Mostly delicate herbaceous plants with creeping or ascending, rarely erect, dichotomously branched stems and small green leaves. The leaves are small, single-veined, entire or fine-toothed, identical or dimorphic, arranged spirally or in four rows. At the base of the leaf blade is a small scale called the tongue. The plant is kept in the soil by dichotomously branched roots, which often develop at the ends of rhizophores. Micro- and megaspores develop on one plant. Gametophytes are formed unisexual.