For Urtica
Boufford, D.E. 1997. Urticaceae. In Flora of North America Editorial Committee. 1997. Flora of North America north of Mexico. Volume 3, Magnoliophyta: Magnoliidae and Hamamelidae. Oxford Univ. Press, New York, NY. 590 pp.
Friis In Kubitzki, K., J.G. Rohwer, and V. Bittrich, eds. 1993. The families and genera of vascular plants. II. Flowering plants – Dicotyledons – Magnoliid, Hamamelid and Caryophyllid families. Springer, Berlin. 653 pp.
Grosse-Veldmann, B., and M. Weigend. 2017. The geometry of gender: hyper-diversification of sexual systems in Urtica L. (Urticaceae). Cladistics (2017): 1-20.
Henning, T, D. Quandt, B. Grosse-Veldmann, A. Monro, and M. Weigend. 2014. Weeding the nettles II: a delimitation of “Urtica dioica L.” (Urticaceae) based on morphological and molecular data, including a rehabilitation of Urtica gracilis Ait. Phytotaxa 162: 61-83.
Woodland, D.W. 1982. Biosystematics of the perennial North American taxa of Urtica. II. Taxonomy. Syst. Bot. 7: 282-290.
Woodland, D.W., I.J. Bassett, C. Crompton, and S. Forget. 1982. Biosystematics of the perennial North American taxa of Urtica. I. Chromosome number, hybridization, and palynology. Syst. Bot. 7: 269-281.
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