Kelloff, Carol Lynn & Judith Skog, Laura Adamkewicz and Charles R. Werth (2002) Differentiation of Eastern North American Athyrium filix-femina Taxa: Evidence From Allozymes and Spores. Amer. Fern J. 92(3): 185-213.
Notes by John Scott:
This paper explores the differences between A. angustum and A. asplenioides, our eastern northern and southern Lady Ferns respectively. Their evidence suggests that these taxa deserve species or subspecies rank. “The two eastern North American taxa,……, have long been perceived as close relatives separable by distinctive characters that area consistent with the vast northern and southern areas they respectively occupy, but that intergrade and recombine to form a hybrid in their relatively narrow region of overlap.” Of particular interest to us in Pennsylvania is that we are in the center of the overlap of the two taxa. All the taxa are diploids; the hybrid is fertile. In my collecting of lady ferns in southeastern Pennsylvania, I have generally be unable to assign plants to either taxon except for the red stemmed A. angustum rubellum that has all the proper characteristics of A. angustum. Perhaps the hybrids in our area are more common than the authors stated: “In the present study, the degree of differentiation between A. angustum and A. asplenioides may appear exaggerate by the omission of localities where the two taxa co-occur. This omission was neither intentional nor an oversight, rather it resulted from a failure to discover such localities despite a substantial effort to do so. A search for co-occurring populations in southern Pennsylvania resulted in finding only a few isolated individuals of A. asplenioides in this highly agriculturalized and urbanized region…….”