Montane. Rocks. Summer.
Near Durango, September, 2010.

Polypodium hesperium is characterized by light green, oblong to long-triangular fronds that are up to 35 centimeters long and 7 wide.  Leaflets are less than 12 millimeters wide and are opposite or somewhat offset from each other.  The bumpy appearance of the leaflet is due to the presence of the spore cases ("sporangia" -- which are grouped into masses called "sori") on the underside of the leaflets (see below).  Fronds are in thick clusters in rock crevices.

As the range map below indicates, this is a rare plant in Colorado; it is a bit more common in New Mexico and Arizona; and it is relatively common but found in only three counties of Utah. 

The Polypodium genus was named by Linnaeus in 1753 and this species was named and described by William Maxon in 1900. Greek gives us "poly" for "many" and "pous" for "foot", referring to the branching rhizomes.

Polypodium hesperium (Western Polypody)
Polypodiaceae.  (Polypody Family)

Montane. Rocks. Summer.
Near Durango, September, 2010.

The lumpy raisin-looking clusters are the sori, less than 3 millimeters in diameter.  The sori are made up of dozens of sporangia, each of which encloses a myriad of spores.  Sori can be located in various places on the underside of Fern leaflets; the sori of Polypodium hesperium are located about mid-way between the center vein and the outer leaflet margin.

It is very difficult to distinguish P. hesperium from the almost identical P. saximontanum.  In the Botrychium key of the Flora of North America, the central point that separates the two is quite minute: P. hesperium lacks a casing (a modified sporangium called a "sporangiaster") that covers the sporangium; P. saximontanum has this structure.  A microscope is needed to observe the sporangiaster! 

The two species are further distinguished from each other by several more variable and, therefore, less reliable characteristics: P. hesperium leaf blades are somewhat triangular and up to 7 centimeters wide; P. saximontanum has oblong fronds to 4 centimeters wide.  P. hesperium is found on a variety of substrates; P. saximontanum is found only on granite.

Range map © John Kartesz,
Floristic Synthesis of North America

State Color Key

Species present in state and native
Species present in state and exotic
Species not present in state

County Color Key

Species present and not rare
Species present and rare
Species extirpated (historic)
Species extinct
Species noxious
Species exotic and present
Native species, but adventive in state
Eradicated
Questionable presence

Range map for Polypodium hesperium