in botany: A genus of the natural order of algae, belonging to the cryptogamia class of plants. There is no calyx, but a vesicular cavity within the substance of the leaf. There is no corolla; the antherae are cylindrical, and sessile, placed on the germen, which is turbinate; the style is filiform, perforating the anthera; and the seed-case is spherical, crowned with the withered anthera; the seeds are hemispherical and pedicellate.