270 



OHIO BIOLOGICAL STRVEV 



Tile fitMnuMit association of the sweet birch (B. loita) witli the 

 hemlock seems to be due to the similarities in their root systems. The 

 seedlings do not develop a tap root, but form a much-branched system 

 of fibrous roots, which spread out freely near the surface, never attain- 

 ing any great depth. They are therefore esi)ecially suited to rocky 

 situations in which penetration is difficult or imi)0ssible, and they are 



Fig. 12. A Cascade in the Hemlock Forest. 



limited to suljstrata furnishing a constant supi^ly of water near the 

 surface. Very few soils, iiowever, can maintain such a condition except 

 when bathed in a lieavily moisture-laden atmosjihere. In such humid 

 habitats both species do well regardless of the substrata, growing almost 

 everywhere and showing a strong tendency to become epiphytes. But 



