Morus L.
Morus is a genus with ca. 13-15 species in the warm-temperate regions of the world. Although widely spread in the Old as well as the New World, the genus is by far most diverse in Southeast Asia. Eleven species occur in China, five of them endemic (Zhekun & Gilbert 2003). No species are native in Belgium but several are cultivated as ornamentals or for their edible berries.
Morus, as currently circumscribed, is non-monophyletic. Subgenus Morus is resolved as a clade and consists of two well-supported clades: one of Asian taxa and one of North American taxa (Nepal & Ferguson 2012). The African and neotropical (South American) members of Morus probably belong to separate genera.
The identity of the species of Morus found in the wild in Belgium is somewhat uncertain. According to Lambinon & Verloove (2012) Morus alba L. and M. nigra L. are cultivated for their edible fruits and sometimes escape. However, both these names may be misapplied in Belgium, at least in part. Large-leaved plants of M. alba with deep and narrow lobes probably refer to M. kagayamae Koidzumi (syn.: M. bombycis Koidzumi) (Tison & de Foucault 2014). According to Mabberley (2008) this is a mere cultivar (‘Macrophylla’) of M. alba. Similarly, claims of the Asian M. nigra may hide records of related species. The North American M. rubra L. looks very similar and is cultivated as an ornamental.
1 Upper leaf surface smooth, lower leaf surface glabrescent. Fruit white or pink === Morus alba
Upper leaf surface rough and hairy, lower leaf surface persistently downy. Fruit black or dark red === M. nigra
Literature
Lambinon J. & Verloove F. (avec coll. Delvosalle L., Toussaint B., Geerinck D., Hoste I., Van Rossum F., Cornier B., Schumacker R., Vanderpoorten A. & Vannerom H.) (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Nepal M.P. & Ferguson C.J. (2012) Phylogenetics of Morus (Moraceae) Inferred from ITS and trnL-trnF Sequence Data. Syst. Bot. 37(2): 442-450. [available online at: http://www.bioone.org/doi/pdf/10.1600/036364412X635485]
Tison J.-M. & de Foucault B. (coord.) (2014) Flora Gallica. Flore de France. Editions Biotope, Mèze : xx + 1196 p.
Wunderlin R.P. (1997) Moraceae. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 3. Oxford University Press, New York-Oxford: 388-399. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=10583]
Zhekun Z. & Gilbert M.G. (2003) Moraceae. In: Flora of China Editorial Committee (eds.), Flora of China, vol. 5. St. Louis: Missouri Botanical Garden Press: 21-73. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=10583]