Description of Oenothera sect. Ravenia:
Distribution: The three species of sect. Ravenia form a distinctive group based on their seed morphology and anatomy (Tobe et al. 1987; Wagner 2005). They have relatively narrow ranges in montane habitats in four disjunct areas of Mexico: 1) northern Coahuila (O. riskindii); 2) southern Coahuila, Nuevo León, and Tamaulipas (O. muelleri and O. tubifera subsp. tubifera); 3) southern Durango (O. tubifera subsp. macrocarpa); and 4) southern Hidalgo, east to central Puebla, and south to extreme northernmost Guerrero (O. tubifera subsp. tubifera), from 1,690 to 3,250 m elevation.
Habit: Perennial herbs, from a large fleshy taproot, lateral roots occasionally producing new shoots; stems several, arising from the rosette, unbranched or with short lateral branches, decumbent to ascending.
Leaves: Leaves forming a basal rosette and also cauline, 2.5-18 cm long, pinnately lobed to sinuate-dentate or subentire, gradually tapering to the base, sessile or with a winged petiole.
Flowers: Apex of bud curved downward by the recurved floral tube, becoming erect before anthesis, sharply quadrangular in cross section, without free sepal tips, floral tube 6.5-15.2 cm long, sepals individually separate and reflexed at anthesis, petals white, fading pink to rose, drying purple.
Fruit: Capsule oblong-lanceoloid, straight or slightly curved, quadrangular in cross section, the angles acute to rounded, the base symmetrical to slightly asymmetrical, dehiscing 1/2 to nearly the full length, sessile.
Seeds: Seeds 40-60, in 1 or 2 rows per locule, obovoid to oblong or oblanceoloid, often somewhat irregular; the testa much thickened above the raphe and at the distal end, the thickened area with a cavity not visible externally or rarely appearing as a distal pore and/or a raphial groove, the abaxial surface with longitudinal ribs.
Chromosomes: Chromosome number: n = 7.