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Liverworts are non-vascular plants, also called hepatics. Originally such plants were grouped together as the Division Bryophyta, within which the liverworts made up the class Marchantiopsida. However, since this makes the Bryophyta paraphyletic, the liverworts are now usually given their own division, Hepatophyta, with a single class, Hepaticopsida. Liverworts appear to have diverged from all other embryophyte plants near the beginning of their evolution.
Most liverworts consist of a prostrate, flattened, branching structure called a thallus (plant body). These liverworts are termed thallose liverworts. Other liverworts produce flattened stems with overlapping scales or leaves in three or more ranks, the middle rank being conspicously different from the outer ranks. These are called leafy liverworts or scale liverworts.
They can be distinguished from the apparently similar mosses by their single celled rhizoids. Other differences are not universal for all mosses and all liverworts, but the lack of clearly differentiated stem and leaves, the presence of deeply lobed or segmented leaves, and the presence of leaves arranged in three ranks all point to the plant being a liverwort. Confirmation of the identifiaction of a moss or a leafy liverwort can only be performed with certainty by microspoical investigation.
Aside from lacking a vascular system, liverworts have a gametophyte-dominant life cycle, i.e. the plant's cells are haploid for most of its life cycle. Sporophytes (i.e. the diploid body) are short-lived and dependent on the gametophyte. This is in contrast to the pattern exhibited by most higher plants and animals. In higher plants, for example, the haploid generation is represented by the pollen and the ovule while the diplod generation is the familiar flowering plant.
Life cycle
The life of a liverwort starts from a haploid spore, which germinates to produce a protonema, which is either a mass of filaments or thalloid (flat and thallus-like). This is a transitory stage in the life of a liverwort. From the protonema grows the gametophore ("gamete-bearer") that produces the sex organs of the liverworts. The female organs are known as archegonia (singular archegonium) and are protected by the perichaetum (plural perichaeta). The archegonia have necks called venters which the male sperm swim down. The male organs are known as antheridia (singular antheridium) and are enclosed by the perigonium (plural perigonia).
Liverworts can be either dioecious or monoecious). In dioecious liverworts, male and female sex organs are borne on different plants. In monoecious liverworts, they are borne on the same plant. In the presence of water, sperm from the antheridia swim to the archegonia and fertilisation occurs, leading to the production of a diploid sporophyte. The sperm of mosses is biflagellate, i.e. they have two flagella that aid in propulsion. Without water, fertilisation cannot occur. After fertilization, the immature sporophyte pushes its way out of the archegonial venter. It takes about a quarter to half a year for the sporophyte to mature. The sporophyte body comprises a long stalk, called a seta, and a capsule capped by a cap called the operculum. The capsule and operculum are in turn sheathed by a haploid calyptra which is the remains of the archegonial venter. The calyptra usually falls off when the capsule is mature. Within the capsule, spore-producing cells undergo meiosis to form haploid spores, upon which the cycle can start again.
In ancient times, it was believed that liverworts cured diseases of the liver, hence the name. This probably stemed from the superficial appearance of some thalloid liverworts, which resemble a liver in outline. An unrelated flowering plant, Hepatica, is sometimes also refered to as liverwort because it was once also used in treating diseases of the liver.
References
- Bold, Alexopoulos, and Delevoryas, 1987, Morphology of Plants and Fungi, Harper-Collins, NY.
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