buckleyi hybrids. The Reginae Group contains cultivars known to be derived from hybrids with S. orssichiana. The Exotica Group is used for the small number of hybrids involving S. opuntioides. Attempts have also been made to classify cultivars by colour. A difficulty is that the flowers of many cultivars exhibit different colours depending on the temperature during bud formation and growth. In particular, temperatures below produce pink tones in otherwise white and yellow cultivars, and deepen the colour in pink and red cultivars. The availability of iron to the plant has also been suggested to affect flower colour. In the United States, cultivars are propagated in large numbers for sale before Thanksgiving Day (the fourth Thursday in November). In Europe, plants are mainly sold later in the year, in the period before Christmas. A single Dutch grower (de Vries of Aalsmeer, the Netherlands) was reported in 1989 as producing 2,000,000 plants per year. Common names Plants are offered for sale under a variety of common names. The earliest English common name was "Christmas cactus". In Europe, where plants are largely produced for sale in the period before Christmas, this remains the most widely used common name in many languages for cultivars of all groups (e.g. in German, in French, and in Spanish). This is also the name used in Canada. In the United States, where plants are produced for the Thanksgiving holiday in November, the name "Thanksgiving cactus" is used; "Christmas cactus" may then be restricted to cultivars of the Buckleyi Group, particularly the very old cultivars such as 'Buckleyi'. In Russia they are known as dekabrist ("decembrist") and rozhdestvennika ("nativity"). The name "crab cactus" (referring to the clawed ends of the stems) is also used for the Truncata Group. "Link cactus" is another common name, describing the way that the stems of the genus as a whole are made up of linked segments. The name "chain cactus" is common in New Zealand, and may also refer to Hatiora or Rhipsalidopsis species. The Easter cactus or Whitsun cactus was placed in the genus Rhipsalidopsis , but was at one time included in Schlumbergera (or one of its synonyms). The name "holiday cactus" has been used to include both Schlumbergera and Rhipsalidopsis cultivars. Care of cultivars When grown as house plants, Schlumbergera cultivars are said to be relatively easy to care for. McMillan and Horobin describe in detail their cultivation in