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 dorsiventrally 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 farmore 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 bymeristematic 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.
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 Jungermanniopsida, with most species in the order Jungermanniales. 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.
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 bryophytes, 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 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).
Archegonia 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 embryomay also be enclosed by a sheath of stem tissue (the perigynium).
After sporophyte 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 gametophyte, 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 specialized 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, amajor producer in mountain streams that is remarkably tolerant of acid mine drainage. Many species are epiphytes, festooning trees as pendent mats in temperate and 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.
Simple thalloid liverworts are placed in the class Jungermanniopsida, with most species in the order Metzgeriales. The straplike or ribbonlike thalli (bodies) grow bymeans of “apical initials” in marginal notches. Branching patterns vary, but thalli generally expand outward, forming circular mats.
In Pallavicinia, a distinct midrib contains “hydroids,” elongated cells specialized for water conduction, analogous to tracheids of higher plants. Archegonia and antheridia in the Metzgeriales are usually scattered on the thallus and are variously protected by sheaths, scales, flaps, and slime hairs.
At maturity, the sporophyte bursts through the calyptra, the capsule splits open into two to four valves, and spores and elaters are released. Some species also produce gemmae. Simple thalloid liverworts grow in the same range of habitats as leafy liverworts and are often intermingled with them.
Complex thalloid liverworts, or chamber liverworts, are placed in the class Marchantiopsida. They have an upper layer of loosely packed green filaments in boxlike “air chambers” and a lower layer of compact food storage cells.
Each air chamber has a pore in its “roof.” The waxy epidermis of the “roof” repels excess water, while the pores permit the gas exchange necessary for photosynthesis. Although pores cannot be opened and closed (as can stomata of higher plants), the complex pores of some species can shrink under dry conditions.
Although water-conducting cells occur in the midribs of some species, chamber liverworts, like all bryophytes, rely primarily on capillary water. Capillary spaces are abundant and occur among ventral scales and within a dense mat of rhizoids, smooth vertical rhizoids and horizontal rhizoids (with “pegged,” wavy walls) that conduct water along the liverwort’s underside.
Chamber liverworts grow by means of apical initials (protected by scales) in marginal notches. Thalli appear to branch dichotomously, repeatedly forking in two’s. However, this growth is pseudodichotomous; in true dichotomies, each apical initial splits into two new initials. As in most bryophytes, colonies “grow ahead and die behind,” so that one colony fragments into two (or more) new colonies.
Reproductive structures of chamber liverworts are highly specialized. In Marchantia, raindrops splash gemmae out of gemma cups. A constriction in the cup speeds up the water’s movement and enhances dispersal.
Also in Marchantia, thallus branches have been modified into umbrella-like gametangiophores; these elevate antheridia and sporangia and thereby enhance rain dispersal of sperm and wind dispersal of spores. In male plants, rain strikes the “umbrellas,” with their sunken antheridia, and splashes out sperm.
In female plants, sporophytes (with reduced setae) are borne on the underside of the “umbrellas”; mature capsules burst through the calyptrae, tear open irregularly, and release spores and elaters.Asexual reproduction (via gemmae) is favored by short days, whereas sexual reproduction (via sporophytes) is favored by long days.
Other chamber liverworts are less specialized than Marchantia. For example, Conocephalumhas female umbrellas only and lacks gemma cups. In Riccia and Ricciocarpus, gametangia and sporangia are sunken into the thalli, and spores are liberated when the thalli decay.
In both genera, spore release by disintegration is synchronized with seasonal change and is just as adaptive as the complex “umbrellas” of Marchantia. Riccia fluitans and Ricciocarpus natans form floating mats on ponds, but other Riccia species dominate the extensive “cryptogamic crusts” on arid plains in Australia.
Marchantia is found on recently burned ground in humid areas, whereas Conocephalum frequently grows on shaded ledges along streams. Chamber liverworts have been extensively used as experimental subjects. Sex chromosomes in plants were first discovered in the chamber liverwort Sphaerocarpos.