Flatmosses are related to the terrestrial Bryophytes.
They are small (typically from 2-20 mm wide and less than 10 cm long) non-vascular plants with multi-cellular rhizoids. Most species have simple, leaf-like scales growing as a flattened thallic rosette. Each thallus cell contains just one chloroplast, which is fused with other organelles to form a large pyrenoid that both manufactures and stores food. Flatmosses develop internal mucilage-filled cavities when groups of cells break down.
They are widely distributed, occuring in almost every available habitat, most often in humid locations, and usually cover large patches of ground, rocks, trees or other reasonably firm substrates. Some species thrive in arid areas, where they curl up in a small tight ball during dry times, and uncurl in the presence of moisture.
Flatmosses have a gametophyte-dominant life cycle, i.e. the plant's cells are haploid for most of its lifecycle. Sporophytes are heterosporous and short-lived, dependent on the gametophyte. The megaspores have scale-like outgrowths near the base of the upper surface of each.