INTRODUCTION:  The cactus family is one of the most common in house plant collections.  There are many members of the family and they are well adapted to indoor culture.  the plural for cactus is cacti.  Cacti are dicots that have a fleshy, green, photosynthetic stem, the cladophyll, and leaves modified as spines.  One group of cacti, Pereskia, has true leaves that are shed during long droughts.  Some cacti are spineless and others have their spines modified as coarse hair.  Most cacti produce perfect flowers.  The flowers vary in size but are showy, usually delicate, and very attractive.  Most cacti are terrestrial but some are epiphytes.  Many cacti can be grafted and the grafts can be made across genera. 
 Cephalocereus senilis is the old man cactus because it is covered in white, hair-like spines. 
 Chamaecereus silvestr is the peanut cactus because of its clusters of short, spiny cylindrical branches. 
 The genera Echinocactus, Notocactus, Rebutia and Lobivia are all called barrel cacti for their barrel-like shapes. 
 Echinopsis multiplex is the Easter-lily cactus.  It has the shape of a barrel cactus and produces long, upright, pink or white, fragrant, trumpet-like flowers.  It flowers freely. 
 Epiphyllum oxypetalum is one of the large, trailing or cascading cacti called night blooming cereus.  (Hylocereus and Selenicereus are also known as night blooming cereus.)  Day blooming cultivars of Epiphyllum are known as the orchid cacti which produce attractive flowers in many interesting sizes, shapes and colors. 
 The genus Mammalaria is very large, containing hundreds of species, many of which are cultivated as ornamentals.  The genus gets its name from the numerous mammary-like or breast-like protuberances occurring in lines along the stem.  Some common mammalarias are the old lady cactus, golden stars cactus, feather cactus, powder puff cactus, lady finger cactus, thimble cactus and rose pincushion cactus.  Mammalarias often cluster in masses. 
 Opuntia is a large genus that contains the pad cacti or prickly pear cacti since the cladophylls are pad-shaped and the fruit are the edible prickly pears.   The cholla is a form of Opuntia that has cylindrical stems and vicious spines.  Bunny-ears is a pad cactus with spineless pads covered with bright golden tufts.  
 Pereskia is a genus of cacti that produce large spines as well as normal leaves.Pereskia grandifolia  
 The genus Schlumbergera contains the Christmas cactus, Thanksgiving cactus and Easter cactus, each named for the season during which it blooms.  Thanksgiving cactus had been classified as Zygocactus, but is not put together with the Christmas cactus in Schlumbergera.  These cacti have soft, wide green cladophylls that appear as joints of the stem. 
General Care of Cacti:  


temperature: Most cacti are tolerant of very hot temperatures but grow well at room temperature.  Some, such as Christmas cactus and orchid cacti, flower better when exposed to cool or cold temperatures, but they can not take freezing temperatures.  Many of the opuntias are very cold tolerant and found growing wild in some of the northern states. 
medium: Well drained medium is essential.  The media recommended for succulents work well for the cacti.  A few types, such as the orchid cacti and Christmas and Thanksgiving cacti, grow better with more organic matter in their medium.
water: Cacti should be watered thoroughly and then allowed to dry down between waterings.  A few, such as the orchid cacti, grow best in high humidity and benefit from misting or wetting the cladophylls.
light: Most cacti grow best in bright light.  They reach maturity more rapidly and flower more freely in bright light.  They will tolerate moderate light but do poorly in low light.  A few, such as Christmas cactus and the orchid cacti will burn in hot, bright light typical of summers in the southwest.  the Christmas cactus, Thanksgiving cactus and orchid cactus all flower best when exposed to short days, long nights.  The Christmas cactus often fails to flower if it does not get a short day, long night treatment coupled with cool  temperatures in early fall.
fertilization: Cacti seem to grow slowly and attempts to promote faster growth with fertilizer tend to fail as over fertilization makes the plants weak and susceptible to disease.  Cacti should be fertilized lightly in spring and summer and little or no fertilizer applied in fall and winter.  Nitrogen is especially problematic when given in excess.
pests and problems: Cacti have insect pests as do other plants.  Mealy bugs seem especially fond of cacti and can destroy the plants.  Over watering or poorly drained containers can lead to rot of the roots.  Stems may be attacked by organisms that cause areas to decay.
grooming: Cacti need very little grooming.
propagation: Cacti are propagated by seeds, separation of pups or offsets and stem cuttings.  Allowing stem cuttings to cure a few days in air before sticking in the propagation bed will reduce rot.  Cacti may be grafted.  They have wide graft compatibility and it is not unusual for a cactus such as the Christmas cactus to be grafted to Pereskia, Opuntia or Cereus.


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