57. 



not at all the same, which undoubtedly accounts for 

 the variation in the distribution of the epiphytes. 

 The vertical distribution of the epiphytes on one tree 

 may thus be compared with the vertical distribution 

 of the vegetation on a mountain.The mosses axd the 

 polypody correspond to the mesophytic vegetation of the 

 middle region of the mountain, while the lichens 

 of the dry regions of the tree are comparable with 

 the xerophytic vegetation of the summit of the mountain 

 where the drying winds prevail .The hygrophyllous 

 vegetation which can not endure high evaporation, if 

 such is pre sent, would be found at the base and would 

 correspond then to the hepatics in regions on the 

 tree where the evaporation is the lowest. 



Not only do climatic conditions affect the dist«» 

 ribution of the fern but even the chemical and physical 

 characters of the bark exert a marked influence on the 

 occurrence and development of the epiphytes, It appears 

 that a substratum rich in nitrogen causes a vigorous 

 growth of the epiphyte; the other elements seem to 

 be supplied in sufficient amounts by the dust of the 

 air, Furthermore, the bark itself in order to make a 

 desirable substratum for the epiphyte must be sufficiently 

 soft to allow the penetration of the young roots of the 

 fern, When that is done , the absorbing roots remain 

 practically imbedded within the bark,while those which 

 earlier fijinctioned for absorption are later found in 



