Trees, 1-10-branched or unbranched, massive with trunk, main branches erect or ascending from near midstem of trunk (secondary branches absent except where injured). Roots diffuse. Stems unsegmented, green, columnar, gigantic, 500-1500 × 25-75 cm; bark gray, well developed only near base and over stem wounds; ribs 19-26 or, on young plants, 11-15, broadly triangular in cross section, rib crests rounded, flat, neither tuberculate nor notched between areoles (slightly raised tubercles on distal stems of old plants); areoles 1 cm apart along ribs, circular, elliptic, or shield-shaped, with dense, tannish white to gray hairs; areolar glands absent; cortex and pith not mucilaginous. Spines (8-15-28(-50) per areole, tannish white to beige or grayish, acicular, rigid, terete, flattened, or angular, hard; radial spines (8-)15-20(-50) per areole, flattened against ribs or spreading, 10-20 mm; central spines (0-)4-7 per areole, diffusely spreading, 30-50 × 10-15 mm. Flowers nocturnal, but remaining open next day, mostly terminal to subterminal on stems (also lateral), from adaxial edges of areoles, funnelform-salverform with short tube, 8.5-14 cm; tepals all strongly reflexed; outer tepals green, 2-2.5 cm, margins whitish, undulate; inner tepals waxy white, 2-3.5 × 1-1.8 cm, margins entire or undulate; ovary tuberculate, minutely scaly; scales triangular, small, axils usually without hairs or spines (sometimes flexible bristlelike spines present); stigma lobes ca. 10, tannish white, ca. 12 mm. Fruits dehiscent by 2 or more vertical splits from apex, dark red, obovoid to ellipsoid, 45-95 × 25-45 mm, apex truncate, occasionally with thin white spines; pulp bright red, sweet; floral remnant persistent. Seeds reddish black, obovoid, 1.5 mm, glossy, smooth; testa cells flat. x = 11.
Steenbergh, W. F. and C. H. Lowe. 1977. Ecology of the Saguaro, II: Reproduction, Germination, Establishment, Growth, and Survival of the Young Plant. Washington. [Natl. Park Serv., Sci. Mongr. Ser. 8.]
|[For Andrew Carnegie, 1835-1919, Scottish-born American philanthropist and patron for systematic studies of cacti]|