1 Leaves elliptic to oblanceolate, mostly < 15 cm long, the apices acute to obtuse; twigs puberulent; spine-covered husk of fruit splitting into 2 sections, enclosing 1 nut; nut circular in cross-section, 7-19 mm in diameter; pistillate dichasia of 1 flower; leaves with stellate trichomes, with few bulbous-based trichomes when young, puberulent, pilose, tomentulose, or tomentose in age (usually rather densely so).

  2 Longest spines of the fruit husk usually > 10 mm long; young twigs glabrous; petiole 8-10 (-15) mm long; [plants of AR, OK, MO, and n. LA]

  2 Longest spines of the fruit husk usually < 10 mm long; young twigs puberulent; petiole 3-7 (-10) mm long; [plants widespread in our area]

1 Leaves elliptic, oblanceolate or lanceolate, 8-30 cm long, the apices acuminate, sometimes only shortly so; spine-covered husk of fruit splitting into 4 sections, enclosing 1-3 (-5) nuts; nut flattened on at least one side, 18-30 mm in diameter; pistillate dichasia of 3 flowers; leaves with or without stellate trichomes; twigs puberulent or glabrous.

    3 Undersurface of leaves densely covered with bulbous-based trichomes when young, essentially glabrous in age; leaves mostly > 15 cm long, generally long-acuminate; twigs glabrous; trees single-trunked; spines of fruit husk weak, easily bent

      4 Leaf margins not ciliate; young twigs glabrous (but not glaucous); burs with (2-) 3 (-5) nuts

    3 Undersurface of leaves persistently and densely tomentose beneath; leaves mostly < 15 cm long, generally short-acuminate; twigs puberulent; trees multi-trunked from base; spines of fruit husk stiff.

        5 Leaf undersurface covered with yellowish brown to yellowish gray scalelike glands (as well as tomentose)

        5 Leaf undersurface with few or no scalelike glands