Shrubs [trees], erect, branched, branches mostly basal, closely parallel [to candelabra-shaped or solitary]. Roots diffuse. Stems unsegmented, dark green or green to blue-green or glaucous gray-green, thick columnar, often somewhat narrowed between growth increments, 300-4500[-700] × [5-]12-16[-100 in P. weberi] cm, sometimes dimorphic with terminal reproductive zone or cephalium bearing specialized, densely spiny areoles; ribs [absent on cephalium in P. militaris or 3-]4-7[-16 in P. pringlei], nearly triangular in cross section to rounded, rib crests flat to crenate; areoles distinct or confluent via felty abaxial grooves along ribs, of [1 or] 2 kinds, circular to shield-shaped and slightly raised to elongate and flat; hairs white to light gray; areolar glands absent; cortex and pith not mucilaginous, blackening when cut, firm, pith often exceeding 5 cm diam. Spines [0-]5-20[-60] per areole, whitish gray to gray, sometimes aging black; radial spines on proximal, nonflower-producing portions of stems, acicular to short and stout, usually less than 3 cm; central spines (0-)1(-3) per areole, usually pointing toward stem base, otherwise longer but similar to radial spines in form and color, (1-)3(-10+ in P. weberi) cm; on flowering areoles radial and central spines not readily distinguishable, bristles numerous [or nearly absent], gray [amber-yellow to golden or reddish brown], wiry, long, slender. Flowers nocturnal [or diurnal], several per areole [or solitary], subterminal to lateral on distal 1-3 m of stem in cephalium of specialized, densely bristly, nearly confluent areoles [or stem areoles not specialized], from adaxial portion of areoles, cylindric to narrowly funnelform, narrowly campanulate, or short funnelform, 3-4.5[-12] cm; tepals spreading [to ascending or erect], margins entire to fimbriate; outer tepals with pink to rose centers and lighter margins [to yellowish, greenish, or rose-maroon]; inner tepals whitish pink [to ivory white, yellowish, rose, or coral]; ovary few scaled [to very scaly], spineless [to very spiny or bristly]; scales soon deciduous [persistent], rose-red to yellowish, soon turning black [or not changing], triangular with prominent bases, fleshy [to papery], tips acute to acuminate, with axillary tufts of whitish to tan hairs [to densely tan woolly]. Fruits indehiscent [ to irregularly dehiscent or dehiscing by vertical slits], reddish [sometimes color hidden by tan to yellowish wool], ovoid to spheric, 20-75 mm diam.; areoles ± absent [or deciduous or persistent], spineless [or spiny to densely covered with wirelike bristles]; pulp slightly sour to sweet, colorless or wine red [purplish or yellowish], often not filling locule; floral remnant absent [or persistent]. Seeds black, ovoid to helmet- or comma-shaped, 2.2-2.8[-6] mm, glossy; testa relatively smooth [to ± papillate], cells usually flat with minute pits at "corners" between cells. x = 11.
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|[Greek pachys, thick, and Cereus, a genus of cacti]|