Osmunda japonica Thunb.
Etymology Japonica refers to Japan.
Description Rhizome: erect, massive, forming a trunk, occasionally branching, hairs and old stipe bases woven together with black, fibrous roots.
Frond: 50 cm high by 40 cm wide, deciduous, dimorphic, fertile fronds earlier, erect, sterile later, arching, blade/stipe ratio: 5:4 for sterile fronds, 1:1 for fertile fronds.
Stipe: stipules (flared leaf base), unique to the family/genus, pale reddish brown, long lax hairs, but soon glabrous, vascular bundles: 1 in a U-shape where the top of the arms continue to curl.
Blade: 2-pinnate, sterile oblong, widest at the bottom, fertile consisting entirely of sporagia, papery, reddish to light brown hairs, soon falling.
Pinnae: 4 to 6 pair, catadromous; pinnules oblong, to 10 cm; margins minutely dentate; veins free, forked.
Sori: none, indusium: absent, sporangia: large, globose, tan or black when mature, spores green.
Culture Distribution: Sakhalin, Japan, Korea, Himalaya, China, Taiwan Thailand and Indochina. Hardy to -20C, USDA Zone 6.
Distinctive Characteristics This brings to mind a cross between O. regalis and O. cinnamomea, the 2-pinnatedness of the former, the dimorphism of the latter. Late season fronds are said to mix sterile and fertile portions. If you know of a photo, please point it out.
Osmunda nipponica Makino

Osmunda japonica, habit  
Taxonomy Ohwi's Flora of Japan seems to describe a different, larger fern.
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