Polystichum acrostichoides (Michaux) Schott

Christmas fern

Etymology Acrostichoides means resembling the Acrostichum fern, a tropical genus, where the sori completely cover the undersurface.
Description Rhizome: erect, stout, pale brown, subterranean, scaly.
Frond: 80 cm high by 12 cm wide, evergreen, somewhat dimorphic, the fertile pinnae (in upper part of frond) narrowed, blade/stipe ratio: 2:1 to 3:1.
Stipe: grooved, green, densely scaly; scales light brown, diminishing in size upwards, vascular bundles: 5 in an arc.
Blade: 1-pinnate, linear-lanceolate, base narrowed, glossy, microscales filiform, dense, on lower surface only.
Pinnae: 20 to 30 pair, oblong to falcate, upper auricles well developed; margins entire to serrulate-spiny with teeth ascending; large natural variation; veins free, forked, anadromic.
Sori: distinct, green at first, confluent later, completely covering lower surface of fertile pinnae (upper, reduced pinnae only), indusium: peltate, shriveling, centrally, sporangia: golden brown, maturity: early summer.
Dimensionality: lower pinnae bend forward and down, out of the plane of the blade.
Phenology: main flush late April-early May, occasional fronds later; spore matures June.
Culture Habitat: forest floor and shady, rocky slopes . Distribution: eastern North America, Mexico. Hardy to -35C, USDA Zone 3.
Distinctive Characteristics The sharp reduction in fertile pinna size (masked in the illustration due to the bent frond at the same point) is unique in the genus, as reflected here.
Synonyms
Nephrodium acrostichoides Michaux
Polystichum acrostichoides
Polystichum acrostichoides. (a) sterile and fertile fronds, (b) portion of immature fertile pinna, (c) mature pinna.  Illustration by V. Fulford from Ferns and Fern Allies of Canada, William J. Cody and Donald M. Britton, 1989, Agriculture Canada, used with permission
Polystichum acrostichoides
Polystichum acrostichoides. stipe base: five vascular bundles, 2 primary, three secondary, in a c-shape opening at the frond or stipe front.  Photo: Tom.
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