298 PLANT PHYSIOLOGY AND ECOLOGY . 



325. Physiographic symmetry. The symmetry of a habitat 

 depends primarily upon the distribution of water in it, and this 

 is greatly affected by soil and physiography. Decisive differences 

 in soil rarely occur within a single habitat, though this is often 

 the case in a series. The strikingly zonal structure or arrange- 

 ment of habitats is nearly always due to differences in water con- 

 tent produced by physiographic factors, slope, exposure, surface, 

 and altitude. All of these have a pronounced influence upon 

 water content and humidity. Consequently, wherever appre- 



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Fig. 122. Physiographic symmetry shown by the valley of Bear Creek 

 near Colorado Springs. I'he bilateral zones of Populus angustifolia 

 are indistinct, owing to the narrow stream, but the thicket zones are 

 marked. 



ciable differences in physiography occur, they produce areas of 

 excess or deficiency in water content about which this factor 

 varies symmetrically. Peaks and hills are typical examples of 

 areas of deficiency; ponds, lakes, and oceans of areas of excess. 

 When such areas are extreme in character and close to each other, 

 the resulting zonation is marked. When they are moderate, 

 particularly if they are widely separated, the zones produced are 

 obsciu"e. 



326. Symmetry in vegetation. The response of vegetation to 

 habitat is so exact that physiographic symmetry every^vhere 

 produces a vegetational symmetry, which is expressed in zones. 



