Pereskia, as traditionally circumscribed, is a genus of 17 tropical species and varieties of cacti that do not look much like other types of cacti, having substantial leaves and thin stems. They originate from the region between Brazil and Mexico. Members of this genus are usually referred to as lemon vines, rose cacti or leaf cacti, though the latter also refers to the genus Epiphyllum. The genus is named after Nicolas-Claude Fabri de Peiresc, a 16th century French botanist.
Species of Pereskia generally resemble other types of plants, such as roses. Pereskia species have large, bright green, privet-like leaves and long spiny stems. Not always succulent plants, they can be classified as shrubs, climbing plants or slightly succulent trees. However, close examination shows spines developing from areoles, and the distinctive floral cup of the cactus family. Unlike Pereskiopsis, Maihuenia, Quiabentia and Austrocylindropuntia which have persistent succulent leaves, Pereskia is the only cactus genus that has persistent non-succulent leaves. It is believed that the ancestor of all cacti resembled Pereskia.
A recent study divided Pereskia into these two clades. It was suggested that the two clades be recognized as distinct species - Pereskia Clade A becoming Leuenbergeria and Pereskia Clade B becoming a more tightly circumscribed Pereskia.
The two clades of Pereskia differ in their geographical distribution, with one exception, Leuenbergeria (Clade A) is found around the Gulf of Mexico and the Caribbean Sea, whereas Pereskia (Clade B) occurs south of the Amazon Basin. Species of Leuenbergeria always lack two key features of the stem present in most of the remaining "caulocacti": like most non-cacti, their stems begin to form bark early in the plant's life and also lack stomata – structures which control the admission of air into a plant and hence control photosynthesis. By contrast, species of Pereskia typically delay forming bark and have stomata on their stems, thus giving the stem the potential to become a major organ for photosynthesis.
The two genera have been placed in separate subfamilies of the Cactaceae: Leuenbergeria being the sole genus in the subfamily Leuenbergerioideae, and Pereskia the sole genus in the subfamily Pereskioideae.