Tilia americana L. (American basswood)

Pl. 455 a–c; Map 2067

Plants medium to less commonly large trees to 25 m tall. Bark deeply ridged and grooved, light brown to gray. Twigs slender to moderately stout, often appearing somewhat zig-zag, green to yellowish or reddish brown, eventually becoming gray, smooth but with abundant small lenticels, glabrous or sparsely to moderately pubescent with minute, stellate hairs, often somewhat shiny. Winter buds axillary (true terminal bud absent, but the uppermost bud often appearing asymmetrically terminal), oblong-ovoid, with a pair of exposed outermost buds scales, these orangish brown to reddish brown or brownish purple, glabrous or more commonly at least sparsely stellate-hairy. Leaves alternate and appearing 2-ranked, long-petiolate, the fall foliage turning bright yellow. Leaf blades 5–16(–20) cm long, 4–13(–19) cm wide, broadly ovate to heart-shaped, truncate to more commonly cordate at the base, but often somewhat asymmetric and sometimes with 1 side truncate to broadly angled; short-tapered to broadly angled at the usually sharply pointed tip, unlobed, the margins sharply and often finely toothed, also minutely hairy, especially in the sinuses between teeth, the upper surface dark green, glabrous and somewhat shiny, the undersurface green or pale green, variously glabrous at maturity except for scattered, minute glands and/or small tufts of hairs in the axils of main veins (then sometimes inconspicuously pubescent with unbranched hairs during development), to persistently and moderately to densely woolly with mostly small, stellate hairs, the venation conspicuously palmate with mostly 5 veins from the blade base. Stipules not persistent at maturity, 6–10 mm long, narrowly strap-shaped to lanceolate, glabrous or hairy. Inflorescences axillary, flat-topped to dome-shaped panicles, these more or less pendant, stalked, the stalk partially fused to and appearing to originate from the midvein of a prominent, bract, this 6–11 cm long, strap-shaped to narrowly oblong-oblanceolate, initially pale green to white and membranous to somewhat leaflike, but soon turning tan to pale brown and papery. Flowers not subtended by bractlets. Calyces bell-shaped, not appearing angled or winged, the sepals free to the base, not overlapping in bud, 4–6 mm long, not persistent at fruiting, triangular-lanceolate, sharply pointed at the tip, relatively thick and leathery, the upper surface white and densely woolly, the undersurface pale green and moderately to densely pubescent with minute, stellate hairs. Petals 7–9 mm long, narrowly elliptic, the tips rounded to nearly truncate, sometimes slightly irregular, the margin otherwise entire or nearly so, white or pale cream-colored, occasionally faintly pinkish-tinged, not persistent at fruiting. Stamens numerous, the filaments not fused into a tube, united at the base into usually 5 groups, often forked near the tip so that the 2 anther sacs appear separate, these small, attached near their midpoints, yellow. Staminodes usually present, usually 1 per fused cluster of stamens, this innermost and positioned opposite the petals, petaloid, slightly shorter than the petals. Pistils with 5 locules, each with 2 ovules. Style 1, the stigma small, capitate, shallowly 5-lobed or 5-angled. Fruits nutlike drupes, 6–10 mm long, broadly ellipsoid to nearly globose, often with a short, slender beak at the tip, indehiscent, the outer shell relatively thick and hard, brown to grayish brown at maturity, the outer wall obscured by dense stellate hairs, 1-locular, with 1(–3) seeds. Seeds 3–7 mm long, subglobose (variously shaped in 2- and 3-seeded fruits), usually with a shallow longitudinal ridge along 1 side, brown to reddish brown, glabrous. 2n=82. May–July.

Scattered to common nearly throughout the state (eastern U.S. west to North Dakota and Texas; Canada, Mexico). Bottomland forests, mesic upland forests, banks of streams and rivers, and bases and ledges of bluffs; also edges of pastures.

Steyermark (1963) and Jones (1968) treated the basswoods native to North America as comprising four closely related species, two of which occur in Missouri. In a detailed study of morphological variation across the range of the complex, Hardin (1990) documented considerable overlap for every key character and concluded that these taxa were more properly treated as varieties. The var. caroliniana (Mill.) Castigl. occurs to the south and east of Missouri, mainly on the Atlantic and Gulf Coastal Plains, and differs in its tendency to produce stalked-stellate hairs and/or fascicles of nonstellate hairs on the undersurface of the leaves. The var. mexicana (Schltdl.) Hardin is scattered but widespread in Mexico and is distinctive in its relatively densely and mostly persistently hairy twigs. Most recently, the morphological, molecular, and ecological studies of McCarthy (2012) resulted in the finding of little congruence between the data sets, as well as the lack of correlation among morphological characters across the distributional range of the complex, and a suggestion that much of the observed variation may be environmentally induced rather than genetically determined. Thus the varietal treatment accepted in the present work remains somewhat controversial.