Boke N. H., Anderson E. F., Structure, Development, and Taxonomy in the Genus Lophophora, American Journal of Botany, 1970, 57(5):569-578, doi: 10.1002/j.1537-2197.1970.tb09850.x
Studies of tubercle, areole and flower development and the anatomy of the stem surface, pollen, fruits and seeds. Anatomical material comes mainly from Lophophora williamsii from Texas. The comparison of northern and southern populations supports recognition of L. williamsii and L. diffusa in the authors' historical treatment. The relationships and hypotheses of species origins presented there are morphological interpretations from 1970.