About Cystopteris tenuis (Michx.) Desv.
Cystopteris tenuis (Michx.) Desv. is sometimes called Mackay's bladder fern or Mackay's fragile fern. For a long time, it was treated as part of the fragile fern superspecies, classified as Cystopteris fragilis (L.) Bernh. var. mackayi Lawson. This species is an allotetraploid that originated via hybridization, and is part of the Cystopteris hybrid complex. Its diploid parent species are Cystopteris protrusa and the hypothesized, believed-to-be-extinct Cystopteris hemifragilis. According to the Flora of North America, C. tenuis is known to hybridize with C. bulbifera to produce the hybrid C. ×illinoensis, with C. tennesseensis to produce the hybrid C. ×wagneri, and with both C. fragilis and C. protrusa to produce unnamed hybrids. Mackay's fragile fern grows on rock or in scree, usually in sheltered locations, in the northeastern United States. It can be told apart from the similar species Cystopteris tennesseensis by its substrate preference: C. tenuis grows on acid substrate, while C. tennesseensis grows on calcareous substrate. C. tenuis also has broader fronds that generally bear alternate pinnae, whereas C. tennesseensis has narrower fronds with generally opposite pinnae.
