Cactus Family
Bruce D. Parfitt & Arthur C. Gibson
Trees, shrubs, subshrubs, or somewhat vinelike, solitary to forming mats or clumps, terrestrial (sometimes deep-seated in substrate) to epiphytic or epipetric, erect to sprawling (rarely scrambling or climbing) or pendent in epiphytic or epipetric taxa, simple to many branched, usually stem succulent. Roots diffuse, taproots, or tuberlike, sometimes adventitious. Stems unsegmented or segmented, segments persistent to easily detachable; long shoots spheric to depressed-spheric or club-shaped to long cylindric, or sometimes flattened cladodes, smooth, tuberculate and/or fluted with ribs; tubercles distinct as nipple-shaped or ridgelike (to triangular or pyramidal) protuberances to coalescent as vertical ribs; ribs 2-30[-40+], if ribs 2, stems winged, if ribs 3 or more, stems ± angled; short shoots (areoles) positioned on crests of ribs, at or near tubercle apices, or in axils of tubercles, commonly bearing persistent spines, also minute, barbed, deciduous spines (glochids) in subfam. Opuntioideae, and abundant, dense hairs (wool) creating a cushionlike appearance. Leaves deciduous to persistent, vestigial or absent, spirally alternate, sessile (petiolate to subsessile in Pereskia and several genera outside the flora), terete or flat, 0-3 cm (to 10 cm in Pereskia); stipules absent. Spines flexible and hairlike or bristlelike to rigid and needlelike or nail-like, terete to angled or flat, mostly hard (rarely corky or papery). Flowers bisexual (rarely unisexual or with bisexual and pistillate flowers on separate plants), nocturnal or diurnal, 1(-several) per areole, arranged in true inflorescence only in subfam. Pereskioideae, or chains of fruits proliferating from fruit areoles (in Cylindropuntia fulgida), sessile (pedicellate in Pereskia), arising from stem areole at apex or axil of tubercle, radially symmetric [bilaterally symmetric]; flower tube 0.2-15[-30] cm; perianth epigynous (perigynous in some Pereskia), deciduous or persistent on fruit; tepals 5-50 or more, intergrading gradually from bractlike or sepal-like outer tepals to petal-like inner tepals; stamens usually 50-1500+ [sometimes fewer], decurrent on inner surface of flower tube; true ovary sunken in stem with tubercles present or absent, areoles conspicuous to obscure or absent; subtending scales persistent or deciduous, sometimes absent; spines present or absent, glochids present only in subfam. Opuntioideae; pistils compound, 1-locular; placentas parietal, 3-14[-20+]; style 1; nectary usually forming chamber around base of style; stigma lobes 3-14[-20+], 1 per placenta. Fruits basically berrylike (variable in succulence), deciduous or long persistent, indehiscent or dehiscent, succulent or leathery, sometimes promptly drying. Seeds (0-)5-3000+, yellowish, reddish, brown, black, or appearing tan or whitish (dark testa completely covered by pale, tough, glabrous or rarely pubescent, tight-fitting aril or "funicular envelope" in subfam. Opuntioideae), pyriform, obovoid, lenticular-reniform, or nearly circular, 0.4-12 mm diam.; testa glossy or dull; rarely with corky arillate appendages (strophioles in Mammillaria tetrancistra). x = 11.
Genera 125-130, species ca. 1800 (34 genera, 189 species in the flora): tropical to temperate regions in North America, West Indies, Central America, South America, including Galápagos Islands, Africa, Madagascar, Sri Lanka; widely naturalized in Old World tropics and subtropics.
The Cactaceae had a New World origin. Only Rhipsalis, a tropical, epiphytic "mistletoe cactus" with seeds dispersed by birds, has colonized forests of Africa, Madagascar, and Sri Lanka, apparently without human assistance. Other genera of cacti, especially Opuntia, have become naturalized in dry regions of Africa, Eurasia, and Australia, where they have been introduced for food, fodder, ornament, and the formerly very lucrative cochineal dye industry. In some places where they vegetatively clone, cacti have become weedy. A number of taxa throughout the New World have been used by humans in various ways for about 12,000 years (E. F. Anderson 2001).
The nomenclature and taxonomic circumscriptions presented here are based on recent, independent studies and differ significantly from the work of L. D. Benson (1982) and older publications. In particular, many taxa previously combined and recognized in the broad sense are now known as biologically distinct taxa, and other taxa appearing as sympatric "varieties" within species on Benson’s distribution maps have since been resolved either as fully isolated biological species or as misidentified or mismapped records of allopatric geographic taxa.
The family is now divided into four subfamilies (R. E. Wallace 1995; E. F. Anderson 2001). Three subfamilies occur in North America: Pereskioideae, Opuntioideae, and Cactoideae. The fourth subfamily, Maihuenioideae, is restricted to South America (Argentina and Chile).
The Cactaceae maintain an unusually complete representation of their phylogenetic history. Subfamily Pereskioideae, with the least-derived traits, consists of trees, shrubs, or scrambling "vines" with broad, seasonally deciduous leaves and terete stems that are only weakly succulent. As broad-leaved plants, their relationship to other cacti is not readily apparent, especially if flowers are absent. The succulent trees, shrubs, mat-forming subshrubs, and few geophytes of subfam. Opuntioideae have mostly very short-lived, terete, cylindric, or conic leaves, usually present only on young growth and flowers. Plants of subfam. Cactoideae have vestigial, usually minute (or absent) leaves and extremely succulent, spheric, barrel-shaped to columnar or snakelike stems. Similarly, flowers of Cactaceae range from the simple, radially symmetric, bee-pollinated flowers of most cacti to the elaborate, bilaterally symmetric, hummingbird-pollinated flowers of, e.g., Schlumbergera, the commercially grown "Christmas cactus" or "Easter cactus." Funnelform bat-pollinated flowers also occur on some of the tallest cacti. The family ranges from extreme xerophytes in deserts to epiphytic mesophytes in rain forest, from sea level to about 4500 m (the altiplano of South American Andes), and from the equator to about 56o north latitude (Opuntia fragilis) and 50o latitude in the Southern Hemisphere (E. F. Anderson 2000). Whereas most cacti are terrestrial, in the tropics several genera are epiphytic on trees. A few tropical genera are also epipetric, and a number of xeric taxa may occur exclusively (or nearly so) in rock fissures.
Hybridization has been an influential force in the evolution of cacti. Many taxa and apparent taxa have their origins in hybridization, especially among the polyploid taxa. In North America, most polyploid cacti are taxa of Cylindropuntia, Opuntia, or Echinocereus.
The long shoots (main axis and branches) of cactus stems are succulent, ranging from hard and rigid to soft and marshmallow-like or watery inside. Surfaces of the long shoots may be smooth, ribbed (fluted), "winged" if the ribs are few, or covered with large tubercles. As used here, external ribs are not to be confused with the internal, long cylindric "ribs" or "rods" of wood, which correspond to the furrows between the external ribs. We will refer to only the external structures as ribs. In subfamily Cactoideae, the seedlings of some genera are tuberculate, even in genera with stems ribbed at maturity. Some phylogenetically transitional taxa, and individuals transitional between seedling and adult (reproductive) stages, exhibit every possible intermediate morphology with the tubercles coalescing into ribs, the ribs then interrupted or notched to various degrees. Further complicating identification in many taxa of subfamily Cactoideae, flowering may begin one or more seasons before attaining their adult stem morphology, e.g., bearing only tubercles instead of the ribs of adults. Stages of such plants have sometimes been misidentified and even named as new species. Some entire populations or taxa are neotenous, i.e., maintaining juvenile morphology throughout the life of the plant, and may look very different from their nearest kin.
Short shoots (areoles) are regularly spaced along the ribs or occur singly at the apices of the tubercles or in the axils of leaves. The areoles are woolly cushions with few to many spines (modified leaves of the shoot); spines are absent in some taxa, populations, and environments. A vernacular term for areoles is "spine-clusters," but fertile areoles of adult cacti may contain additional meristematic regions, such as the areolar grooves of Coryphantha, from which flowers are produced. Photosynthetic leaves or bracts, when present, subtend the abaxial edges of areoles (one per areole). Areolar glands are extrafloral nectaries highly modified from greatly reduced spines. The glands, when present, usually occur toward the adaxial edges of areoles and appear domed or in some genera peglike. Glochids and hairlike spines are homologous transition-forms or "short-cuts" between normal spines, ontogenetically derived from short-shoot leaves, and normal hairs that are uniseriate proliferations of short-shoot epidermis.
Flower buds develop in the youngest areoles at the apex of the plant, or older areoles away from the apex, then often forming a ring around the apex. In most cacti, areoles are circular to oblong. In a few genera, they become elongate to hourglass-shaped: the vegetative, usually spiny, portions at tubercle apices and the reproductive (flowering or fruiting) portions in the axils of the tubercles---the two parts connected by a narrow, often fuzzy, groove on the adaxial side of the tubercle (e.g., Coryphantha). Several genera have a short groove or adaxial extension of the areole reaching only part way to the axil of the tubercle, with the flower produced at the adaxial end of the extension. In some genera (e.g., Mammillaria) the connecting groove is absent; the axillary reproductive regions are separate areoles distinct from the vegetative areoles.
In cactus flowers and fruits, the true ovaries are hidden in the tips of specialized long shoots, which strongly resemble inferior ovaries but often with areoles, spines, and/or bracts. In this treatment, "ovary" refers to the apparent inferior ovary---the proximal portion of the cactus flower that becomes the fruit. The fused stem ripens with the true ovary to become fruit wall.
The terms adaxial and abaxial in this treatment relate to the stem axis, especially when describing position within or adjacent to areoles. Virtually all other literature on Cactaceae uses the terms upper and lower, respectively.
Measurements apply to fresh or very well-preserved material. Flower colors are for fresh material. Shrinkage in dried specimens can be more than 50%. Stems of normally green species are often purplish or reddish under stress.
L. D. Benson (1982) reported that Rhipsalis baccifera (J. S. Müller) Stearn was "introduced or native" formerly in southern Florida on Cape Sable, east of Flamingo, and west of Kendall, and that the last individual was apparently destroyed by a hurricane in 1965. No more recent records have been located to indicate that the species is presently established in the flora. The species resembles mistletoe (Phoradendron) in habit (however, epiphytic not parasitic), stem diameter, and fruit size and color. The leafless areoles are minute, bearing hairlike spines; the flowers are rotate, translucent white, with very few tepals (ca. 7-10) and stigma lobes (ca. 3).
SELECTED REFERENCES
Anderson, E. F. 2001. The Cactus Family. Portland. Barthlott, W. and D. R. Hunt. 1993. Cactaceae. In: K. Kubitzki et al., eds. 1990+. The Families and Genera of Vascular Plants. 4+ vols. Berlin etc. Vol. 2, pp. 161-197. Benson, L. D. 1969b. Cactaceae. In: C. L. Lundell. 1942-1969. Flora of Texas. 3 vols. in parts. Dallas and Renner, Tex. vol. 2, pp. 221-317. Benson, L. D. 1982. Cacti of the United States and Canada. Stanford. Gibson, A. C. and P. S. Nobel. 1986. The Cactus Primer. Cambridge, Massachusetts. Hunt, D. R., comp. 1999. CITES Cactaceae Checklist, ed. 2. Kew. Hunt, D. R. and N. P. Taylor, comps. 1990. The genera of Cactaceae: Progress towards consensus. Bradleya 8: 85-107. Wallace, R. S. 1995. Molecular systematic study of the Cactaceae: Using chloroplast DNA variation to elucidate cactus phylogeny. Bradleya 13: 1-12. Wallace, R. S. and A. C. Gibson. 2002. Evolution and systematics. In: P. S. Nobel, ed. 2002. Cacti: Biology and Uses. Berkeley. Pp. 1-21.
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