The two species are easy to distinguish when cultivated side by side, but they are sometimes difficult to identify from herbarium specimens. Analysis of flavonoids (S. Zona and R. Scogin 1988) can reliably distinguish the two species.
PLANT: Unbranched trees to 25 m tall, usually with persistent dead droop¬ing leaves forming a skirt. LEAVES: fan-shaped to 1 m broad, palmately dissected into 40-60 segments, their margins commonly with slender filaments; petioles to 1 m long, flattened, projecting into the blade as a winged arrowhead point, the margins usually spine-bearing. INFLORESCENCE: 3-4 m long. FLOWERS: numerous, perfect, white, 7-10 mm long; calyx lobes erect; corolla lobes reflexed; stamens 6; pistil 3-lo¬culed. FRUITS: dry, hard, ellipsoid drupes, 5-10 mm long. NOTES: 2 spp.; AZ, CA; nw Mex. (for G. Washington). Bailey, L. H. 1936. Gentes Herbarum 4:53-82. REFERENCES: Mason, Charles T., Jr. 1999. Arecaceae. Ariz.-Nev. Acad. Sci. 32(1).