Humulus L.
Humulus is a genus with – depending on species boundaries – three to up to ten species from the northern hemisphere. One species, Humulus lupulus L., is a common native plant in Belgium. It is also locally cultivated (hop) for flavouring beer or as an ornamental. A second species, Humulus japonicus, is cultivated for ornament and occurs as an invasive escape in many parts of the world (including parts of southern Europe).
1 Annual. Abaxial side of leaf blades stiffly hairy, especially on the nerves. Petioles usually longer than the blades, the lowermost blades (5-)7(-9) lobed. Anthers non-glandular === Humulus japonicus
Perennial. Abaxial side of leaf blades glabrous or softly pubescent. Petioles usually shorter than the blades, the lowermost blades 3-5 lobed. Anthers glandular (native) === H. lupulus
Literature:
Davies E.L. (1957) Morphological complexes in hops (Humulus lupulus L.) with special reference to the American race. Ann. Missouri Bot. Gard. 44: 271-294.
Nelson E.C. (1997) Moraceae. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 3. Cambridge University Press, Cambridge: 86-102.
Small E. (1978) A numerical and nomenclatural analysis of morpho-geographic taxa of Humulus. Syst. Bot. 3: 37-76.
Small E. (1997) Cannabaceae. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 3. Oxford University Press, New York-Oxford: 381-387. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=105522]
Tembrock L.R. & McAleer J.M. (2016) A revision of native North American Humulus (Cannabaceae). J. Bot. Res. Inst. Texas 10(1): 11-30. [available online at: https://www.researchgate.net/publication/311226829_A_revision_of_native_...