This is a widespread variable species of woodlands on dry acid soils across most eastern states; 2n = 16 and 32 (FNA 20). However, it is absent in the upper mid-west, and uncommon to rare in much of w. Ky. and w. Tenn. Cauline leaf blades are entire, sessile and cordate-clasping, or with petioles strongly dilated to a cordate-clasping base, unlike other species in Sect. Heterophylli (W). In Ky. plants are usually covered with dense short stiff hairs (hirtellous to hispidulous). However, the species may sometimes be glabrescent below the inflorescence, and then more easily confused with shortii. Moreover, undulatum appears to form occasional hybrids with shortii (e.g. from BOUR, BULL, MCRA and POWE), urophyllum (WOLF), cordifolium (CART and MCRE), "lowrieanum" (PERR), laeve (WAYN, WHIT), dumosum (BULL), oolentangiense (FNA 20) and perhaps other species. Unusually narrow-leaved plants have been named A. undulatus var. loriformis Burgess (e.g. from NELS at MICH).