Mesembs are annual to perennial succulents, ranging from minute to compact and tufted to prostrate to decumbent; herbs, dwarf shrubs, subshrubs or shrubs, they are rarely spiny. The leaves are opposite, rarely alternate, simple, free at the base to almost completely united into single, conical to ± spherical bodies, often sheathing the stem. Their shape may be flat to 3-angled to cylindrical. All leaves may be similar (plant homophyllous) or different types of leaf pairs may be present (plant heterophyllous), members of a leaf pair may be similar (plant isophyllous) or dissimilar (plant anisophyllous), heterocellular (with distinct differences in cell sizes) or homocellular (cells of similar size), ephemeral to persistent, with or without bladder cells, sometimes tuberculate or punctate. The flowers are bisexual, regular, in cymose inflorescences, sometimes representing the entire plant, to solitary. The calyx is composed of 4 to 6 lobes, sometimes forming a short tube above the ovary. The corolla is composed of many more or less linear petals of staminodial origin, either free or shortly fused at the base. Stamens are numerous, often surrounded by few to many filamentous staminodes. The floral nectaries are either koilomorphic: represented by usually 4 or 5 shell-shaped depressions (in Mesembryanthemoideae), or lophomorphic: raised into a crested ring, continuous or broken into ± 5 crested glands (in some Ruschioideae), or flat, in the more primitive Ruschioideae. The ovary is inferior to half-inferior (in Mesembryanthemoideae), (3, 4)5(-25)-locular; placentas ranging from axile through basal to parietal; ovules usually many; stigmas usually as many as locules (seed chambers), subulate to filiform, sometimes fused at the base, sometimes plumose. The fruit is a loculicidal capsule that splits open when dry or mostly when wetted by means of hygroscopic expanding keels, the fruit is rarely schizocarpic (splits into separate carpels) with winged or nut-like mericarps, or it is fleshy and indehiscent. The seeds are more or less kidney-shaped, glossy to textured, black to brown to pale cream; in the seed, the embryo is curved around the perisperm (food reserve tissue).