Background:

Mentzelia (Loasaceae) is primarily distributed in western North America. The genus is classified into six monophyletic sections, among which species of section Bartonia are particularly poorly understood.

Questions:

What species of Mentzelia section Bartonia occur in Mexico? What are the species distribution ranges and what are their defining attributes?

Study species:

Mentzelia section Bartonia.

Methods:

Field, herbarium, distribution, and scanning electron microscopy studies were conducted to assess species diversity. Seed coat microsculpture patterns were evaluated to confirm species identities and understand their distributions throughout Mexico.

Results:

Six species from section Bartonia were recognized, with taxa distributed into either the Chihuahuan or Sonoran deserts. Mentzelia longiloba var. pinacatensis is the only Mexican endemic.

Conclusions:

Although this work comprehensively addressed the species of section Bartonia in Mexico, two groups will continue to present identification challenges: the M. longiloba and M. mexicana-M. saxicola complexes. Overlapping and variable characters will continue to complicate species identification in the M. longiloba complex, while the late-season loss of primary leaves will obscure species identification between M. mexicana and M. saxicola.

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