Columbia subject headings:
Ferns--Canada--Identification.
Ferns--United States--Identification.
Pteridophyta--Canada--Identification.
Pteridophyta--United States--Identification.
Harvard subject headings:
Ferns--Canada--Identification.
Ferns--United States--Identification.
Pteridophyta--Canada--Identification.
Pteridophyta--United States--Identification.
Princeton subject headings: