Chap. IV] TROPICAL DISTRICTS CONSTANTLY MOIST 309 



A peculiar form of vegetation is produced by the Hymenophyllaceae 

 (Fig. 146), never absent from the deepest shade of the forest, which, though 

 they often clothe the bases of 

 tree-trunks as epiphytes, yet 

 also occur on the ground and 

 on rocks, and in any case do 

 not assume the peculiar char- 

 acters of epiphytic plants. The 

 Hymenophyllaceae (Hymeno- 

 phyllum and Trichomanes) il- 

 lustrate the great humidity of 

 the virgin forest better than any 

 other plants, as they have many 

 features in common with aquatic 

 plants. Their delicate leaves 

 usually consist of only one layer 

 of cells, excepting over the 

 veins, absorb water by their 

 whole surface, and shrivel up 

 quickly whenever the atmo- 

 sphere is not wellnigh saturated 

 with water - vapour. As in 

 aquatic plants, the greatly re- 

 duced roots play merely a 

 subordinate part as organs of fixation, or may be entirely absent : . 



Fig. 146. Hymenophyllaceae which are epiphytic 

 on tree-ferns in the tropical rain-forest of America at 

 Blumenau, South Brazil, i. Trichomanes angustatum, 

 Carm. 2. Trichomanes sinuosum, Rich. 



iii. LIANES OF THE RAIN-FOREST. 



The most peculiar components of the rain-forest, those which first strike 

 travellers and are most frequently mentioned by them, are lianes and 

 epiphytes. Both these forms of vegetation, it is true, also occur in other 

 forests, and are not confined to the tropics, but, as has already been shown-, 

 the tropical rain-forest is the original home of nearly all the higher epi- 

 phytes, even of those that occur in open dry tracts of country, and 

 woody lianes have in the tropical rain-forest, if not their sole place of 

 origin,, yet certainly the site of their most luxuriant development and of 

 their greatest diversity of form. Lianes and epiphytes exhibit a connexion, 

 to this extent, that a forest rich in woody lianes is rich usually in 

 epiphytes also, and the representatives of both these guilds frequently 

 belong to the same families. The origin of both forms is to be traced 

 back to the same factors, the struggle for light assisted by abundance 

 of moisture ; they are connected by intermediate forms, and many epiphytes 

 have apparently been evolved from lianes. 



1 See in particular the cited works of Prantl and Mettenius. 2 See p. 198. 



