Trees or shrubs, erect to ascending or reclining, usually branched in basal portion, older plants with closely parallel, erect branches forming narrow crown; trunk absent or present, 8-12+ cm diam. Roots diffuse. Stems unsegmented, green to bluish or blue-green, columnar, [0.5-]3-10 m × [2.8-]5-8 cm, glabrous; ribs 3-30, low, rounded, furrows straight [or wavy], rib crests sometimes notched between areoles [ribs strongly tuberculate in 1 Brazilian species]; areoles usually closely spaced along ribs [often confluent in flowering region], circular to elliptic, woolly; hairs usually white or tawny [to blackish] soft, to 8[-50] mm, longest on flower-producing areoles along each rib or on one side of stem in distal region; areolar glands not apparent; cortex and pith extremely mucilaginous. Spines 6-31 per areole, yellow [to brown or black], often aging gray, acicular, straight, terete, [2-]10-15(-25)[-40] × 0.25 mm, smooth; radial and central spines not distinguishable. Flowers nocturnal or crepuscular, lateral to subterminal on stems, from closely adjacent or confluent areoles, funnelform to narrowly campanulate, [2.5-]5-6[-9] × 2-5[-7] cm; flower tube straight or slightly curved; outer tepals greenish [blue, red, or purplish], margins entire or minutely denticulate; inner tepals white [pinkish to reddish], 9-26 × 7.5 mm, margins entire; ovary smooth, scales absent (or 1-2), spines absent; stigma lobes usually 8-12. Fruits dehiscent along 1 side or apparently indehiscent, red or purple [to blue-green], [spheric or] depressed-spheric [to ovoid or oblong], 20-45 × 30-50 mm, fleshy, scales and spines absent (rarely 1-2 scales); pulp color variable; floral remnant usually persistent. Seeds black or dark brown, snail-shaped, 1.2-2.6 mm, smooth, shiny; testa cells flat. x = 11.
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