Classification of the cactus family is considered to be highly problematic and difficult due to the effects of convergent or parallel evolution. Systematic studies have shown that there are four major lineages of cacti that are recognized at the subfamily rank. Of these, the two primitive groups (Maihuenia [two species] and Pereskia [sixteen species]) are found mainly in South America and have persistent true leaves, a limited degree of succulence, and are not as well adapted to heat and water stress as are members of the two remaining groups, the subfamilies Opuntioideae and Cactoideae.

The Opuntia group has approximately three hundred species of cacti or more, and typical members of subfamily Opuntioideae have well-defined stem segments, an unusual kind of specialized seed structure, and a unique type of reverse-hooked small spines (glochids), which penetrate the skin and are easily dislodged from their attachment points in the areoles. These cacti include the prickly pears and chollas (pronounced CHOY-yas), which produce showy flowers that are often pollinated by bees, and develop fleshy (occasionally dry) edible berries called tunas. This group is the most widespread of any in the Cactaceae, ranging from southern Canada southwards almost continuously to the cold habitats of Patagonia in southernmost Argentina. They are found from sea level to high elevation (4,000 meters and higher) and have adapted to a wide variety of habitat types. Species of Opuntia introduced into suitable habitats in Africa and Australia have become noxious invasive pests, and removing these escaped cacti has become an significant ecological concern.

The largest subfamily of the cacti (subfamily Cactoideae) contains approximately thirteen hundred species and provides examples of extreme morphological variation in the family. There is a tremendous range of variation in spine form and color, flowering types, formation of ribs and tubercles, and other morphological characters in members of this subfamily. Plants in this group range from plants with narrow upright stems that sometimes branch (a columnar habit) to plants with globular or ball-shaped forms (the barrel cactus types) to epiphytic plants that either hang from trees in a pendant fashion (such as in Schumbergera or Rhipsalis) or are lianas and climb up the surfaces of trees with a vining habit. The most easily recognizable cactus species, the saguaro (Carnegiea gigantea), is the state flower of Arizona and the symbol of the American desert. It is also among the tallest of all cacti, with individual plants recorded over 15 meters (50 feet) in height. Some cacti produce special alkaloid compounds, notably the peyote cactus (Lophophora williamsii), which is known for its hallucinogenic properties.

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