Key to the genera

1 Flowers haplostemonous; petals with one principal vein, membranaceous; ovary with one pendent ovule; seed without dark testa.

  2 Gronoviaceae

2 Flowers in racemes with coflorescences; sepals caducous; bract and bracteoles remain attached to fruit upon maturity (N. Mexico - SW USA).
Petalonychoideae - Petalonyx (5ssp)
2* Flowers in cymoids or thyrsoids, sepals persistent, fruit free of bract.
3 Gronovioideae
3 Erect subshrub with narrow, sinuate-lobate leaves; sepals and petals linear; connective protracted into long appendage (N. Mexico SW USA).
Cevallia sinuata
3* Liana or climber with cordate to subpalmately lobed leaves; sepals spatulate; petals lanceolate; connective not differentiated.
4 Leaves cordate, margin entire; inflorescence branches dichasial; petals laciniate (Hispaniola).
Fuertesia domingensis
4* Leaves subpalmately lobed; inflorescence branches monochasial; petals entire (Peru - Mexico).
Gronovia (2 spp)

 1* Flowers polyandric or obdiplostemonous; petals with 3-5 principal veins; ovary with one to many ovules on parietal placentae; seeds with dark testa.

  5 Loasaceae
5 Petals +/- flat and membranaceous; staminodia not in antesepalous groups if present.
6 Mentzelioideae
6 Flowers obdiplostemonous (S. Mexico).
Schismocarpus pachypus
6* Flowers polyandric.
7 Seeds tuberculate (both Americas).
Mentzelia (ca. 60 spp)
7* Seeds striate (Guatemala - USA).
Eucnide (ca. 15 spp)
5* Petals deeply cymbiform or carnose if flat; staminodia in antesepalous groups always present.
8 Loasoideae
8 Flowers tetramerous; inner staminodia with a differentiated, lobed apex; petals usually with longitudinal lamellae; urticant hairs always absent.
9 Klaprothieae
9 Outer 4-6 staminodia united to form a flat, yellow floral scale; erect shrub from underground xylopodium (N.Peru).
Xylopodia klaprothioides
9* All staminodia free nearly to base; erect annual or perennial herbs or shrubs without differentiated underground structures.
10 Erect shrub (Polynesia).
10* Erect annual or basally decumbent, perennial herbs (S. & C. America, islands).
8* Flowers penta- to octamerous; inner staminodia undifferentiated or variously thickened and appendaged but never with a lobed apex; petals without longitudinal lamellae; urticant hairs often present.
11 Loaseae
11 Flowers erect; floral scales pale yellow, white or cream coloured, +/- flat, dorsal threads filiform if present; petals white, greenish or cream coloured.
12 "Lower Loaseae"
12 Floral scales from more than 3 staminodia, 4-5 free staminodia; plant without urticant setae (N. Chile).
Huidobria (2 spp)
12* Floral scales from three staminodia, 2 free staminodia, plant with or without urticant setae.
13 Sepals much larger than petals; floral scales > 5x as long as wide; desert shrub without urticant setae (Africa, Arabia).
Kissenia (2 spp)
13 * Sepals smaller than petals, floral scales +/- as long as wide or up to 2x as long as wide; plants with urticant setae (S. & C. America).
14 Erect desert shrubs; leaves less than 4 cm long; petals spreading; floral scales with dorsal threads much exceeding the scale neck in length (Peru).
Presliophytum (3 spp)
14* Tall rain forest herb (to 4m); leaves more than 15 cm long; petals semi-erect, floral scales without threads but with lobed apex (N. Colombia to Costa Rica).
Chichicaste grandis
11* Flowers usually pendent, very rarely erect; floral scales brightly coloured, usually red and yellow or green, and / or with various morphological elaborations such as dorsal calli, wings, flags or a double arch.
15 "Higher Loaseae"
15 Inflorescence ebracteate; seed testa with asymmetrical cells (Brazil, Hispaniola).
Aosa (6 spp)
15* Inflorescence bracteose or frondose; seeds deeply pitted, irregularly fibrous, reticulate or rugulate, symmetrical (Andean and southern South America, 1 sp in Brazil).
16 Every flower on the branches of the inflorescence subtended by one bract only (primary flower usually with two bracts); floral scales with conspicuous dorsal sacs and apical wings, often with dorsal callus.
Nasa (ca. 100 spp)
16* Every flower of the inflorescence subtended by two bracts; floral scales rectangular or with double arch and / or dorsal threads or flags but never with conspicuous dorsal sacs or apical wings or dorsal calli.
17 Fruits always narrowly twisted anticlockwise; seeds winged or with fibrous testa or angular but never deeply pitted with fenestrate anticlinal walls nor rugulose; floral scales rectangular, equalling the free staminodes in size.
Blumenbachia s.l. (12 spp)
17* Fruits straight or twisted, if twisted always antidromous (alternatingly clockwise and anticlockwise); seeds deeply pitted with fenestrate anticlinal walls, very rarely with irregularly rugulose testa; floral scales with double arch and often with flags or floral scales reduced in size (<< than free staminodes).
18 Annual, winding herb; fruits completely inferior, narrowly cylindrical, +/- straight, more than 10x as long as wide; flower erect, petals dark yellow (Chile).
Scyphanthus (1-2 spp)
18* Perennial or annual herb; fruits partially superior to completely inferior, cylindrical, ovoidal, clavate or globose, never more than 7x as long as wide, often twisted; petals white, green, yellow or red ; flowers usually pendent or deflexed (if flowers erect:  plant a tiny rosette herb and petals cream coloured).
19 Fruits straight or twisted, opening with longitudinal slits, v. rarely tiny rosette herb with oblong, cream-coloured petals and fruits opening with apical valves.
                 Caiophora (ca. 50 spp)
19* Fruits straight, opening with apical valves.
Loasa (ca. 35 spp)

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