Plants with only a single leaf-pair during most of the year are presented, their morphological features are discussed and the ecological situations in which they grow are investigated. The seven genera with such members, namely Argyroderma (2 species), Conophytum (14 species), Didymaotus (1 species), Dinteranthus (3 species), Lithops (2 species), Pleiospilos (2 species), and Tanquana (1 species) represent different evolutionary lineages; even within a given genus, the unbranched species can belong in different groups. The ecological adaptation cannot be related to any one distinct feature; instead, the success of the extremely reduced growth form in very shallow soils or tiny pockets can be understood as an exploitation of lack of competition from other plants requiring more space for larger root systems in these places. A key to the species included is given, maps for the genera and the relevant species are supplied.