The diverse New World shrub genus Ceanothus (Rhamnaceae) is made up of two deeply divergent, reciprocally monophyletic groups that are treated as subgenera in most recent systematic and flora treatments, Ceanothus subgenus Cerastes, and Ceanothus subgenus Ceanothus. The two groups are phylogenetically divergent and appear to have simultaneously diversified in western North America, particularly the California Floristic Province. Ceanothus subgenus Cerastes is easily distinguished from Ceanothus subgenus Ceanothus by its sclerophyllous (versus thin) leaves, stomata sequestered in pits (versus on the leaf surface), and swollen (versus papery) stipules. Low levels of genetic divergence among species, combined with highly variable, intergrading morphology makes taxonomic work in Ceanothus subgenus Cerastes difficult. To aid research in this diverse and ecologically important group of plants, we present a taxonomic conspectus of Ceanothus subgenus Cerastes, including formal recognition of Cerastes at the rank of subgenus, and recognition of all infraspecific taxa at the rank of variety. We discuss problems with defining taxa in Ceanothus subgenus Cerastes, and call attention to the need to distinguish between the subgenera in Ceanothus research. Twenty-five species and 10 varieties are recognized in Ceanothus subgenus Cerastes.
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