The columnar cacti of Mexico have confounded botanists for years, and their nomenclature is confusing with numerous names. Nathaniel Britton and Joseph Rose (1919-1923, 2: 25-60) listed 48 species of Cephalocereus, from throughout the Western Hemisphere. The first major attempt to determine the more natural affinities of the columnar cacti was that of Franz Buxbaum (1958), who described the tribe Pa-chycereeae and placed in it Carnegiea, Cephalocereus, Le-maireocereus, Mitrocereus, Neobuxbaumia, and Pachycereus, a strictly North American group. Only 3 years later however, Buxbaum (1961) added another seven genera to the tribe, which he then subdivided into five subtribes. Many people followed Buxbaum's classification until the major work of Arthur Gibson and Karl Horak. Their detailed studies of the chemistry and anatomy of the columnar Mexican cacti have resulted in several major changes from the Buxbaum system. Although they commented that "Cephalocereus and its numerous segregate genera is still a major unresolved systematic problem" (Gibson and Horak 1978, 1044), they nonetheless provided important data for others to use.
The International Cactaceae Systematics Group has also struggled with the placement of species within Cephalocereus. In its first consensus attempt (Hunt and Taylor 1986), Cephalocereus was listed as a single-species genus closely allied to Carnegiea, the latter including Haseltonia and Neobuxbaumia. The second publication of the group (Hunt and Tay
lor 1990) modified the arrangement of related groups so that Cephalocereus included parts of Haseltonia and Pilocereus, with Carnegiea including Neobuxbaumia and Neodawsonia. Wilhelm Barthlott and David Hunt's treatment of Cactaceae in The Families and Genera of Vascular Plants represented a further step in the group's efforts to understand the Mexican columnar cacti; in 1993 Cephalocereus contained part of Haseltonia, Neodawsonia, and Pilocereus, with Neobuxbaumia recognized as a distinct genus. The treatment here follows this latest conclusion of the International Cactaceae System-atics Group, which must still be considered unsettled.
Cephalocereus (type, Cactus senilis = Cephalocereus senilis) was described by Ludwig Pfeiffer in 1838, the name derived from the Greek cephale, head, thus headed cereus, referring to the hairy pseudocephalium. A pseudocephalium differs from a true cephalium in bearing flowers only laterally, with the ribs still visible, and the floral areoles covered by long hairs (Gibson and Nobel 1986, 116). Five species of night- and summer-flowering columnar cacti are included in the genus.
Cephalocereus Pfeiffer 1838 Pilocereus Lemaire 1839, illegitimate name Haseltonia Backeberg 1949 Neodawsonia Backeberg 1949
Subfamily Cactoideae, tribe Pachycereeae. Plants columnar, erect, branched or unbranched, 10-12 m (33-39 ft) high. Stems to 40 cm (16 in) in diameter, light green, becoming gray with age, often nearly obscured nearthe tips by dense spines. Ribs vertical, 12-30 or more. Areoles close set, dimorphic. Central spines as many as 5, yellowish to gray, to 4 cm (1.6 in) long. Radial spines numerous, bristle- or hairlike, white, usually densely covering the stem. Flowers produced by pseudocephalia of varying types, unarmed, of medium size, open at night, tubular to bell shaped; pericarpels and floral tubes with small scales; areoles with wool and short hairs. Fruits ovoid, with small scales and wool; perianth parts persistent. Seeds pear shaped, smooth, black. Distribution: southern Mexico in Puebla, Oaxaca, Hidalgo, Chiapas, and Guanajuato, often forming dense, spectacular forests.
Cephalocereus apicephalium e. y. Dawson 1948 Neodawsonia apicephalium (E. Y. Dawson) Backeberg 1949 Neodawsoniaguiengolensis Bravo 1956 Neodawsonia nana Bravo 1956
Plants with some basal branching, occasionally from above as well, 1-3 m (3.3-9.8 ft) high. Stems gray-green, to 7 cm (2.8 in) in diameter. Ribs 22-25, narrow. Central spines 1-4, expanded at their base, directed downward, reddish yellow to black, 2-4 cm (0.8-1.6 in) long. Radial spines 9-12, very slender, gray to white, 1.5-1.8 cm (0.6-0.7 in) long. Pseudo cephalia apical, covered with long, wrinkled, woolly hairs to 5 cm (2 in) long; with pseudocephalia from previous years persisting as woolly rings below the new year's growth for several years. Flowers narrowly bell shaped, rose with a yellowish tint, 5-6 cm (2-2.4 in) long, to 3 cm (1.2 in) in diameter; pericarpels with small, triangular, white scales. Distribution: limestone cliffs in Oaxaca, Mexico.
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