Cactaceae - Cactus Family
Greek: kaktos = a prickly plant (Spanish artichoke) from Sicily. Linnaeus (1737)
The Cactaceae is one of the most distinctive familes of dicotyledenous flowering plants,
with around 90 genera and 2500 species, ranging in size from less than half an inch to tens of feet in height and exlusively native to the American continent including the West Indies. The solitary exception to the New World distribution is that of the epiphyte Rhipsalis baccifera which is distributed in Africa, Madagascar and Sri Lanka as well as in America. Most species of Cactaceae are spiny succulents, although there are a few woody shrubs (Pereskia) and more or less succulent epiphytes. The family contains numerous genera, which seem to be constantly under revision.
The Cactaceae are distinguished by modified buds that have become transformed into specialised areoles from which grow multiple spines and glochids (top right). These should be distinguished from solitary spines or thorns, sometimes branching above the plant body, on other plants e.g. spiny Euphorbias. In some species spination is minimal, or occurs only during some phases of the plant's life or vestigal spines are present as hairs. Leaves where present are alternate, but usually reduced or absent. Fruit are generally berries with multiple seeds, which may be surrounded by juicy flesh or by a more or less dry membrane and may have external spines.
Betalain pigments are present and characteristic of the family Cactaceae and of the Order Caryophyllales in which they are included, as is absence of anthocyanin pigments found in other families of flowering plants. Betalains and anthocyanins never occur in the same plant. Betalains are responsible for the red or violet colouration of some cacti when stressed.
Ferulic acid in the cell walls of Cactaceae is also a characteristic.
Several species are cultivated for their juicy fruit (e.g. Opuntia ficus-indica, Selenocereus sp., Hylocereus sp.) and through introduction Cactaceae are found throughout the tropics. In some places they have become invasive weeds.
Many species are cultivated as ornamentals and are grown from seed in vast numbers. Some species e.g. Echinocactus grusonii are far more common in cultivation than in their habitat, much of which has been destroyed. All species are protected as CITES 1 or CITES 2 plants, which presents a barrier to free trade even for nursery grown plants and artificial hybrids with no natural distribution.
Cactus taxonomy appears to follow a record of their assumed evolutionary history. However, as there is little evidence of cacti in the fossil record, this logical sequence is based mainly on morphology. Recent DNA studies support the general groupings, but suggest some taxonomic rearrangements of species. Cacti generally have 22 chromosomes.
- Subfamily Pereskioideae: broad leaves, no glochids.
Pereskia, Maihuenia
(or Maihuenia may be separated into Subfamily Maihuenioideae.)
- Subfamily Opuntioideae: fleshy round leaves present or absent, glochids present.
Austrocylindropuntia, Brasiliopuntia, Cylindropuntia, Cumulopuntia, Grusonia, Micropuntia, Opuntia, Pereskiopsis, Pterocactus, Quiabentia, Tacinga, Tephrocactus, Tunilla.
- Subfamily Cactoideae: no leaves*, no glochids.
Ariocarpus, Astrophytum, Aztekium, Carnegiea, Cereus, Cleistocactus, Coryphantha, Echinocactus, Echinocereus, Echinomastus, Epithelantha, Escobaria, Ferocactus, Geohintonia, Hylocereus, Leuchtenbergia, Lobivia, Lophophora, Mammillaria, Matucana, Neolloydia, Obregonia, Parodia, Pelecyphora, Pilosocereus, Pediocactus, Rebutia, Rhipsalis, Sclerocactus, Schlumbergia, Stenocactus, Strombocactus, Thelocactus, Trichocereus, Turbinocarpus, Uebelmannia and all other genera.
*Except that some species have seed leaves.
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Example of cactus spine cluster
Opuntia engelmannii
Ariocarpus fissuratus
Rhipsalis pilocarpa
Zygocactus truncatus
Pereskia grandiflora
more images of cacti
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Plant Families with Succulent Species 
