Scientific Name: Polypodium virginanum
Common Name: Common Polypody Fern
(The information for this species page was gathered in part by Ms. Kristin Baer for Biology 220W Spring 2009 at Penn State New Kensington)
Appearance and Growth Habit
The common polypody fern (Polypodium virginanum) is also known as the “rock cap fern” and the “rock polypody fern.” As these common names imply, this fern is most often found growing on rock surfaces usually in moist, shady woods. It is found in rocky woods all across eastern and midwestern North America. Its fronds are evergreen, four to twelve inches in length, and have ten to twenty, alternate leaflets. The leaflets are lance-shaped with broad bases and pointed tips. This fern grows in scattered, random clumps across a suitable rocky surface. Some of these clumps may be quite small (even representing single individuals) while others may be quite extensive with large numbers of colonial plants. The overall suitability or quality of the rock microhabitat determines the extent and number of root and rhizomes that can be supported. The ability of this species to grow in sites devoid of soil gives it a substantial ecological and selective advantage over other ferns on these bare rock surfaces. Further, the ability of this species to tolerate extreme desiccation and to rapidly rehydrate when moisture becomes available adapts it well to the extreme moisture fluctuations of their rock surface habitats.
Ferns reproduce by spores rather than seeds. In many species, including the common polypody, the organs that house the dust-sized spores (the “sporangia”) are found on the undersides of the fertile frond leaflets. The sporangia form discrete clusters called “sori.” The sori of the common polypody fern are distinctively orange-brown when mature and lack the protective covering (the “indusium”) seen in some other fern species. Spores are dispersed by the wind, and, if they land in a suitable microhabitat, they develop into the small, gamete (sperm and ova) producing life stage called the “gametophyte.” Sperm produced by the gametophyte swim through water films to reach and fertilize an ovum on the same or a different gametophyte. There are a variety of mechanisms to prevent sperm from fertilizing an ovum on the same gametophyte, thus protecting genetic diversity. The fertilized ovum (the zygote) grows into the new fern individual (the “sporophyte”).
The evergreen fronds of the common polypody fern are consumed in the extreme winter by ruffed grouse, wild turkey, and white-tailed deer. The value of these fronds as a food source, though, is quite limited.
The polypody fern’s role in the ecology of our forests is as a primary colonizer of exposed rock. The accumulation of fern fronds and influence of root and rhizome exudates on the rock surface help to catalyze the weathering of the rock and accelerate the formation of soil which then opens the rock microhabitat to other, more rapidly growing plants.