DITHYREA Harvey, London J. Bot. 4: 77, pl. 5. 1845.
Ihsan A. Al-Shehbaz
Tribe: Physarieae B. L. Robinson in A. Gray & S. Watson, Syn. Fl. N. Amer. 1(1): 100. 1895.
Name derivation: Greek di, two or double, and thyros, shield, in reference to the spectacle-shaped fruits.
Type species: Dithyrea californica Harvey.
Herbs, annual or rhizomatous perennial, densely pubescent. Trichomes stalked stellate to subdendritic, sometimes as simple clavate papillae on fruits. Multicellular glands absent. Stems erect or decumbent, branched below and above. Basal leaves petiolate, rosulate, simple, entire, dentate, sinuate or rarely pinnatifid; cauline leaves petiolate to subsessile, not auriculate, simple, entire, or dentate. Racemes many flowered, ebracteate, densely corymbose, elongated considerably or not elongated in fruit; rachis straight; fruiting pedicels basally with 2 well-developed glands, divaricate to slightly reflexed, persistent. Sepals linear to narrowly oblong, free, connivent, caducous, erect and forming a tube, pubescent, unequal, base of lateral pair saccate. Petals white to lavender, erect, longer than sepals; blade lingulate to oblanceolate or obovate, obtuse to rounded at apex; claw differentiated from blade, subequalling sepals, glabrous, unappendaged, entire. Stamens 6, exserted, erect, tetradynamous; filaments wingless, unappendaged, glabrous, free, not dilated at base; anthers narrowly linear, sagittate, not apiculate. Nectar glands confluent, lateral annular, median glands present. Ovules 2 per ovary; placentation apical. Fruits silicles, breaking up at maturity into 2 1-seeded units, didymous, strongly angustiseptate, not inflated, unsegmented; valves suborbicular or slightly broader than long, thick papery, not veined, pubescent with branched or clavate simple papillae, keeled, indurated around the margin, enclosing seeds, wingless; gynophore to 0.5 mm; replum concealed by valve margin; septum complete, not veined; style to 1 mm, pubescent, persistent; stigma broadly capitate, entire. Seeds aseriate, oblong, flattened, wingless; seed coat smooth, not mucilaginous when wetted; cotyledons accumbent. x = 10.
Two species: N Mexico, SW United States.
References: Aigner (2003), Payson (1918), Rollins (1979).