A liverwort is a flowerless, spore-producing plant - with the spores produced in small capsules. The introductory WHAT IS A BRYOPHYTE? page noted that bryophytes have a gametophyte stage and a sporophyte stage. The spore capsule (possibly with a supporting stalk, or seta) is the sporophyte and this grows from the gametophyte stage.
The aim of this page is simply to describe the features you can see in a liverwort. You will see much, but by no means all, of the variety to be found in the liverworts. While the identification of liverworts often requires the use of a microscope, you can learn a lot just by using your eyes and a handlens that magnifies 10 times. In the reference button you’ll find some books with good colour photographs of Australian liverworts. Looking through them will give you a good introduction to liverwort diversity.
The following references are very useful for more detail about this great diversity, from the macroscopic view to the microscopic level. Much of the following information on this page has come from these books.
What's in a name?
The English word "wort" means "small plant" and it turns up in names such as Pennywort and Bladderwort. The term liverwort originated from the fact that the early herbalists thought that one of the liverworts had some resemblance to a liver - and some use as medicine for liver ailments. Hence the word liverwort for a "liver-like small plant".
As well as the term liverwort you may also see the alternative term hepatic used and this comes from the Greek word "hepatikos" - meaning liver. Do not confuse the ordinary English word hepatic (meaning liverwort) with the genus name Hepatica. The latter is in fact in the flowering plant family Ranunculaceae! That family also contains the genus Ranunculus, the plants of which are commonly called Buttercups.
We'll start with definitions of the two broad liverwort groups - leafy and thallose. After that there'll be a few general points that apply to both types of liverworts. However, liverworts show such diversity that it is not possible to make too many general statements that apply to both the leafy and thallose species. After that, there are links to sections that go into more detail about the different types of liverworts.
In broad terms, a liverwort gametophyte will show one of two forms, depending on the genus. In a leafy liverwort the gametophyte consists of leaves on stems. In a thallose liverwort you'll see a flattish, green sheet - possibly wrinkled or lobed. Such a sheet is called a thallus and, in many cases, that's all there is to the gametophtyte. However, in some thallose genera there's a bit more to the gametophyte, as shown in the accompanying photograph of a Marchantia . Those umbrella-like structures, which have grown out from the flat, green thallus, are still part of the gametophyte. Those umbrellas will be explained at the end of this page.
Thallus and thallose
Don’t confuse these two words. The first is the name of the flattish green sheet and the second is an adjective that describes that sheet-like growth form.
The thallus of a thallose liverwort can be anything from thin and translucent to thick and opaque, depending on the genus. The liverwort of the early herbalists was Marchantia polymorpha and one characteristic of all species of Marchantia is that the thalli are thick and opaque. Such a thallus is many cells thick and the cells in different layers within the thallus have different functions. Thallose liverworts in which there is such a differentiation of cell function are called complex thallose liverworts. On the other hand, thallose liverworts in which there is no such differentiation of cell function are called simple thallose liverworts. The upper surface of the thallus is almost always some shade of green. The genus Cryptothallus provides the exception - with a whitish thallus. This genus is unusual in that it does not photosynthesize, but relies on a fungus for its food. There's some more about this in the ECOLOGY-FUNGI SECTION. Thus far, Cryptothallus has been found in several European countries, Greenland and Costa Rica .
Many people are familiar with the thick, green and somewhat leathery sheets of the complex thallose species Lunularia cruciata . This is a cosmopolitan species that is very commonly found in gardens, parks and in pots in nurseries and glasshouses. The complex thallose genera Lunularia and Marchantia are probably the most widely noticed liverworts. But the simple thallose species outnumber the complex thallose species. However, the simple thallose liverworts are far less robust and are therefore easily overlooked. Finally, there are far more species of leafy liverworts than there are of thallose liverworts.
Many people will have seen leafy liverworts - in their own gardens, in public parks, in the bush - but have simply thought of them as just another moss. You might easily confuse leafy liverworts with mosses (which also have a leaves-on-stems growth form). You might even mistake some of the thin, thallose liverworts for mosses - because those liverworts have thin, translucent thalli and moss leaves are typically thin and translucent. You might confuse the thick, thallose liverworts with hornworts (which also have a flattish-sheet growth form). However, once you’ve read this page as well as the WHAT IS A MOSS? and WHAT IS A HORNWORT? pages, you will have all the information to let you tell the three bryophyte groups apart. For convenience, the distinguishing features of all the bryophytes are summarised on the page that lets you answer the question: WHICH BRYOPHYTE IS IT?
The division of the liverworts into leafy and thallose is very useful and is used by all bryologists. However, it is important to note that there are a few liverworts, classified as thallose, which come very close to leafy in appearance. The liverworts show a great variety of gametophytic form (far greater than that shown by mosses or hornworts). Regardless of whether a liverwort is leafy or thallose, the gametophyte is the dominant stage - in terms of both bulk and longevity. Sporophytes are fairly ephemeral. This is markedly different to the flowering plants where the sporophyte is the dominant stage.
All liverworts produce mucilage, which helps liverworts absorb and retain water. The mucilage is produced by the gametophytes, either internally in slime cells or externally in slime papillae. The latter are simply very tiny outgrowths, possibly stalked, from the gametophyte. Amongst the thallose liverworts there are genera (such as Riccardia, right) in which the mucilage is produced by slime papillae and genera (such as Marchantia) with internal slime cells. In the leafy liverworts mucilage is produced in slime papillae, which may be found on stems or leaf tips, depending on the species. Liverworts produce mucilage at the growing points and this mucilage protects the growing points from drying out.
Since liverworts are photosynthesizing plants, their cells contain chloroplasts. In addition to chloroplasts, the cells of about 90% of liverwort species contain oil bodies . These vary in size, shape and number per cell, depending on species and are therefore useful for identification. While often colourless, brown and blue oil bodies are also found. There are species in which the oil bodies are found in the majority of cells, while in others they are confined to isolated cells. In some cases the oil bodies are persistent and can be found in dried, herbarium specimens but in many species the oil bodies disintegrate when a specimen is dried for storage in a herbarium and the oil bodies are then permanently lost. The compounds found in these oil bodies are various terpenoids and the amount produced various between species. In many cases the functions of these compounds are unknown, but they do give distinctive aromas or tastes to various liverworts.
Another feature common to virtually all liverworts is the presence of rhizoids. These are anchoring structures, superficially root-like, but without the absorptive functions of true roots. Liverworts in the genus Haplomitrium lack rhizoids and have a rhizome-like growth, with both erect and subterranean stems. As a rule liverwort rhizoids are single-celled, with just a few species having multi-celled rhizoids.
The male and female gametes (sperm and eggs) are produced on the gametophyte (in antheridia and archegonia, respectively) and a fertilized egg will develop into a spore-bearing sporophyte. Thus the spores are part of the sexual reproduction cycle. There's more about this in the REPRODUCTION SECTION.
Liverwort sporophytes may develop in a variety of ways, which cannot be summarised simply, so they'll be covered in more detail in the sections dealing with the different types of liverworts. However, there are some common features that can be given here. First, there is always a spore-containing capsule. This capsule is spherical to cylindrical and is blackish when mature. The mature capsules spilt open to release the spores. Within the capsules of most species there are elaters as well as spores. . ELATERS are microscopic, spiral-like structures which usually twist or untwist in response to changes in humidity and often help in spore dispersal. Moss spore capsules don't have elaters, but you do find elaters (or pseudo-elaters) in hornwort capsules. Also, the spore capsules in almost all moss species open by a definite mouth, rather than by splitting.
In the leafy liverworts the sporophyte consists of a spore capsule atop a flimsy stalk (or seta). The seta is attached to a stem of the gametophyte. In thallose liverworts the sporophyte may appear in various ways, including the same capsule-on-seta form that is found in leafy liverworts. However, in some genera the spore capsules appear on quite complex supporting structures. In the genus Marchantia the spore capsules are formed on the undersides of the umbrella-like structures shown in this photo. The umbrella and the flat, green thallus it grew from are gametophytic and the sporophytes are just the capsules, hidden within that yellow fluffy stuff (the yellow fluffy stuff is usually the remains of elaters and spores after the capsule has split) that you can see in the photo. The stem holding up the umbrella is not a seta holding up a capsule.
You can find out more about the different types of liverworts by following the links below. These sections give descriptions of the features you can see in many genera. While not an exhaustive coverage, you'll be able to get a good overview of the structural variety to be found in the liverworts. Moreover, you'll find out more about the similarities and differences between the different types of liverworts.