Asperula

Tutin, T. G., Heywood, V. H., Burges, N. A., Moore, D. M., Valentine, D. H., Walters, S. M. & Webb, D. A., 1976, Flora Europaea. Volume 4. Plantaginaceae to Compositae (and Rubiaceae), Cambridge University Press : 6

Sect. Cynanchicae includes many very polymorphic diploid and tetraploid races based on x = 10, which are connected with each other by transitional (perhaps hybridogenous) populations. The taxa are frequently very closely related to one another and the present treatment is provisional. A comprehensive study of the section is urgently needed.