Mature thalloid liverwortsassume theform of a flattened and undifferentiated body of greenish plant tissuethatlacks either stems or leaves. The thalloid body of these liverwortsgrows by developing lobes up to 15 mm. across (rarely more). Becausemultiple lobes can develop from the same thalloid body, the largerthalloid liverworts can form bodies, or mats of ribbon-like bodies,spanning several inches in diameter (1 inch =2.5 cm.). However, smaller thalloid liverworts may span only a fewmillimeters across. The lower (ventral) side of thalloid liverwortsdevelop two kinds of rhizoids: short peg-like rhizoids in rows and longfibrous rhizoids. While rhizoids resemble the roots of vascular plants,they are not connected to a network of veins in the plant body. Therhizoids of liverworts are single-celled, unlike the multicellularrhizoids of mosses. The short peg-like rhizoids are thought to play alimited role in absorbing water and possibly nutrients from thesubstrate and transporting them to the thalloid body of the liverwort.Because the water and nutrients have to pass through cell membranes,this process is less efficient than what is encountered in vascularplants. The long fibrous rhizoids allow the thalloid body to adhere tothe underlying substrate. Minute air bubbles are visible in the bodiesof some thalloid liverworts; the cells of liverworts usually containoil bodies that may protect them from herbivores, fungi, or bacteria.The thalloid bodies of these liverworts are gametophytes that canproduce antheridia (male sexual organs) and archegonia (female sexualorgans) on either the same plant (monoecious) or different plants(dioecious). This can vary with the liverwort species, although mostthalloid liverworts are monoecious. Across differentspecies of thalloid liverworts, there is considerable variety in thestructures that are associated with the production of the sexualorgans. In the simplest arrangement, both types of sexual organs areproduced within the thalloid body of the liverwort; they are located ator near the upper (dorsal) surface. These sexual organs may, or maynot, be associated with conspicuous tubercles, cavities, orflask-shapedstructures on the upper surface. When environmental conditions aremoist and a film of water is present on the thallus, 2-tailed sperm arereleased from the antheridia and they swim about until they either dieor find the ovules of the archegonia. The sperm may be attracted bychemicals that are released by the archegonia, although to some extentthis process is random. After fertilization occurs, the ovules of thearchegonia develop into spores (sporophyte). These spores are notreleased until the thalloid body of the liverwort forms cavities abovethem, or until the thalloid body breaks apart and dies. The spores arethen transported by water or animals. Other thalloid liverworts developmore elaborate structures for the archegonia and sometimes also theantheridia. For example, the thalloid liverwort,Marchantiapolymorpha,forms an umbrella-like structure on a long stalk to facilitate thedistribution of sperm from the antheridia. Because the antheridiadevelop on the dorsal surface of this structure, the sperm can bedistributed in part by water currents and raindrop logistics.Similarly, a structure resembling a palm tree (several elliptic lobesat the apex of a stalk) develops in this liverwort to facilitate thespread of itsspores. The archegonia are located along the lower sides of the lobeson this structure. After fertilization of the ovules occurs, the tinyspores aredistributed primarily by wind. Other thalloid liverworts produce theirantheridia near the upper surface of their thalloid bodies (sometimesindicated by tubercles), but their archegonia are produced on stalkedstructures resembling umbrellas or palm trees. Some thalloid liverwortsare able to reproduce asexually by forming gemmae (small clonal plantbodies that resemble buds) along the margins of their lobed bodies. Thegemmae eventually separate from the mother plant, forming new plantswith rhizoids in other locations. Water currents can spread the gemmaefor considerable distances. A very small number of thalloid liverworts,such as Marchantia polymorpha, also form gemmae cups on the uppersurface of the thalloid body. The gemmae in these cups are surroundedby a short fringed membrane, and they are distributed to new locationsby water currents and raindrop logistics.
The other major group,the leafy liverworts, superficially resemble mosses. Like mosses, leafyliverworts have leafy stems, rather than thalloid bodies. Most leafyliverworts are quite small in size – their leafy shoots typicallyspan about 0.5–6 mm. across and 6-80 mm. in length. The leafy stems canbe ascending to erect, or they sprawl across the substrate as a low matof leafy stems (the latteris more common). Colonies of plants are often produced. Usually, thestems branch readily, but sometimes they are unbranched or sparinglybranched (this varies with the species). The leaves of leafy liverwortsare arranged very differently from the typical moss species. All leafyliverworts have pairs of opposite lateral leaves along their stems, andmany species also have underleaves on the lower side of their stems.Unlike mosses, the leaves are often lobed. The leaves of theseliverworts can assume various shapes, including linear, oval,orbicular, reniform (kidney-shaped), and more complex forms. The upperlateral leaves of some species are divided into distinct upper lobesand lower lobes (lobules), causing each leaf to appear like 2 leaveswith one above the other. The leaf margins of leafy liverworts usuallylack teeth, although a minority of species have leaf margins that aretoothed or fringed to resemble teeth. Fibrous rhizoids areproduced from the axils of the underleaves, if they are present,otherwise they are produced sparingly along the lower side of thestems. These rhizoids are used to anchor theplant to the underlying substrate. Like other bryophytes, leafyliverworts produce male sexual organs (antheridia) and female sexualorgans (archegonia) on the same plants (monoecious) or different plants(dioecious). Thus, the leafy stems of these liverworts aregametophytes. The antheridia are usually produced on short lateralstemswith modified lateral leaves (or leafy bracts); the antheridia aretypicallylocated on the lower axils of these leaves. The archegonia, incontrast, are produced at the tips of stems inside an erect perianth.The perianth consists of 3 modified leaves (or leafy bracts) that arejoined together, forming atubular-angular bud about 1-2 mm. high. The base of the perianth may bepartially hidden by other modified leaves (leafy bracts). Whenenvironmentalconditions are moist and a film of water covers the surface of plants,two-tailed sperm are released from the antheridia. The sperm swimaround until they find the ovules of the archegonia and fertilize them,assuming they don't die first. Chemicals may be released by thearchegonia to attract the sperm, although to some extent they swimaround at random. After fertilization, a sporophyte develops from theperianth, consisting of a short-lived spore-bearing capsule on a stalk.The capsule is usually ovoid or globoid in shape and black or brown,while the stalk is typically translucent white. Because there is nogreen photosynthetic tissue nor pores in the sporophyte, it isdependent on the gametophyte for its source of water, carbohydrates,and minerals. At maturity, thecapsule of the sporophyte divides into 4 parts to release the tinyspores to the wind. Some leafy liverworts are capable of reproducingasexually by forming gemmae along the margins of their leaves. Thesesmall bud-like gemmae eventually become detached from the leaves, afterwhich they are capable of forming new clonal plants at differentlocations.