Cystopteris

Glands in Cystopteris may be shed during the growing season, and after the end of June, glands are usually absent, on C. laurentiana especially, except on the indusia (be careful not to mistake indusial fragments for glands).

1. Blades elongate-triangular or ovate, distinctly widest near or at the base of the blade; minute stalked glands (use 30–40×) abundant to scarce on indusia, and usually on rachis, midveins of pinnae and pinnules; intact or vestigial bulblets on costae and rachis.

2. Blades, at least after mid-June, of two types: sterile short-triangular blades and much longer fertile blades, in both cases widest at the base; minute stalked glands (use 30–40×) abundant; fully developed (but readily deciduous) bulblets frequent on larger leaves.

C. bulbifera

2. Blades monomorphic, ovate to lanceolate, widest a little above the base; minute stalked glands occasional, mostly on indusia or in axils of costae; vestigial bulblets (small, hairy bodies at the junction of the rachis and midrib of pinnae) occasional.

C. laurentiana

1. Leaf blades narrowly elliptic, mostly widest at or just below the middle of the blade; minute stalked glands absent; bulblets always absent.

3. Leaves clustered 1-4 cm behind rhizome apex; pinnules nearest the rachis on the lowest pinnae cuneate to a short petiolule ca.0.5–1 mm long.

C. protrusa

3. Leaves clustered at rhizome apex (rhizome essentially not protruding); pinnules nearest the rachis on the lowest pinnae essentially sessile (petiolule less than 0.5 mm long).

4. Dark pigment of petiole base usually extending up to base of blade and often into the rachis; pinnules nearest the rachis on the lowest pinnae cuneate or rounded at base; veins of the pinnules extending to the margins both into the teeth and into some of the sinuses.

C. tenuis

4. Dark pigment confined to petiole base; pinnules nearest the rachis on the lowest pinnae obtuse or truncate; veins extending to the margins at the teeth or tiny notches along the margins, not at sinuses.

C. fragilis

All species found in Cystopteris

Cystopteris bulbiferaBULBLET FERN 
Cystopteris fragilisFRAGILE FERN 
Cystopteris laurentianaLAURENTIAN FRAGILE FERN 
Cystopteris protrusaFRAGILE FERN 
Cystopteris tenuisFRAGILE FERN 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 12, 2017. http://www.michiganflora.net/genus.aspx?id=Cystopteris.