Calystegia collina occurs in the Coast Ranges of California from Lake County to Santa Barbara County and is considered closely related to, but specifically separable from, C. malacophylla. The northernmost taxon in the C. collina complex, subsp. tridactylosa, differs from other taxa in significant morphological characters and is geographically disjunct. Further investigation may suggest that subspecies tridactylosa should be elevated to the rank of species. The contiguous distributions of subspecies collina and oxyphylla are detailed, and subspecific rank is justified based on morphological considerations despite range overlap and the existence of some intermediates. Subspecies apicum, formerly included in subspecies venusta based on sepal similarity, is segregated based on differences in leaf margin morphology, leaf size, and overall differences in pubescence. The revised concept of subspecies venusta only includes plants with strongly sinuate leaf margins.
Brummitt, Richard K. and Namoff, Sandra M.
"A New Subspecies of Calystegia collina (Greene) Brummitt (Convolvulaceae) in the Coast Ranges of California and Notes on the Distribution of the Species,"
Aliso: A Journal of Systematic and Evolutionary Botany:
2, Article 4.
Available at: https://scholarship.claremont.edu/aliso/vol31/iss2/4