Smithsonian Contr. Knowl. 5(6): 60. 1853.
Ariz., N.Mex., Tex., Mexico.
Varieties 2 (1 in the flora).
Apodanthera undulata in the flora area is recognized by its reniform, undulate-margined, and white-backed leaves and yellow petals. Variety australis McVaugh (A. aspera Cogniaux) differs in reduced vestiture and pistillate inflorescence; it occurs in Mexico (Aguascalientes, Guanajuato, Hidalgo, Jalisco, Michoacán, Nayarit, Oaxaca, Querétaro, San Luis Potosí, Zacatecas). According to McVaugh, the two varieties are allopatric.