Distinguishing Cryptogramma Ferns

There are only two common Cryptogramma spp. in our area (PNW). With minimal experience you can recognize the genus but the two species are not easy to distinguish. A casual photo will usually not be sufficient to be sure.

After much experimenting, I have nailed a simple method for distinguishing them based on their hydathodes (specialized water exuding tissue patches on leaves), though it does require a good closeup photo. C. cascadensis - backlight of leaves (either surface), looking for a narrow pale stripe leading to each leaflet tooth (https://www.inaturalist.org/observations/176449907). C. acrostichoides - glancing light on front of leaves (adaxial surface), looking for a pit that is shorter, wider, and deeper than C. cascadensis (https://www.inaturalist.org/observations/175736494). There are several other character differences (see below) but this is the one that I find most reliable and easiest to see in photos, though you do have to get the lighting just right.

Here is a link with more details for distinguishing these two. https://westerncascades.com/2015/08/23/distinguishing-parsley-ferns-cryptogramma-spp/

C. acrostichoides is more common at lower elevations and has more leathery (though still thin) opaque sterile fronds usually with a more waxy lustrous surface (C. cascadensis is found only at higher elevations and leaves are more herbaceous, dull, and translucent when dried).

C. acrostichoides fronds tend to persist over winter and cling to the base of the plant as dried up leaves in the spring (C. cascadensis fronds are deciduous and do not persist over winter, although they may form a mulch-like mat under the plant where they fell). When C. acrostichoides is just starting to leaf out in the spring, last year's fronds will sometimes be partially green and later they will be dried up but still adherent at the base.

Habitat - both on talus and rock cracks in near full sun, but acrostichoides usually with moss or lichens (longer stability rock) and cascadensis often on bare talus. At lower elevations, acrostichoides is much more common (https://docs.google.com/presentation/d/141MdDB6BOWgQVMU0mTxu4KIRg74QjBtJpNJm_yqH2U8/edit?usp=sharing). C. acrostichoides is more common in dry areas than C. cascadensis, though both are often found on rocky slopes with a lot of sun and they overlap in habitat. Consistent with this, C. acrostichoides ranges to east of the Cascade crest (C. cascadensis does not or is rare) and also ranges further south.

C. acrostichoides may have paler color on leaf bottom than top (C. cascadensis does not). C. cascadensis leaves are thinner and more delicate looking.

C. acrostichoides: the groove on top of the sterile frond petiole and rachis has sparse pale tan hairs (C. cascadensis does not) but these are hard to see - you need a very close picture or a hand lens and honestly I don't usually see the difference, or perhaps the hairs are shed later in the season. See https://westerncascades.com/2015/08/23/distinguishing-parsley-ferns-cryptogramma-spp/ for excellent photo comparison.

Both have lower flat sterile fronds and upper erect pinched-looking fertile fronds, though the ratio of the two vary from plant to plant according to how well it is flourishing. The fertile fronds look that way because the leaf (pinnule) is rolled back at the margins to protect the spores (a false indusium).

Posted on July 31, 2022 12:39 AM by jhorthos