2. Humulus L. (hops)

Plants annual or perennial herbs, with long taproots or stout rhizomes respectively, not strongly aromatic, the staminate and pistillate plants appearing similar vegetatively. Stems twining, branched (except in very depauperate individuals), usually finely ridged or angled, solid at maturity, sparsely to densely rough-pubescent with stiff, prickly 2-branched hairs. Leaves with the petioles shorter than to longer than the blades, sometimes twining, pubescent with short-stalked, 2-branched hairs. Stipules lanceolate, sometimes fused laterally and then appearing solitary rather than paired. Leaf blades ovate to nearly circular in outline, unlobed or palmately 3–9-lobed, deeply cordate at the base, the lobes mostly somewhat narrowed toward the base, tapered to a sharply pointed tip, the margins sharply toothed and sometimes hairy, the surfaces variously hairy and glandular. Staminate inflorescences appearing as open axillary or less commonly terminal panicles with small flower clusters at the branch tips, the flowers with stalks 0.5–3.5 mm long, the branch points and flowers subtended by small bracts. Pistillate inflorescences short, dense spikes continuing to elongate as the fruits mature, pendant at maturity, conelike, the solitary or paired flowers sessile or nearly so, the bracts elliptic or ovate to broadly ovate, sometimes hairy on the surface and along the margins, also with glandular dots and/or short-stalked glands cup-shaped at the tip. Staminate flowers 1.5–3.0 mm long, the sepals lanceolate to ovate or oblong-ovate, with pale, thin margins, glabrous or less commonly sparsely hairy, also with glandular dots. Fruits broadly ovoid to nearly spherical, sometimes slightly flattened, enveloped by the persistent enlarged bracts and membranous calyx. Three species, North America, Europe, Asia, widely cultivated and introduced nearly worldwide.

The branched hairs consist of two short, rigid branches spreading in opposite directions. These are raised from the surface on short, multicellular bases containing cystoliths (calcium carbonate inclusions) that appear as small tubercles. This arrangement apparently is an adaptation to help anchor the plants as they climb; they impart a strong, prickly roughness to the surfaces of the stems and leaves.