lowland brittle fern

Unlike most species of the genus Cystopteris, this fern is exclusively terrestrial, often forming large, dense colonies. It is also largely a spring ephemeral. Some fronds may remain by late summer, but most have disappeared. The specific name, protrusa, refers to the fact that the rhizome protrudes a short distance beyond the current year's fronds to form the following year's leaf buds. This protrusion is a positive diagnostic in the field. Sori are round and covered in a bladder-like indusium.

Taxonomy. Cystopteris protrusa was originally considered only a variety of Cystopteris fragilis. However, it is now known that this is a diploid species, while C. fragilis (ss) is a polyploid species of hybrid origin. C. protrusa is a parent of the hybrid species Cystopteris tennesseensis and Cystopteris tenuis.