Most of the fern species native to western Washington have fertile and sterile fronds that look pretty much the same. Blechnum spicant (deer fern) and two others are the exception, exhibiting dimorphic fronds. In the photograph above notice the two fronds in the foreground with narrow pinnae versus the sterile fronds in the background with pinnae that meet at the base ("broadly sessile" in Hitchcock). Other species that exhibit dimorphic fronds include Crypotgramma acrostichoides (American rockbrake) and Cryptogramma cascadensis (Cascade rockbrake).
Pinna = (pl. pinnae) one of the primary divisions or leaflets of a pinnate leaf (Harris).
Click on the photo below to enlarge and see the sori in a continuous line following a pair of veins that run parallel to the costa. The sori are described as "coenosorus" and the indusium is described as "appearing almost like an inrolled pinna-margin" in Hitchcock. Coenosorus = condition where sporangia are in a continuous line and discrete sori are not formed (Advanced Learner's Dictionary of Botany). Sorus = (pl. sori) a cluster of sporangia (spore-bearing case or sac) on the surface of a fern leaf. Costa = a rib or prominent mid-vein. Indusium = a thin epidermal outgrowth from a fern leaf that covers the sorus.
