As noted by Thomson (2000a), the genus Pteridium is well-demarcated morphologically and taxonomically within the family and not easily confused with other genera of Dennstaedtiaceae. Within the genus, P. aquilinum sensu stricto generally is separated from the other species by its fronds, which tend to be less than 2 m long, with the lower pinnae no more than 3-pinnate (the whole lamina usually 2-pinnate-pinnatifid to 3-pinnate-pinnatifid), and with the ultimate segments more or less symmetrical at the base (not decurrent along one side). Within P. aquilinum, infraspecific taxa tend to differ in pubescence patterns, segment shape, segment width, pseudoindusial width, and subtle differences in laminal shape and division pattern. As there is no modern monograph of the species, circumscription and recognition of subspecies and varieties remain equivocal.
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