1_Sargent, C. S. (1905) Manual of the Trees of North America Boston: Houghton, Mifflin and Co.
2_Little, E. L., R. O. Woodbury, and F. H. Wadsworth (1964, 1974) Trees of Puerto Rico and the Virgin Islands (2 vols.) USDA Agriculture Handbooks No. 249 and 449. (note that 2.1 and 2.2 in the reference list refer to the two volumes)
3_Flora of North America Editorial Committee, eds. (1993--) Flora of North America North of Mexico, 7+ vols., New York. Online at <http://www.efloras.org/flora_page.aspx?flora_id=1>
4_Atlas of Florida Vascular Plants. <http://www.plantatlas.usf.edu>
5_USDA, NRCS (2006) The PLANTS Database, 6 March 2006, National Plant Data Center, Baton Rouge, LA 70874-4490 USA. <http://plants.usda.gov>
6_Virginia Tech dendrology fact sheets <http://www.cnr.vt.edu/dendro/dendrology/main.htm>
7_Britton, N. and A. Brown (1913) An Illustrated Flora of the Northern United States and Canada, 2nd Ed. New York: Scribner's.
8_Hickman, J. C., ed. (1993) The Jepson Manual: Higher Plants of California  Berkeley: University of California Press. Online at <http://ucjeps.berkeley.edu/interchange/I_treat_indexes.html>
9_Texas A&M Horticulture Network <http://aggie-horticulture.tamu.edu>
10_UNC Herbarium: Ilex collina Alexander =Nemopenthus collinus (Alex.) Clarke (NCU 00005854)
11_UNC Herbarium: Robinia hispida var. kelseyi (NCU 00007726)