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158 FERNS OF NORTH AMERICA.
Polypodium calcarcum, P u r sh , FI. Am. Sept., ii. p. 6 5 9 (not of Smith and
Willdenow).
H ad. — Open, rocky woods, not rare in Canada and the Northern
United States, and extending to the mountains of Colorado, to Oregon,
Unalaska, Labrador, and perhaps Greenland. It is found also throughout
Northern Europe and Asia, from the British Isles to Kamtschatka, the
southern limits being the Pyrenees and Northern Italy in Europe, and
Tibet and Cashmere in Asia.
D e s c r i p t i o n . — The oak-fern has a cord-like creeping root-
stock, scarcely a line in diameter, but often a foot or more in
length. It creeps several inches in advance of the growing
fronds, the newer portion bearing a few thin ovate chaffy scales,
and producing rudimentary stalks which grow up and bear fronds
the coming year. The stalks are continuous with the root-stock,
as in the Aspidia, and not articulated with it as they are in
Polypodium. This is the best technical distinction between Phe-
gopleris and Polypodium : the former being, as Mr. John Smith
has termed it, desmobryoid; and the latter, eremobryoid.
The stalk is erect, very slender, greenish in the living plant,
but stramineous in the dried specimen. The lowest portion is
commonly somewhat flexuous, dark brown, and clothed with a
few thin ovate scales like those of the root-stock.
The frond is thin-membranaceous, perfectly smooth, and of
a clear leaf-green. It is broadly triangular in shape, and measures
from four to eight or ten inches in breadth, and nearly as
much in length. It is divided into three spreading parts, which
' See the explanation on p. 116 , aute.
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