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Excerpts from Jim Conrad's
Naturalist Newsletter
from the July 19, 2009 Newsletter, issued from the Siskiyou Mountains west of Grants Pass, Oregon:
NARROWLEAF SWORDFERNS
In this area the most commonly found fern is Bracken, Pteridium aquilinum, which also is the most abundant fern worldwide. I introduced you to it from Mexico at https://www.backyardnature.net/chiapas/bracken.htm.
I've also introduced you to our second-most common fern, the Western Swordfern, Polystichum munitum, at https://www.backyardnature.net/n/x/sword-fn.htm.
There I report that Western Swordfern is very closely related to, and can be hard to distinguish, from the third-most-common fern here, the Narrowleaf or Cliff Swordfern, POLYSTICHUM IMBRICANS, which now you can see above.
A close-up showing the round, spore-producing fruit-dots, or sori, and spiny-margined leaflets, or pinnae, is below:
Both of these swordferns are closely related to the East's Christmas Ferns. Around here, if you see a big swordfern in a moist valley it's surely the Western Swordfern. The Narrowleaf Swordfern is a tougher, wirier, more slim-fronded species than the Western. The pictured Narrowleaf occurs on a very dry, exposed talus slide below a roadcut. Narrowleaf Swordferns prefer drier habitats while Western Swordferns prefer moister ones. However, in in-between habitats the Narrowleaf species looks lusher while the Western species looks leaner. Then they can be hard to distinguish.
If you have sori in the stage shown in the picture, the cellophane-like "caps" atop each sorus -- the indusia -- are fringed with tiny hairs in the Western Swordfern, but are hairless or broadly toothed on the Narrowleaf. Also there are differences in size of scales on the fronds' stems, or petioles. But often there are no indusia, and the scales are of in-between sizes. Sometimes in in-between habitats with in-between ferns, I just can't distinguish them.
Narrowleaf Swordferns occur from British Columbia south through Washington and Oregon into California.