THE PTERIDOPHYTA : FILICALES, THE FERNS 513 



Hymenophyllum tunbridgense 



The genus Hymenophyllum has about 300 species, mostly tropical, although 

 two, H. tunbridgense and H. iinilaterale (zvi/soni), occur in Britain, chiefly in 

 mountain districts. The geographical distribution of these species is very 

 remarkable. . They are plants of the Southern Hemisphere with a wide dis- 

 tribution in Australia, South Africa and South America, reaching Europe 

 through the Canary Islands and extending (H. zvi/soni) as far as Norway. 

 This extraordinary northward extension of their range is marked by a very 

 strict selection of habitats, for in Western Europe they are confined to the 

 most sheltered habitats, in districts with the most equable climate. The 



Fig. 509. — HymenophyUum tunbridgense. Complete plant showing 

 rhizome and leaves. About half natural size. 



species concerned are very closely related and are possibly no more than 

 varieties of one form. 



The stem is very slender, rarely more than 6 in. long and often much 

 shorter, and bears two ranks of leaves which vary from i to 12 in. in length 

 (Fig. 509). In outline the leaves vary from entire, with pinnate venation, to 

 highly divided leaves which may be once, twice or thrice pinnate. There is a 

 good deal of webbing, the lamina running decurrently along both the main 

 raches and those of the pinnae, forming a narrow, continuous membrane. 

 The filmy texture is a very marked characteristic, the laminae being trans- 

 lucent and for the most part only one cell thick, which probablv accounts 

 for the preference shown by these plants for moist and shady habitats. A 

 few forms which occur in drier places have a covering of felted hairs, but 

 there are no true ramenta. 



The internodes of the stem are relatively long, and branching is carried 



