Bryophyta

(botany) A small phylum of the plant kingdom, including mosses, liverworts, and hornworts, characterized by the lack of true roots, stems, and leaves.

A division that consists of some 23,000 species of small and relatively simple plants commonly known as mosses, granite mosses, peat mosses, liverworts, and hornworts. The bryophytes display a distinct alternation of sexual and asexual generations; the sexual gametophyte, with a haploid chromosome number, is the more diversified. The sporebearing, diploid sporophyte is reduced in size and structure, attached to the gametophyte, and partially or almost completely dependent on it.

The gametophytes may consist of leafy stems or flat thalli. They have no roots but are anchored to the substrate by hairlike rhizoids. Vascular tissue is at best poorly differentiated, with no lignification of cells. Growth results from the divisions of single cells (rather than meristematic tissues) located at stem tips or in notches at the margins of thalli. The sex organs are multicellular and have a jacket of sterile cells surrounding either the single egg produced in flask-shaped archegonia or the vast number of sperms produced in globose to cylindric, stalked antheridia. The sperms swim by means of two flagella. The sporophyte commonly consists of a capsule that produces a large number of spores, a stalklike seta, and a swollen foot anchored in the gametophyte. The spores, nearly always single-celled, are dispersed in the air, except in the case of a small number of aquatics. They germinate directly or produce a juvenile stage called a protonema. See also Reproduction (plant).

The division can be divided into five classes: Sphagnopsida (peat mosses), Andreaeopsida (granite mosses), Bryopsida (true mosses), Hepaticopsida (liverworts), and Anthocerotopsida (hornworts). The mosses have radially organized leafy gametophytes that develop from a protonema and have multicellular rhizoids with slanted crosswalls. The liverworts and hornworts are mostly flat and dorsiventrally organized and have no protonematal stage; the rhizoids are unicellular. Though obviously related, as evidenced by similar sex organs and attachment of a simplified sporophyte to a more complex and independent gametophyte, the classes differ greatly in structural detail. See also Andreaeopsida; Anthocerotopsida; Bryopsida; Hepaticopsida; Plant kingdom; Sphagnopsida.