Stenocereus

	Taxon	Illustrator ⠉
	Carnegiea gigantea Stenocereus thurberi Echinocactus horizonthalonius	John Myers John Myers John Myers

Trees or shrubs, erect, arching, or procumbent, mostly branched from base. Roots adventitious if plant procumbent. Stems unsegmented, often more narrow at branch bases and where showing growth increments, green to bluish green [or whitish from surface wax or purple-tinged from pigment], columnar, ribbed, [50–] 100–500+ × [5–] (9–) 11–18 [–20] cm; ribs 4–20, rounded, margins nearly flat to sinusoidal or strongly tuberculate, with transverse folds between areoles or not; areoles 0.5–2.5 cm apart, circular to horizontally elliptic, young hairs whitish or tan to reddish-brown, aging darker; areolar glands present or absent; cortex mucilaginous or not, mucilage throughout and slippery or restricted to sacs in outer cortex, green to white or yellowish; pith mucilaginous or not, 1–8 cm wide. Spines to 28 per areole, hemispherically arranged, initially rose to magenta, becoming darker or fading to grayish white, relatively thin and brittle to stout and very hard; radial spines to 3.5 cm; central spines usually weakly defined or absent, sometimes broad and downward pointing, to 7.5 cm. Flowers diurnal or nocturnal, produced only once on areole [or not], terminal to lateral, funnelform [to tubular]; flower tubes2–11 cm; outer tepals with dark green to purplish centers but light margins, margins entire; inner tepals white to rose-red [or yellow]; ovary globose to barrel-shaped, similar to locule shape; scales persistent, reddish or green with red tips, small, triangular; hairs and spines often present; stigma lobes 5–15, inserted or exserted; nectar chamber open. Fruits indehiscent or sometimes splitting irregularly, dark red to purplish green or green, spheric [to ovoid], 30–100 mm, fleshy to juicy [or somewhat dry], bearing deciduous spine clusters; pulp red, special pigment cells present; floral remnant persistent or deciduous. Seeds brownish black or black, oblong to subspheric with oblique hilum, 0.7–3 mm, dull or rarely glossy; testa cells convex or nearly flat, with prominent to faint, raised waxy striations or not. x = 11.

Distribution

s Ariz., Mexico, West Indies, coastal Central America, n South America, cultivated and naturalized elsewhere

Discussion

Species ca. 20 (1 in the flora).

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).