1. Aesculus glabra Willd. (Ohio buckeye)

Pl. 425 k–m; Map 2586

Plants small to more commonly medium to large trees to 20 m tall, occasionally flowering as shrubs 3–5 m tall (these sometimes caused by flood damage followed by resprouting). Bark relatively smooth on young trees, developing shallow fissures and small scaly plates with age, dark grayish brown, often becoming lighter gray on older trees. Leaf blades palmately compound with 5–11 leaflets, these 5–16 cm long, the upper surface glabrous or sparsely to moderately short-hairy along the veins, yellowish green to bright green, the undersurface sparsely to densely pubescent with short, curly, sometimes tangled or woolly hairs, sometimes only along the veins or as small tufts in the vein axils, green to dark green or noticeably pale. Inflorescences 10–15 cm long. Calyces 3–8 mm long, more or less bell-shaped, relatively symmetrical at the base, yellow to greenish yellow, the lobes more or less similar. Corollas only slightly zygomorphic, greenish yellow, sometimes the upper pair of petals marked with orangish or reddish spots and/or central region, the petals 10–19 mm long, the upper pair slightly longer and more or less oblanceolate, gradually tapered to the stalklike base, the lower pair slightly shorter and with the blade broadly oblong to oblong-ovate or nearly circular, more abruptly tapered to the stalklike base. Stamens 7, the filaments 15–23 mm long, strongly exserted, hairy below the midpoint, the anthers orange. Fruits 2–4(–5) cm long, ovoid to obovoid or nearly globose, the outer wall leathery, light brown to brown, usually with abundant, irregular, spinelike tubercles, these sometimes shed with age, otherwise slightly roughened or warty. 2n=40. April–May.

Scattered to common nearly throughout the state, but apparently absent from most of the Mississippi Lowlands Division (eastern U.S. west to Minnesota, Nebraska, and Texas; Canada). Banks of streams and rivers, bottomland forests, mesic upland forests, and bases of bluffs; also margins of pastures and old fields.

Steyermark (1963) noted that during his monographic research on the genus, Hardin (1957a, b) annotated several specimens from Missouri as representing putative hybrids of intermediate morphology between A. glabra and A. flava Sol. (the latter parent under the name A. octandra Marshall). Some of these have been called A. ×marylandica Booth ex Dippel. The yellow buckeye, A. flava, occurs from western Pennsylvania southwest along the Ohio River to southern Illinois. It differs from A. glabra in its more strongly zygomorphic flowers, stamens that are not or only slightly exserted, and nonspiny fruits. The hybrids are characterized by somewhat exserted stamens, scattered gland-tipped hairs mixed in with the nonglandular hairs on the calyx and flower stalk, somewhat more zygomorphic corollas, and fruits with irregular clusters of spines. For a discussion of putative hybrids between A. glabra and A. pavia, see the treatment of that species.

Steyermark (1963) treated A. glabra in Missouri as comprising three varieties and one additional form. He did not agree with the earlier treatment of Hardin (1957a, b), who accepted only two varieties. However, Hardin argued persuasively that plants attributed to var. leucodermis, characterized by paler bark and pale undersurface of the leaflets, appeared sporadically throughout the species range and that these two characters did not correlate in many specimens. He studied plants at the type locality of var. leucodermis in Arkansas, where he observed the existence of a large putatively interbreeding population of A. glabra and A. pavia. Hardin also noted that pubescence density, which was said to characterize f. pallida, varied too much within populations to be useful taxonomically. These views are accepted in the present treatment, with acknowledgment that occasional specimens are difficult to determine to variety.