The Encyclopedia Americana (1920)/Liverworts

LIVERWORTS, or HEPATICÆ, a group of cryptogamous plants, forming one of the two divisions of the class Bryophyta or Muscincæ (moss-worts), and closely related to the true mosses (Musci), with which some of the species are apt to be confounded. They are either spread out in the form of a simple lobed thallus showing differentiation into a dorsal (upper) and a ventral (lower) surface, or they are composed of a small ramified stem bearing sessile leaves in two or three ranks. Root-like bodies (rhizoids) attach the plant to its substratum. Many liverworts reproduce themselves by means of brood-cells (thallidia or gemmæ), formed asexually in cups on the surface, in leaf-margins, etc. They are also reproduced sexually by means of club-shaped antheridia, containing the male elements (antherozoids), and flask-shaped archegonia, containing each an egg-cell or oösphere. These sexual organs occur in groups either in small depressions or special outgrowths of the thallus, or as so-called flowers at the tips of the leafy shoots, or in the axils of their leaves. The spore-capsule is formed after fertilization within the archegonium, and the spores are often provided with hygroscopic elaters which assist in their dispersal. On germination a spore produces, not the common liverwort plant, but a very small filamentous protonema. There are four families of liverworts, namely, Ricciaceæ, Marchantiaceæ, Anthocerotaceæ and Jungermanniaceæ. The first includes the duck-weed-like crystalwort (Riccia natans); the second the exceedingly common Marchantia polymorphia, formerly used as a basis for medicine for ailments of the liver (whence the name “liverwort”); and the last, which is much the largest family, comprises all the leafy, as well as some thalloid forms. The Hepaticæ are generally distributed over the world, and prefer situations similar to those occupied by the mosses. There are about 4,000 species, of which about 3,500 belong to the Jungermanniaceæ. See authorities on cryptogamic botany, especially Cooke's ‘British Hepaticæ’ (1893); and Strasburger, ‘Text-book of Botany’ (1903).