The name liverwort means "Liver Herb" after a 19th century medieval dogma called the Doctrine of signatures
, where the appearance of a plant was believed to offer clues to the plants use.
Liverworts are considered to be the first plant to make the transition from the sea to land, some 400 million years ago, and share a common ancestry with green algae (Chlorophyta). The Hepatophyta division consists of about 8,500 species found throughout the world from the arctic to the tropics. Although some do grow in dry places and a few are aquatic, most are well adapted to the moist habitats in which they are found.
Liverworts can vary in size from less then 1 mm to 50 mm or more, and generally lie prostrate on the substrata. In appearance they can look leaf-like (leafy liverworts
) or form large flat sheets (thallose liverworts
) and are represented by approximately 60 families. Thallose liverworts are quite distinctive and are easily recognised, but the leafy ones are a different story being easily mistaken for mosses and it can take careful examination to tell the difference. See the "Difference
" page to help with identifications.
Liverworts of this group bear leaf-like structures, on a branched or unbranched stem, that are of a prostrate form with leaves which are generally only a single cell
thick. Often the leaves are divided into two or more lobes, and sometimes the lobes are folded to form various shapes. The leaves are most often arranged in two rows, but in many species there is a third row of much smaller leaves, which are often only visible with a hand-lens or microscope.
Liverworts of this type have a plant body with no stem-leaf structure, but consist instead of a "thallus
" a large flat plate, one to several cells thick. The thallus may be subdivided into lobes, and these may vary in width from 2-3 mm to 100 mm or more according to the species. In some species there is a thickened midrib, and some may have pores
(small holes) dotted about their surface.