Thallus

Thallus, a plant body which is either nonsegmented, or, if segmented, segmented only into similar members - i.e., in some cases branched, but exhibiting no differentiation into root and shoot, or into stem and leaf. Thus among the lower Thallophyta (q.v.), in which there is no alternation of generations, the thallus may be a single spherical cell, or a spherical coenobium with multilateral symmetry; or it may be filamentous, with or without branches. The thallus of the higher Algae and Fungi is far more complex; but it is a structure which is represented outside the sub-kingdom Thallophyta, as by the dorsiventral dichotomous gametophyte of most Hepaticae (q.v.), by fern prothallia, by the archisperm in all spermaphytes, and by the main body of the minute rootless duckweed, Wolffia arrhiza.