Flora of North America species comparison

Polystichum acrostichoides is a common species most closely related to P. munitum (G. Yatskievych et al. 1988), which also occurs extensively on forest floors.

The dimorphic pinnae of Polystichum acrostichoides are not unique to the genus; they are found also in some Asian species. Numerous variants have been named, mostly as forms, but none are of taxonomic consequence. Hybrids are known with P. braunii (P. × potteri Barrington) and P. lonchitis (P. × hagenahii Cody). The latter hybrid is rare, known only from its type locality in Ontario, where it grows with both parents. It is recognized by its intermediate morphology (leaves wider than P. lonchitis, narrower than P. acrostichoides, with slightly contracted sorus-bearing pinnae) and malformed sporangia and spores. Polystichum × potteri is much more widespread, from Nova Scotia, New Brunswick, and Quebec through New England to Pennsylvania. It resembles P. braunii but has narrower leaves bearing malformed sporangia.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

The hybrid between Polystichum lonchitis and P. acrostichoides (= P. × hagenahii Cody) is discussed under P. acrostichoides. The hybrid with P. braunii (= P. × meyeri Sleep & Reichstein) is discussed under P. braunii. In the Georgian Bay area of Ontario, P. lonchitis hybridizes with Dryopteris goldieana to produce the peculiar × Dryostichum singulare W. H. Wagner (W. H. Wagner Jr., F. S. Wagner et al. 1992).

The spiny spores of P. lonchitis are distinctive and distinguish this from dwarfed forms of other 1-pinnate species.

(Discussion copyrighted by Flora of North America; reprinted with permission.)