a major producer in mountain streams that is remarkably tolerant of acid mine drainage. Many species are epiphytes, festooning trees as pendent mats in temperate and
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Classification of Liverworts | |
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Subclass Jungermanniae | |
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Order Calobryales |
Order Metzgeriales |
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Family: |
Families: |
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Haplomitriaceae |
Allisoniaceae |
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Order Jungermanniales |
Aneuraceae |
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Families: |
Blasiaceae |
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Acrobolbaceae |
Fossombroniaceae |
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Antheliaceae |
Metzgeriaceae |
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Arnelliaceae |
Pallaviciniaceae |
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Calypogeiaceae |
Pelliaceae |
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Cephaloziaceae |
Treubiaceae |
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Cephaloziellaceae |
Subclass Marchantiae |
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Chonecoleaceae |
Order Marchantiales |
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Geocalycaceae |
Families: |
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Gymnomitriaceae |
Aytoniaceae |
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Gyrothyraceae |
Cleveaceae |
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Herbertaceae |
Conocephalaceae |
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Jubulaceae |
Corsiniaceae |
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Jungermanniaceae |
Lunulariaceae |
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Lejeuneaceae |
Marchantiaceae |
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Lepidoziaceae |
Monosoleniaceae |
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Mastigophoraceae |
Oxymitraceae |
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Mesoptychiaceae |
Ricciaceae |
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Plagiochilaceae |
Targioniaceae |
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Pleuroziaceae |
Order Sphaerocarpales |
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Porellaceae |
Families: |
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Pseudolepicoleaceae |
Riellaceae |
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Ptilidiaceae |
Sphaerocarpaceae |
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Radulaceae | |
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Scapaniaceae | |
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Trichocoleaceae | |
Source: Data are from U.S. Department of Agriculture, National Plant Data Center, The PLANTS Database, Version 3.1, http://plants.usda.gov. National Plant Data Center, Baton Rouge, LA 70874-4490 USA.
Source: Data are from U.S. Department of Agriculture, National Plant Data Center, The PLANTS Database, Version 3.1, http://plants.usda.gov. National Plant Data Center, Baton Rouge, LA 70874-4490 USA.
tropical rain forests. Although leafy liverworts are (like most bryophytes) typically perennial, their substrata are often "temporary" on a scale of years (fields, flood plains), decades (logs), or centuries (old-growth trees). Propagules (spores and gemmae) enable them to "shuttle" to new substrata as they become available.
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