For Apocynum
Endress, M.E., U. Meve, D.J. Middleton, and S. Liede-Schumann. 2018. Apocynaceae. In Kadereit, J., & V. Bittrich. 2018. The families and genera of flowering plants. XV. Flowering plants. Eudicots. Apiales, Gentianales (except Rubiaceae). Springer, Cham.
Woodson, R.E., Jr. 1930. Studies in the Apocynaceae. I. A critical study of the Apocynoideae (with special reference to the genus Apocynum). Ann. Missouri Bot. Garden 17: 1-212.