Plants with sunken growth never shed their leaves completely but keep their leaf tips level with the ground most of the time, being difficult to detect in habitat. In Aizoaceae, 73 species in the 12 genera Aloinopsis (1), Argyroderma (2), Conophytum (21), Dinteranthus (1), Fenestraria (1), Frithia (2), Gibbaeum (3), Lithops (36), Psammophora (1), Tanquana (2), Titanopsis (2), and Vanheerdea (1) possess this life form exclusively or predominantly; in Fenestraria (1), Frithia (2), and Lithops (36), only subterranean growth is known. Protection from predators and reduction of evaporation, supported by the development of mostly xeromorphic features in the outer epidermal wall, appear to be the main adaptive traits in all taxa, combined with the ability of the plants to inhabit small niches of soft soil in often stony terrain, making use of the lack of competition from bigger plants with more extensive root systems. Completely different strategies have been found, confirming that no single way of evolution for subterranean plants exists. In most genera, the sunken growth is found in few species only in marginal parts of the distribution area, but Lithops has spread over a very wide area with different climatic conditions, presenting all states of speciation and hence difficult to handle in the hierarchal taxonomic system. Conophytum, on the other hand, has evolved a number of species, too, but these are most often adapted to specific soils like quartz and seem to represent parallel evolutionary lines.