xiv CONTENTS 



Page 



C. Saturation Deficit 134 



Saturation Deficit in the Habitat of Plant Communities .... 136 



D. Evaporation 137 



Measurement of Evaporation 138 



Evaporating Power of the Air as a Habitat Factor 140 



E. Ratio of Precipitation to Evaporation 143 



N. S. Quotient 143 



5. Wind 145 



Mechanical Effects of Continuous Winds 146 



Dune Formation 152 



Wandering Dunes 153 



Phj^sio logical Effects of the Wind 154 



Total Effect of the Wind 154 



Wind and Plant Communities 154 



Wind and Boundaries of Forest 157 



CHAPTER VI 



Edaphic or Soil Factors: Chemical 159 



Chemical Soil Theory 159 



Physical Soil Theory 160 



A. Colloid-chemical Viewpoint 161 



Saturated and Unsaturated Colloids 162 



Protective Colloids 163 



Ionic Exchange 163 



Cola's Osmotic Theory 164 



B. Soil Acidity 165 



Expression of Hydrogen Ion Concentration 166 



Active or Specific Acidity 167 



Measurement of pH 167 



Treatment of Soil Samples for j)H Determinations 168 



Significance of Hydrogen Ions 169 



Plant Communities and pH Values 170 



Lines of pH Values 172 



Profiles of pH Values 172 



Buffering of Soils 174 



Measurement of Buffering 176 



Buffer Effect and Plant Communities 176 



Soil Types and pH 178 



pH Values of Aquatic Communities 178 



C. Nutritive Salts and Plant Communities 179 



Toxic Effects 180 



Antagonistic Effect of the Ions 180 



Law of Relativity 180 



1. Calcium 181 



Indirect Action of Calcium 182 



Direct Action of Calcium 183 



Calcium Determination 184 



Gypsum Vegetation 185 



