The difficulty, too, of separating the membrane entire from the frond, to which, by the pressure of the capsules, it is closely applied, is so considerable, that, since the publication of my remark already quoted, its existence has been doubted by a botanist, whose opinion, especially in whatever regards this order of plants, is of peculiar weight, and in opposition to which I should not retain full confidence in my own observations, though frequently repeated, were they not so distinctly confirmed by Mr. Bauer's excellent drawing.
I first observed the involucrum six years ago in living plants of Woodsia hyperborea, and have since repeatedly ascertained its existence in dried specimens of the same species, and of Woodsia ilvensis. These two plants are indeed so nearly related, that I find myself unable to construct for them clear specific characters; and therefore, in proposing them here as distinct species, I am, from want of sufficient materials to determine the question, rather following the prevailing opinion than my own.
To the characters and synonyms which follow, I have not thought it necessary to add descriptions of the two supposed species, these having been given by several of the authors referred to, and in every respect correctly, except what regards the involucrum.
Sori dorsales, subrotundi.
Involucrum calyciforme apertum margine crinitum: includens Capsulas pedicellatas: receptaculo communi elevato nullo.
Filiculæ, frondibus cæspitosis, pinnatim divisis; pilis simplicibus squamulisque angustis instructæ.
1. ilvensis. W. frondibus bipinnatifidis, pinnis oblongis, pinnulis confluentibus multifloris: inferioribus subrepandis: infimis subæqualibus.
Polypodium ilvense, Swartz, Synop. Fil. 39. Willden. Sp. Pl. 5, p. 198. Schkuhr, Crypt. 16, t. 19.
