4. Juglans microcarpa Berlandier in J. L. Berlandier and R. Choval, Diario Viaje Comis. Limites. 276. 1850.
Little walnut, nogal, nogalito, namboca
Juglans rupestris Engelmann ex Torrey
Shrubs or small trees , to 10 m. Bark medium gray, split into ± rough ridges. Twigs with distal edge of leaf scar notched, glabrous or bordered by poorly defined velvety zone; pith light to dark brown. Terminal buds globose to short-ovoid, not flattened, 3-5 mm. Leaves 12-29 cm; petiole 1-3(-4) cm. Leaflets 17-25, lanceolate or narrowly lanceolate, weakly to strongly falcate, 5.2-6.3(-9.6) × 0.8-1.1(-2.2) cm, margins entire or toothed, apex long-acuminate; surfaces abaxially with capitate-glandular hairs (sometimes becoming sparse late in season except along veins), often scattered scales, axils of proximal veins usually, not always, with prominent tufts of fasciculate hairs, adaxially with capitate-glandular hairs (late in season hairs sometimes becoming sparse except along veins); terminal leaflet usually small. Staminate catkins 3-7 cm; stamens 20-25(-35) per flower; pollen sacs 0.8-1 mm. Fruits 1-3, globose, 1.4-2.3 cm, smooth, with capitate-glandular hairs; nuts globose to depressed-globose, 1.1-1.7 cm, grooved, surface between grooves smooth.
Flowering spring (Mar-Apr[-Jun]). Along creeks and rivers; 200-2000 m; Kans., N.Mex., Okla., Tex.; Mexico (Chihuahua, Coahuila, and Nuevo León).
Specimens of Juglans microcarpa with larger leaflets (parenthetical numbers above) may result from introgression with J . major . These have sometimes been treated as J . microcarpa var. stewartii (I. M. Johnston) W. E. Manning, but W. E. Manning (1978) reported this variety only from Mexico. Intermediates between J . microcarpa and J . nigra (q.v.) are also known.