Greek penios—thread, referring to the narrow stems.
Body narrow, ribbed, more or less cylindrical, climbing or scrambling and with few branches. Roots thickened. Flower nocturnal or diurnal, tube long and narrowly cylindrical with areoles; mostly summer. Pericarpel and flower tube with bristles or spines. Fruit ovoid, fleshy, red with spines and/or bristles at first.
Rare in cultivation. Known mostly through P. serpentinus (Lag. & Rodruiges) N.P. Taylor — perhaps better known under the name Nyctocereus serpentinus (Lag. & Rodruiges) Britton & Rose from Mexico which is widely cultivated not only as an ornamental plant but also for its edible fruits.
Roots tuberous; stems thin and dark coloured, striped or speckled; spines generally pressed to the stem.
About 20 species from N Mexico, SW North America and C America.
Source: (1997). Cactaceae. In: . Horticultural Flora of South-eastern Australia. Volume 2. Flowering plants. Dicotyledons. Part 1. The identification of garden and cultivated plants. University of New South Wales Press.
