both commercial and domestic conditions. Their specific recommendations include: Growing medium: Free-draining, humus-rich, somewhat acid growing media are used for commercial production, such as a mixture of peat or leafmould and an inert material such as grit, sharp sand or polystyrene beads. It is recommended that plants should be grown in relatively small pots; half-height pots are suitable. Watering: They are more tolerant of drought than many house plants, but can be damaged by both under- and over-watering. Keeping the growing medium just moist throughout the year avoids either extreme. Light: They can be damaged by exposure to more than small amounts of sunlight. Members of the Buckleyi Group, such as the old-fashioned Christmas cactus with pendant flowers, are more tolerant of high light levels than members of the Truncata Group, such as most of the modern cultivars. Too much light causes stems to take on a reddish colouration; however, very low light levels will prevent flowering. Day length is important in controlling flowering; continuous darkness for at least 12 hours is necessary to induce bud formation. A period of about 8 days with 16 hours of darkness at has been shown to cause flower buds to form. Lower temperatures slow this process. The advice sometimes given to withhold water to produce flower buds has been shown to be incorrect. Propagation: Both commercially and in the home, propagation can be achieved by using short pieces of stem, one to three segments long, twisted off rather than cut. Cuttings are allowed to dry for 1–7 days, forming a callus at the broken end, and then rooted in an open growing medium. Temperatures above and up to in long day/short night conditions speed rooting. Pests and diseases In cultivation, these plants have been described as "remarkably free from pests and diseases". Two significant insect pests are aphids on young shoots, buds and flowers, and root mealybugs which attack below soil level. Stems and roots can be rotted by diseases caused by fungi and similar organisms; these include infections by species of Fusarium (a fungus), and Phytophthora and Pythium (both water moulds). Approved chemical treatments can be used in the case of insect attack or these diseases. Aphids, mealybugs and other invertebrate pests can spread viruses. Symptoms vary with the species, but a loss of vigour is usual. Cactus virus X has been isolated from S. truncata. There is no treatment for virus