The Cactus Classification of the International Cactaceae Systematics Group

In 1984 a new approach to cactus classification was begun. Prior to that, each cactus classification, with the exception of that of Britton and Rose, was basically the work of an individual. Stimulated by the reports of exciting new research at the congresses of the International Organization for Succulent Plant Study (IOS) and by the commitments of Barthlott and Hunt to produce treatments of cacti for the planned reference works, The Families and Genera of Vascular Plants and The European Garden Flora, it was proposed that the Cactaceae Section of the IOS create a working party "to explore the possibility of reaching a consensus on the generic classification of the family" (Hunt and Taylor 1986,65). It was also deemed important to have an '"international standard' for the taxonomy and nomenclature of cacti of economic or conservation interest, for cataloguing herbaria and living collections, for research reports and so on." The matter was discussed at the eighteenth IOS Congress in Frankfurt, Germany, in the summer of 1984, with members of the section strongly supporting the plan. A further meeting of the newly created IOS Cactaceae Working Party, now called the International Cactaceae Systematics Group, was held later in 1984 at the Royal Botanic Gardens, Kew, to plan procedures and a timetable. A general statement of purpose was also developed (Hunt and Taylor 1990,86) that included the following objectives:

the publication, from time to time, of synopses of the classification and genera of groups... based on the consensus of current opinion among IOS specialists and correspondents and subject to modification in the light of new or reconsidered evidence. By encouraging consultation before the publication of major taxonomic changes, IOS seeks to obviate name-changes which are unlikely to prove widely acceptable.

IOS seeks to take into account as wide as possible an expression of views, giving all qualified persons an opportunity to propose amendments to the current IOS listings, and for such proposals to be debated and put to a ballot of all participants. It is emphasized that the adoption of IOS listings is entirely optional and in no way mandatory.

A third meeting was held in 1985 at the IOS Inter-Congress in Zurich. A first draft of a list of genera based on the recent systems of Hunt (1967) and Barthlott (1979) was presented for discussion, which was intense and productive. The pub lished list (Hunt and Taylor 1986) was to serve "as a framework for continuing discussions among the authors themselves and for wider debate." Indeed, that is exactly what occurred. Further meetings were held in 1987 (California), 1988 (Germany), 1989 (Italy), and 1990 (Switzerland), the last a critical year. A new version of the consensus classification was drafted and mailed to cactus specialists throughout the world for their comments and votes upon several proposals that were included with the draft. Following tabulation of the postal ballots and further discussions at the twenty-first IOS Congress in Zürich, a new list of the genera of Cactaceae was published (Hunt and Taylor 1990); in it were 93 genera. The International Cactaceae Systematics Group has continued to meet nearly every year, sometimes with IOS Congresses and Inter-Congresses, sometimes independently, depending on the availability of key participants. By 1994, several aspects of tribal organization were changed and the number of accepted genera increased to 104. Major changes in the organization of the family above the rank of genus have also been proposed, with work continuing.

Several important publications have been based on the consensus classification. The cites Cactaceae Checklist (Hunt 1992,1999a) in addition to The European Garden Flora (Hunt 1988,1989a) and The Families and Genera of Vascular Plants (Barthlott and Hunt 1993) are some examples of the use of the new system.

Some have criticized the work of the International Cactaceae Systematics Group as merely classification by committee. However, modern scientific research is so complex and the data so numerous that nowadays it is nearly impossible for individuals to perform detailed and creative taxonomic studies using all available information. A team approach to such a vast and complicated project as a natural classification of the cacti is logical and necessary. The group has involved specialists in morphology and anatomy as well as experts in electron microscopy, pollen, chromosomes, chemistry, and DNA and other macromolecules. Specialists in various groups of cacti or cacti of particular regions have been included, or their advice and comments solicited. Not only has the International Cactaceae Systematics Group been diverse in specialties, it has also brought together botanists, horticulturists, and dedicated amateurs. An important aspect of the group's success has been that participants are free to disagree with one another, but respectfully and professionally. Moreover, many have formed deep friendships through social interactions following the discussions, stimulating joint research projects. Another benefit of the efforts of the group has been the identification of groups of cacti that are particularly puzzling and in need of further study.

Members have concentrated on some of these enigmatic taxa, increasing our overall knowledge of the family.

It has been my privilege to participate in most sessions of the International Cactaceae Systematics Group. Though I have not agreed with all decisions, I respect and welcome the results. The task is not complete, but most feel that there have been major gains in our understanding of the natural relationships of cacti and the construction of a system of classification that reflects it. I would not pretend in The Cactus Family to work independently of the many specialists and colleagues with whom I have been associated in this project since 1984. Rather, I defer to the judgment of the International Cactaceae Systematics Group, and the classification system used in this book is essentially that of the experts who make up the group. I have been merely one participant in this rewarding effort.

The classification of the family Cactaceae developed by the International Cactaceae Systematics Group recognizes four subfamilies—Pereskioideae, Maihuenioideae, Opuntioideae, and Cactoideae—with Cactoideae further divided into nine tribes. Each of the subfamilies and tribes is described here, together with a list of genera in each. The genera and species of cacti are listed alphabetically in the following part, The Cacti.

Cactaceae A. L. de Jussieu 1789, conserved name

Subfamily Pereskioideae K. Schumann 1898

Plants treelike, shrublike, or climbing, with crassu-lacean acid metabolism (cam) in the stems and C3 metabolism in the leaves. Stems round in cross section, not ribbed or tuberculate. Leaves present. Spines present. Flowers solitary or in inflorescences, diurnal; pericarpels with scales or leaves that are sometimes persistent; floral areoles rarely with spines; floral tubes absent. Fruits indehiscent, berrylike, often with a juicy fruit wall. Seeds more or less round, 1.7-7.5 mm (to 0.3 in) in diameter, brownish black, shiny, not wrinkled or keeled, not expanded around the small hilum-micropylar region. Distribution: from southern Mexico throughout the Caribbean and Central America, and in much of South America east of the Andes.

The subfamily Pereskioideae represents the group of cacti with the most primitive or ancestral features in the Cactaceae, and several characteristics demonstrate the family's relationship to other members of the order Caryophyllales. Leuenberger (1986) has made the definitive study of Pereskioideae.


Subfamily Maihuenioideae P. Fearn 1996

Caespitose shrubs with C3 metabolism only. Stems succulent, short cylindrical to globose. Leaves small, terete, persistent. Spines usually 3 per areole. Flowers terminal, solitary. Fruits somewhat fleshy, with small scales. Seeds almost round, 3-4 mm in diameter, shiny. Distribution: restricted to Argentina and Chile. Maihuenia

Subfamily Opuntioideae K. Schumann 1898

Plants variable in habit, treelike, shrublike, or caespitose. Stems usually segmented into distinct joints or cladodes. Leaves present, ephemeral, round in cross section, small. Glochids present. Spines present, variable. Flowers usually lateral, sessile, solitary, diurnal; pericarpels with leaves, areoles, glochids, and spines; floral tubes short or absent. Fruits berrylike, indehiscent, sometimes becoming dry at maturity. Seeds round to oval, 3-12 mm (to 0.5 in) in diameter, covered by a funicular envelope (also called a bony aril). Distribution: from Canada throughout most of North America, the Caribbean, Central America, and nearly to the southern tip of South America. Austrocylindropuntia Brasiliopuntia Consolea Cumulopuntia Cylindropuntia Grusonia Maihueniopsis Miqueliopuntia Opuntia Pereskiopsis Pterocactus Quiabentia Tacinga Tephrocactus Tunilla

Subfamily Cactoideae

Plants highly variable in habit, treelike, shrubby, caespitose, climbing, or epiphytic. Roots fibrous or tuberous. Stems usually not segmented, globose to columnar, ribbed or tuberculate; reproductive zones differentiated or undifferentiated. Leaves vestigial or absent. Glochids absent. Flowers sessile, diurnal or nocturnal; pericarpels scaly to naked; floral tubes short to elongate. Fruits indehiscent or dehiscent, fleshy or dry, variable in size and shape. Seeds highly variable, 0.4-5 mm in diameter, sometimes with appendages, and with variable testa ar chitecture. Distribution: throughout most of the Western Hemisphere with one species, Rhipsalis baccifera, also in Africa, Madagascar, on islands of the Indian Ocean, and in Sri Lanka.

Tribe Calymmantheae R. S. Wallace 2001

Plants shrubby or treelike. Stems columnar, segmented, ribbed and winglike, somewhat tuberculate. Areoles very evident. Spines straight, stiff, and whitish. Flowers with receptacle tubes partially covering the perianth while in bud; pericarpels and receptacular tubes with small scales and woolly areoles. Fruits fleshy, indehiscent, with few or no areoles. Distribution: the Andes of northern Peru. Calymmanthium

Tribe Hylocereeae Buxbaum 1958

Climbing or epiphytic shrubs. Roots adventitious. Stems flattened or with few ribs; reproductive zones undifferentiated. Glochids absent. Leaves not evident. Flowers borne laterally, medium to large, often nocturnal but sometimes diurnal; pericarpel areoles naked, spiny, bristly, or hairy. Fruits fleshy, indehiscent. Seeds medium to large, hilum and micropyle fused, with a mucilage sheath covering the entire seed. Distribution: tropical forests of Central America. Disocactus Epiphyllum Hylocereus Pseudorhipsalis Selenicereus Weberocereus

Tribe Cereeae Salm-Dyck 1845

Plants treelike or shrubby, sometimes climbing. Stems unsegmented, elongate to globose, ribbed, spiny; reproductive zones usually differentiated into terminal or lateral cephalia. Flowers usually borne laterally, nocturnal or diurnal; pericarpels usually with a few scales or naked. Fruits berrylike, fleshy, dehiscent or indehiscent, often with persistent floral parts that turn black. Seeds small to large, oval, lacking conspicuous surface sculpturing; hilum and micropyle conjunct, appendages absent. Distribution: mostly in eastern South America. Arrojadoa Brasilicereus Cereus Cipocereus Coleocephalocereus







Tribe Trichocereeae Buxbaum 1958

Plants treelike to shrubby. Stems normally unseg-mented, globose to columnar, usually ribbed, ribbed-tuberculate, or tuberculate; reproductive areas differentiated as lateral or terminal cephalia or undifferentiated. Flowers borne laterally to subapically, small to quite large, nocturnal or diurnal, regular or bilaterally symmetrical; pericarpels with scales or hairs. Fruits fleshy, berrylike, sometimes dehiscing longitudinally. Seeds small to medium, variable in shape, hilum and micropyle conjunct or fused, appendages usually absent, strophiole present in some. Distribution: south of the equator in South America and the Galápagos Islands. Acanthocalycium Arthrocereus Brachycereus Cephalocleistocactus Cleistocactus Denmoza Discocactus Echinopsis Espostoa Espostoopsis Facheiroa Gymnocalycium Haageocereus xHaagespostoa (Haageocereus x Espostoa)






Oreocereus Oroya







Tribe Notocacteae Buxbaum 1958

Plants mostly solitary, rarely treelike or shrubby. Stems unsegmented, mostly globose, ribbed, ribbed-tuberculate, or tuberculate; reproductive areas undifferentiated. Flowers arising from the woolly apices though subapical, small to medium, diurnal, regular to bilaterally symmetrical; pericarpels with small scales, areoles with bristles or hairs. Fruits usually dry, rarely berrylike, dehiscent or indehiscent. Seeds small to medium, variable in shape, hilum and micropyle conjunct, strophiole often present, some with mucilage sheath. Distribution: southern South America.








Neowerdermannia Parodia

Tribe Rhipsalideae A. P. de Candolle 1828

Epiphytes or lithophytes, usually pendulous, sometimes creeping or shrublike, never climbing. Stems segmented, round, angled, or flattened in cross section. Areoles sunken. Flowers borne laterally, diurnal but remaining open at night, small, regular to tubular to bilaterally symmetrical; pericarpels naked and rarely with areoles. Fruits fleshy, berrylike, indehiscent. Seeds small to medium, lacking definitive surface architecture, hilum and micropyle fused, mucilage sheath present. Distribution: mainly in eastern South America but with some species extending into Central and North America. One species, Rhipsalis baccifera, is found also in the Old World.

Barthlott and Taylor (1995) have contributed significantly to the classification of the tribe Rhipsalideae. Hatiora Lepistnium Rhipsalis Schlumbergera

Tribe Browningieae Buxbaum 1966

Plants large, treelike or shrubby. Stems segmented or unsegmented, columnar, ribbed, usually heavily spined; reproductive areas undifferentiated though changing in spination. Flowers borne laterally, medium to large, usually nocturnal; pericarpels with imbricate scales, areoles with spines or bristles. Fruits fleshy, indehiscent, scaly, spiny, or naked. Seeds medium to large, often rugose; hilum and micropyle conjunct, appendages none, mucilage sheath sometimes present. Distribution: Andean area of South America and the Galápagos Islands. Armatocereus Browningia Jasminocereus Neoraimondia Stetsonia

Tribe Pachycereeae Buxbaum 1958

Plants large, treelike or shrubby. Stems unseg-mented, columnar, ribbed; reproductive areas undifferentiated or differentiated into apical or lateral cephalia. Flowers borne laterally, subapically, or from the cephalia, small to medium, regular or bilaterally symmetrical, varying in shape, nocturnal or rarely diurnal; pericarpels scaly or rarely naked, areoles spiny or bristly. Fruits fleshy, spiny, dehiscent or indehiscent. Seeds variable, medium to large, hilum and micropyle conjunct or fused, appendages absent. Distribution: mainly in Mexico and the southwestern United States but also in the Caribbean, Central America, and South America to Venezuela.

Gibson and Horak (1978) have made studies of anatomy in relation to the classification of the tribe Pachycereeae. Acanthocereus Bergerocactus Carnegiea Cephalocereus Corryocactus Dendrocereus Echinocereus Escontria Isolatocereus Leptocereus xMyrtgerocactus (Myrtillocactus x Bergerocactus)


Neobuxbaumia xPacherocactus (Pachycereusx Bergerocactus) Pachycereus

Peniocereus Polaskia

Pseudoacanthocereus Stenocereus

Tribe Cacteae

Plants solitary or caespitose. Stems unsegmented, mostly globose, but some short columnar, ribbed, ribbed-tuberculate, or tuberculate. Areoles usually oval, bandlike, grooved, or dimorphic. Reproductive areas undifferentiated. Flowers subapical, small to medium, diurnal, regular to rarely bilaterally symmetrical; pericarpels scaly to naked. Fruits berrylike, fleshy, indehiscent to dehiscent or simply disintegrating. Seeds small to large, variable in shape and in testa relief; hilum and micropyle disjunct or rarely conjunct; appendages absent with the exception of rare strophioles. Distribution: United States, Mexico, Canada, the Caribbean, Venezuela, and Colombia. Acharagma Ariocarpus Astrophytum Aztekium Cochemiea Coryphantha Echinocactus Echinomastus Epithelantha Escobaría Ferocactus Geohintonia Leuchtenbergia Lophophora Mammillaria Mammilloydia Neolloydia Obregonia Ortegocactus Pediocactus Pelecyphora Sclerocactus Stenocactus Strombocactus Thelocactus Turbinicarpus

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