A Darkling Path through the Ferny Ferns

Be there dragons here?

It's November and The Monthly Fern series is winding down. Looking back, I realized that most of the ferns I chose are distinctive—they're aquatic or have dimorphic leaves or are primitive lycophytes or grow large enough to inveil a romantic tryst. So this month's post will feature the ferny ferns (my term)—the ones we immediately recognize as ferns. However, figuring out which specific kind isn't guaranteed. If only ferns had flowers—so showy and diverse! Instead we must rely on leaves (1).

As the days shorten it would seem that writing descriptions for our Guide to South Dakota Plants would be appealing, especially given my current subjects—ferns and their relatives. But they can be difficult, sometimes inscrutable even. Of course they aren't the ones to blame. We are—specifically we botanists who seek order in their labyrinthine world.

I try to make my plant descriptions user-friendly, as our intended audience is broad—professionals, academics, students, enthusiasts, and eager novices (2). This is much easier online. There will be many photos so I can shorten the text and minimize technical terms. Even so, there remain features that must be explained, for example the lovely lacy leaves of the ferny ferns.

The much-divided leaves of ferny ferns are the basis for "fernlike"—for example, "Western Yarrow leaves are fernlike (SAplants).

Fern descriptions typically start with the plant—height, form (erect, spreading, sprawling), behavior (colony-forming, clumped), and other fairly straight-forward things. Leaves are next. Position (basal, on the stem), color, dimensions, and overall shape are easy to describe. But then ... we're faced with the dreaded degree of dissection. How many times is the leaf divided? Are there true segments? Are the segments themselves divided and are these divided as well? Here the guides I've been using as examples diverge, perhaps out of confusion. Suddenly the way forward becomes unclear; the path darkens considerably.

Entering the darkling world of leaf division.

"Leaf Division" from Fern Structure (USDA Forest Service).

In my web wanderings, I found a figure showing degrees of leaf division (above). It seems clear, though one needs to know that "pinnate" means divided and "-fid" means "nearly". For example, "pinnatifid" means nearly once-divided—division doesn't quite reach the midrib of the leaf as it does in "pinnate".

I intend to use this figure, perhaps as a pop-up, but will replace "pinnate" with "division" thereby eliminating the need for translation. "fid" situations will be accommodated with "nearly", for example "nearly twice-divided".

My version—actually a common approach, not my invention. 

Declaration of degree of division is followed by description of the ultimate segments—their shape, size, hairiness, margins, and such. This can provide much-welcomed help with identification. 

Ready for a test? Using the photos below, describe leaf division in Male Fern, Dryopteris filix-mas, and characterize the ultimate segments.

Male Fern's clumped ascending leaves can be more than a meter long. Аимаина хикари

Once-, nearly twice-, or twice-divided? Note the toothed (but not spiny) margins of the ultimate segments (click on image to view). Nick Turland

The sources I use all say that leaves of Male Fern are nearly twice-divided (pinnate-pinnatifid). But you needn't feel bad if you chose a different answer—you are correct. Male Fern leaves are usually once-divided at the tip, often twice-divided near the base, and nearly twice-divided in between. But adjacent segments can differ as the photo shows. Some are true segments, with division reaching all the way to the midrib. Others don't quite make it.

With no obvious path through of this shadowy world, let's change the subject.

It's not uncommon for fern identification to be difficult, as even experts acknowledge (e.g., Cobb et al. 2005):

"Many ferns are distinguished by the finer details of the blade and how it is divided, and descriptions of fern blades can seem difficult and frustrating to beginners" (italics mine; I too get frustrated, and take offense at being labeled a beginner).

This is where a truly user-friendly guide can help, with lookalikes and tips for identification.

Be discriminating in your choice of guides.

In South Dakota we have an especially fine (= difficult) example of lookalikes—Fragile Fern vs. Oregon Cliff Fern. They grow on the same types of sites and look oh-so-similar. Both have nearly twice-divided leaves (often twice-divided at the base) and their ultimate segments have rounded tips and toothed margins.

Fragile Fern, Cystopteris fragilis (MWI).

Oregon Cliff Fern, Woodsia oregana (MWI).

Several small but distinctive features can help with identification (10x magnification recommended). Ultimate segments of Fragile Fern are not glandular and usually hairless, and the margins are irregularly toothed. In contrast, Oregon Cliff Fern segments are glandular hairy (more so on the underside), and the margins are regularly toothed.

Fragile Fern, with irregularly toothed segments (MWI).

In Oregon Cliff Fern, segments are regularly toothed (MWI).

Those familiar with these ferns in the wild have another tip, and something that's easier to see. Our Cliff Ferns (Woodsia species) often have persistent dead leaf stalks. This isn't the case for Fragile Fern.

It's not unusual for a mature Cliff Fern to have more dead stalks than leaves (Andre Zharkikh).

You can relax now. No more tests. We're very close to the end with reassuring light visible ahead. And if you found leaf division tedious and difficult, think how I must feel after attempting to explain it! Sometimes I have to remind myself that I love plants.

Notes

(1) Replacing fern terminology—frond, stipe, pinnae, e.g.—with the more familiar terms used for angiosperms—leaf, leaf stalk, leaflet—has become fairly common (for example, Flora of North America). Others adhere to tradition, explaining terms in a glossary or introduction (for example, Cobb et al. 2005).

(2) I'm not enough of an expert to write descriptions of South Dakota plants myself. Instead I rely on the knowledge of others, both in printed manuals and online. The majority of photos also are by others, available online through Creative Commons licenses.

(3) Some readers may be thinking, "Just find fertile leaves with sori!" (spore clusters). After all, we've been told repeatedly that sori are distinctive. But those of Fragile Fern and Oregon Cliff Fern are hard to distinguish at maturity. Fragile Fern does have distinctive pocket-like indusia, but only when young (see photo of leaf segments in post).

Sources

All fern art created with NightCafe AI Art Generator.

Cobb, B, et al. 2005. Peterson Field Guide to Ferns, 2nd Ed. Northeastern and Central North America. Provides excellent lookalike information and tips.

Minnesota Wildflowers, a guide to the flora of Minnesota. This was the first online guide I found, and remains the most user-friendly of those I've seen (there aren't many, online guides being relatively new). Fortunately South Dakota and Minnesota share many plant species, and this will be our main source of photos.

USDA Forest Service. Ferns. Highly recommended.