This species occurs in from the Transvolcanic Belt of Mexico south to the Sierre de Oaxaca. It is apparently quite common at higher elevations. The practically unlobed leaves are distinctive. This species is also notable for its confused taxonomic history—for the past 80 years it has passed under the name H. hemsleyana due to the mistaken belief that the original H. longipetala was a synonym for H. micrantha.
Y. Okuyama kindly sent me this living material (H. longipetala var. longipetala, the typical phase of the species), originally collected in Morelos.