252 ON WOODSIA. 



sede all changes in nomenclature, may be inferred from 

 the genus Poly podium alone, though reduced nearly one 

 half by its present character, still including 157 species, 

 or upwards of a seventh part of the whole order. 



The expediency of subdividing Polypodium, as well as 

 some of the other genera mentioned, especially Acrostichum, 

 is indeed obvious, not merely on account of their great 

 extent, but also from the striking differences in habit ex- 

 isting among the species referred to each. 



I have, some time ago, 1 had an opportunity of remarking, 

 that the plants referred to Polypodium, P. ilvense and 

 iu/perboreum, form a distinct genus, from the peculiar 

 structure of their involucrum, even the existence of which 

 had escaped preceding observers. 



This genus I have named in honour of my friend Mr. 

 Joseph Woods, whose merits as an accurate and skilful 

 English botanist are well known to many of the members 

 of this Society : and the object of the present communica- 

 tion is to illustrate it by some additional observations on 

 its structure, and by a very perfect drawing, for which I 

 am indebted to the friendship of Mr. Francis Bauer. 



The character distinguishing Woodsia from all other 

 genera of Ferns hitherto established, consists in its invo- 

 lucrum being inserted under the group of capsules, or, as 

 it is technically called, the sorus, which it completely sur- 

 rounds at the base ; while it is in every stage open at the 

 172] top, having its margin divided into a number of capillary 

 segments, which from their length and incurvation en- 

 tirely conceal the young capsules, and in a great measure 

 the full-grown. 



That so singular a structure should have been hitherto 

 unnoticed, even though both species of the genus have 

 been described and figured since the publication of Dr. 

 Smith's memoir, is not perhaps to be wondered at : for 

 the membranaceous base of the involucrum is completely 

 concealed by the capsules, and the marginal hairs, which 

 alone are visible, exactly resembling the pubescence of the 

 frond, have been universally confounded with it. 



» Prodr. Fl. Nov. lloll. i, p. 158, Obs. IV. 



