Hepatics are primarily divided into two groups, the leafy or foliose, and the thalloid or frondose. Owing to the phenomenon known as the alternation of generations, the liverwort “plants”, as they are usually seen and recognised, exhibit only one half of the life cycle of the organism. Briefly, the foliose hepatics are mosslike plants with a stem and leaves, and usually a third row of leaves on the under surface of the stem. The stems do not have woody fibres or vascular tissue, but there is a cortex and medulla. The variations in these and in the pattern of branching play a part in formulating genera. Unlike those of the mosses, the leaves are without a midrib and are often bilobed. The size and colour of the leafy hepatics vary considerably from minute terrestrial plants to large cushions or masses of creeping or pendulous leafy stems, in light or dark shades of green and brown or even in tones of rose.
The thalloid hepatics, less numerous than the foliose, consist of a thallus, a plant body not differentiated into stem and leaves, but flat and more or less broad. The thallus may be multi-layered, as in the order Marchantiales, or one-layered, as in Metzgeriales. “Rootlets”, called rhizoids (also common to the foliose hepatics), help to attach the thallus to the ground or substratum on which it grows.
There are at least three genera, Fossombronia, Noteroclada, and Treubia, which are intermediate between the foliose and frondose groups in that a more or less thalloid axis has lobes or leaflike appendages.