6.
Quercus hypoleucoides
A. Camus, 4: 124. 1932.
Silverleaf oak
Quercus hypoleuca
Engelmann
Trees or shrubs , evergreen, to 10 m. Bark black, deeply furrowed. Twigs dark reddish brown, 1.5-3 mm diam., pubescent. Terminal buds light chestnut brown, ovoid, 2.5-4.5 mm, glabrous except for ciliate scale margins, occasionally with tuft of hairs at apex. Leaves: petiole 1.5-13 mm, densely pubescent. Leaf blade narrowly ovate to ovate or elliptic, 45-120 × 15-40 mm, base cuneate to rounded, margins strongly revolute, entire or spinose with up to 11 awns, apex acute to attenuate; surfaces abaxially densely tawny- or white-tomentose, adaxially noticeably rugose, glabrous. Acorns annual or biennial; cup deeply saucer- or cup-shaped, 4.5-7 mm high × 6-13 mm wide, covering 1/3 nut or less, outer surface pubescent to sparsely puberulent, inner surface pubescent to floccose, scales appressed, blunt; nut ellipsoid to oblong, 8-16 × 5-10 mm, glabrous, scar diam. 2.5-5.5 mm.
Flowering spring. Common in moist canyons and on ridges; 1500-2700 m; Ariz., N.Mex., Tex.; n Mexico.
Quercus hypoleucoides reportedly hybridizes with Q . gravesii ( Q . × inconstans E. J. Palmer [= Q . livermorensis C. H. Muller]) (see C. H. Muller 1951). Several specimens from Pima County, Arizona, fall outside the range of typical Q . hypoleucoides , suggesting hybridization with the Mexican Q . mcvaughii Spellenberg (R. Spellenberg 1992).
