THE FILMY FERNS. 
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2. THE ONE-SIDED FILMY FERN. 
Hymenophyllum unilaterale. 
The two Filmy Ferns resemble each other closely 
in many important respects. But Unilaterale 
or Wilsoni , as it is also called, is chiefly dis- 
tinguished from Tunbridgense by having the 
leafy filaments on the primary branches of the 
fronds produced on the upper surface only of the 
branches, instead of on both sides, above and be' 
neath, which is the case in the Tunbridge Filmy 
Fern. In U7iilaierale, as in Tunbridgense, what 
corresponds in ordinary ferns to the leaflets of 
the fronds are branched veins, with filmy, semi- 
transparent, wing-like borders. The distinction 
between the two ferns is somewhat slight. But 
both are found growing under the same conditions, 
often in company, their thin wire-like rhizomes 
densely interlacing with each other, and with the 
roots of the mosses which cover the moist rocky 
surfaces of the habitats of the Hymenophyllums . 
When the natural conditions under which the 
Filmy Ferns grow can be imitated in the fern 
