Schlumbergera is a small genus of cacti with six to nine species found in the coastal mountains of south-eastern Brazil. These plants grow on trees or rocks in habitats that are generally shady with high humidity, and can be quite different in appearance from their desert-dwelling cousins. Most species of Schlumbergera have stems which resemble leaf-like pads joined one to the other and flowers which appear from areoles at the joints and tips of the stems. Two species have cylindrical stems more similar to other cacti. Common names for these cacti generally refer to their flowering season. In the Northern Hemisphere, they are called Christmas cactus, Thanksgiving cactus, crab cactus and holiday cactus. In Brazil, the genus is referred to as (May flower), reflecting the period in which they flower in the Southern Hemisphere. Most of the popular houseplants are cultivars of Schlumbergera, rather than species, with flowers in white, pink, yellow, orange, red or purple. The Easter cactus or Whitsun cactus, placed in the genus Rhipsalidopsis, is also called a holiday cactus and has flowers in red, orange, pink and white. The cultivars of the Christmas cactus fall into two main groups: The Truncata Group contains all cultivars with features derived mainly from the species S. truncata: stem segments with pointed teeth; flowers held more or less horizontally, usually above the horizontal, whose upper side is differently shaped from the lower side (zygomorphic); and pollen which is yellow. They generally flower earlier than members of the Buckleyi Group and, although common names are not applied consistently, may be distinguished as Thanksgiving cactus, crab cactus or claw cactus. The Buckleyi Group contains all cultivars with at least some features clearly showing inheritance from S. russelliana: stem segments with rounded, more symmetrical teeth; more or less symmetrical (regular) flowers which hang down, below the horizontal; and pollen which is pink. They generally flower later than members of the Truncata Group and are more likely to be called Christmas cactus. Description In the wild, the species of Schlumbergera grow either on trees (epiphytic) or on rocks (epilithic) and can form sizeable shrubs with woody bases; a height of up to has been reported for one species (S. opuntioides). They are leafless, the green stems acting as photosynthetic organs. The stems are composed of segments, which take one of two forms. In most species the segments are strongly flattened (cladodes), being made up