Riella plants are small, usually 2 inches (5.1 cm) or less, and thalloid, with an appearance like an immature alga. The plant consists of an erect central axis ("stem") that is commonly forked, but only sparingly, and the plants are bright green. The stem bears a thin dorsal lamina or "wing", which being on only one side gives the plant an asymmetrical appearance. This lamina is ruffled or undulate. Early descriptions of the species Riella helicophylla overemphasized the spiralled form of the lamina, which does not occur in other species. One species, the Algerian Riella bialata bears two wings along its stems instead of the usual single lamina. The lamina has a thickness of only a single layer of cells, with all the cells thin-walled and chlorophyllose. In addition to the lamina, the central stem bears delicate, small, leaf-like scales in three series along its lateral and ventral aspects. The leaf scales are dimorphic and contain scattered oil cells, with a single oil body per cell. Riella is thus the only member of its order with oil cells, although other genera in the class Marchantiopsida commonly have them.