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|C. protrusa fronds|
Cystopteris protrusa is a common fern of eastern North America. Throughout much of its range, it is the most common Cystopteris. It is commonly known as the lowland brittle fern or lowland fragile fern. 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.
Unlike most species of this genus, 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.
C. 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. Also see Cystopteris hybrid complex.
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