Cystopteris tenuis is common in eastern North America and less frequent at the northern and western perimeter of its range. In the center of its distribution (Minnesota, Iowa, Illinois, Wisconsin, Indiana, Ohio, Pennsylvania), the narrow, elliptic pinnae angled toward the blade apex and the rounded teeth make C . tenuis relatively distinct from C . fragilis and C . protrusa (although the early season, sterile leaves of C . protrusa often resemble those of C . tenuis ). In the west and especially in the northeast, C . tenuis and C . fragilis are difficult to distinguish. For the most part, C . fragilis is confined to higher latitudes and elevations than C . tenuis , but the two species can be sympatric and occasionally form sterile tetraploid hybrids. Cystopteris protrusa and C . tenuis are infrequently sympatric, but where they are, sterile triploid hybrids can occur. Hybrids between C . tenuis and C . tennesseensis are recognized as C . × wagneri (R. C. Moran 1983). Hybridization between C . tenuis and C . bulbifera has also been reported (R. C. Moran 1982b). This hybrid, C . × illinoensis R. C. Moran, is known only from the type and needs to be studied further.
Cystopteris tenuis is sometimes known as Mackay's bladder fern or Mackay's fragile fern. It was long considered to be a part of the superspecies for fragile ferns, as Cystopteris fragilis (L.) Bernh. var. mackayi Lawson.
This species is an allotetraploid of hybrid origin (see Cystopteris hybrid complex). The parent diploid species are Cystopteris protrusa and the hypothesized Cystopteris hemifragilis, believed to be an extinct species.
C. tenuis is known to hybridize with C. bulbifera to produce the hybrid C. Xillinoensis, with C. tennesseensis top produce the hybrid C. Xwagneri, and with C. fragilis and C. protrusa to produce unnamed hybrids (as per Flora of North America).
Mackay's fragile fern grows on rock or in scree, generally in sheltered spots, in the northeastern United States. It may be distinguished from the somewhat similar Cystopteris tennesseensis by the fact that it grows on acid substrate, while the tennesseensis grows on calcareous substrate. The fronds of tenuis are broader, with generally alternate pinnae, while tennesseensis is narrower, with generally opposite pinnae.
Cystopteris tenuis is sometimes known as Mackay's bladder fern or Mackay's fragile fern. It was long considered to be a part of the superspecies for fragile ferns, as Cystopteris fragilis (L.) Bernh. var. mackayi Lawson.
C. tenuis in native habitat This species is an allotetraploid of hybrid origin (see Cystopteris hybrid complex). The parent diploid species are Cystopteris protrusa and the hypothesized Cystopteris hemifragilis, believed to be an extinct species.
C. tenuis is known to hybridize with C. bulbifera to produce the hybrid C. Xillinoensis, with C. tennesseensis top produce the hybrid C. Xwagneri, and with C. fragilis and C. protrusa to produce unnamed hybrids (as per Flora of North America).
Mackay's fragile fern grows on rock or in scree, generally in sheltered spots, in the northeastern United States. It may be distinguished from the somewhat similar Cystopteris tennesseensis by the fact that it grows on acid substrate, while the tennesseensis grows on calcareous substrate. The fronds of tenuis are broader, with generally alternate pinnae, while tennesseensis is narrower, with generally opposite pinnae.
