For Crassula
Bywater, M. and G.E. Wickens. 1984. New World species of the genus Crassula. Kew Bull. 39: 699-728.
Moran, R.V. 2009b. Lenophyllum. In Flora of North America Editorial Committee. 2009. Flora of North America north of Mexico. Volume 8, Magnoliophyta: Paeoniaceae to Ericaceae. Oxford Univ. Press, New York, NY. 585 pp.
Mort, M.E., C.P. Randle, P. Burgoyne, G. Smith, E. Jaarsveld, and S.D. Hopper. 2009. Analyses of cpDNA matK sequence data place Tillaea (Crassulaceae) within Crassula. Plant Syst. Evol. 283: 211-217.
Thiede, J., and U. Eggli. 2007. Crassulaceae. In Kubitzki, K., C. Bayer, and P.F. Stevens, eds. 2007. The families and genera of vascular plants. IX. Flowering plants – Eudicots – Berberidopsidales, Buxales, Crossosomatales, Fabales p.p., Geraniales, Gunnerales, Myrtales p.p., Proteales, Saxifragales, Vitales, Zygophyllales, Clusiaceae alliance, Passifloraceae alliance, Dilleniaceae, Huaceae, Picramniaceae, Sabiaceae. Springer, Berlin. 509 pp.