CHAPTER XXVL 



HYMENOPHYLLUM, LinncEus. 

 ( Hy m- en- opli- y F -lum. ) 

 Filmy Ferns. 



HE genus HymenojyJiyllum, Avhicli derives its name from hymen. 

 a membrane, and jjhyllon, a leaf, on account of the peculiar 

 transparency of the leaves of tlie plants contained in it, and 

 which in the formation of the tribe Hymenojjhyllece is equal 

 in importance to Trichomanes^ is a group of mostly dwarf 

 and sometimes very minute plants, for the most part resembling Mosses more 

 than Ferns. They are natives of tropical and temperate climates, one species 

 only (//. hmbridgeiise) being indigenous in England, where it is frequently 

 found clothing trunks of trees and surfaces of damp rocks. The plants are 

 usually provided with long-creeping, thread-like rhizomes (prostrate stems) 

 of a wiry nature. Their fronds, delicately membranous and often of a lurid or 

 olive-green colour, are simple (uncut), like those of II. cruentum.^ or slightly 

 lobed, like those of H. asplenioides, in one section ; smooth and from once 

 to four times pinnatifid (cleft partly to the midrib), like those of //. clemissum 

 and H. polyantJios, in another; whereas those of a third section are of a hairy 

 nature, as in II. oeruginosum and //. obtusum, and more or less ciliated on 

 the edges, as in ciliatum and //. valvatum. The distinguishing characters 

 of the species, however, reside in the position of the sori (spore masses) : 

 these are always disposed on the very margin of the frond, m which they 

 are either more or less sunk or exserted (thrust out), and always terminate 

 a vein. These spore masses are furnished with an mferior involucre (covering) 



