BioStor
 | Botanical Observations on the Mexican Flora, Especially on the Flora of the Valley of Mexico John W Harshberger (1898) Proceedings of The Academy of Natural Sciences of Philadelphia 50: 372-413 |
 | A list of the ferns and fern allies of North America north of Mexico, with principal synonyms and distribution William R Maxon (1901) Proceedings of The United States National Museum 23: 619-651 |
 | Observations on the Soil Acidity of Ericaceae and Associated Plants in the Middle Atlantic States Edgar T Wherry (1920) Proceedings of The Academy of Natural Sciences of Philadelphia 72: 84-111 |