9. Carya tomentosa (Poir.) Nutt. (mockernut hickory, white hickory)

C. tomentosa var. subcoriacea (Sarg.) E.J. Palmer & Steyerm.

Pl. 431 a–c; Map 1931

Plants trees to 35 m tall. Bark dark gray, shallowly to deeply furrowed and splitting into plates but remaining firmly attached (not exfoliating). Twigs 3–5 mm thick, reddish brown, the terminal bud 9–14 mm long, grayish and densely hairy, the bud scales strongly overlapping, the bractlets around the axillary buds fused, at least toward the base. Leaves with the petiole and rachis usually densely hairy, with (3–)7(9) leaflets. Leaflets 5–21 cm long, 3–9 cm wide, oblanceolate to obovate, straight (appearing symmetrically tapered), the margins finely to coarsely toothed, glabrous or with evenly scattered hairs, the upper surface glabrous or with a few widely scattered hairs and/or small, circular, yellow, peltate scales, especially when young, the undersurface with sparse to moderate, unbranched and branched (the branches appearing fasciculate) hairs along and between the veins, and also with scattered to moderate, small, circular, whitish (translucent) and reddish, peltate scales. Staminate catkins noticeably stalked. Fruits 3.0–4.5 cm long, 3–4 cm wide, globose to somewhat ellipsoid or obovoid, not or only slightly flattened, smooth (rarely with faint ridges along 1 or more of the sutures), the husk 3–8 mm thick, splitting, splitting up to about 80% of its length (sometimes some of the splits reaching the base), with small golden yellow scales that wear off with age. Nut noticeably flattened (oval in cross-section), the shell 4–6 mm thick. Seed sweet. 2n=64. April–May.

Scattered to common nearly throughout the state but apparently absent from the western portion of the Glaciated Plains Division (eastern U.S. west to Iowa, Kansas, and Texas). Mesic to dry upland forests, tops of bluffs, banks of streams and rivers, margins of ponds, lakes, and sinkhole ponds, and occasionally bottomland forests; usually on acidic substrates.

There has been controversy about the proper name for this species. Some authors have maintained that the correct name should be C. alba (L.) Nutt. The basis for this name, Juglans alba L., was based on a mixture of plants here treated as C. ovata and C. tomentosa. Rehder (1945) noted this and suggested that the epithet C. alba should be rejected as a name of ambiguous application, and the recent proposal by Ward and Wiersema (2008) was approved at the 2012 International Botanical Congress. On the other hand, Wunderlin et al. (1985) concluded that the transfer of the epithet from Juglans into Carya established a priority for the name over C. tomentosa, based on their interpretation of the type element within the Linnaean name. However, this opinion has now been superseded by the official rejection of Juglans alba at the recent International Botanical Congress.

In Missouri, C. tomentosa is known to hybridize occasionally with C. illinoinensis (C. ×schneckii Sarg.). Names have been given to many variants of the mockernut. Plants with stalked fruits have been called f. ficoidea (Sarg.) E.J. Palmer & Steyerm.; plants with a tapering apex on the fruit have been called f. ovoidea (Sarg.) E.J. Palmer & Steyerm., and plants with thick leaflets and elongate nuts have been called var. subcoriacea. None of these trivial variants seems worthy of formal taxonomic recognition.