During the nineteenth century, the ribbed columnar cacti, numbering in the hundreds, were generally classified as species of Cereus. In the early twentieth century, however, Cereus, in the broadest sense, was subdivided into many smaller and more homogeneous units, initially by N. L. Britton and J. N. Rose (1909, 1919-1923). The phylogenetic relationships of North American columnar species were clarified by studies of silica bodies in the epidermis and hypodermis covering the stems of certain Mexican species, distinctive pigment cells, called pearl cells, in the fruit pulp, and sugar-bearing oleanane triterpenes in stem tissues (A. C. Gibson and K. E. Horak 1978). Species possessing all three derived characters were removed from Lemaireocereus, Machaerocereus, Rathbunia, Hertrichocereus, Ritterocereus, and Marshallocereus and placed into the genus Stenocereus, which was further emended by removing species without the shared characters.

Several of the Central American species assigned to Stenocereus by E. F. Anderson (2001) are too poorly studied to know whether or not they have the diagnostic characters for the genus. A carefully done DNA phylogeny for all taxa with possible inclusion in Stenocereus is needed, especially to define more precisely the phylogenetic lineages and patterns of speciation (R. S. Wallace and A. C. Gibson 2002).

PLANTS: Columnar trees or shrubs, to 15+ m tall.

STEM: erect, arching or procumbent to prostrate, usually branched, the trunk, when present, to 40 cm in diameter, glabrous; ribs 4-20, rounded and vertically continuous to strongly tuberculate.

LEAVES: of long shoots minute or obsolete.

AREOLES: circular to elliptic, 0.5-4 cm apart on rib. SPINES terete to angular, divergent, to 28 per areole; central-most spines stout or absent, but sometimes flattened and deflexed, to 7.5 cm long; peripheral spines weaker, to 3.5 cm long.

FLOWERS: nocturnal or diurnal, solitary, subterminal or lateral, perfect, actinomorphic to zygomorphic, campanulate, funnelform to salverform, (4-)6-12 cm long, (3-)5-10 cm wide; ovary cylindric, ovoid to globose, bearing fleshy bracts, usually green with reddish apices, triangular, subtending axillary trichomes and up to 18 spines per areole; floral tube scaly; inner tepals white to rose, rarely yellow or red, oblong to obovate, obtuse; stamens many, included to strongly exserted, the filaments slender; style white; stigma lobes up to 15, white.

FRUITS: green to red, glabrous, scaly, often spiny (spine clusters deciduous from ripe fruits), glabrous, globose to ovoid, 3-8 cm long, indehiscent or splitting irregularly; pulp white, red to purplish, fleshy, mostly edible.

SEEDS: numerous, brownish, dull, verrucose, or less commonly, black glossy, finely pitted, 0.7-3 mm long.

NOTES: 18-20 spp.; sc AZ, Mex., coastal C. Amer., n S. Amer. and W. Ind. (Greek: Steno = narrow, with reference to stems + Cereus = an old generic name for columnar cacti).

REFERENCES: Pinkava, Donald J. 1995. Cactaceae. J. Ariz. - Nev. Acad. Sci. 29(1): 2, 6.