CONTENTS xiii 



CHAPTER XXIV. 



SYMBIOTIC NITROGEN FIXATION 



Early Theories 290 



Early Observations on Root Tubercles 291 



Species 292 



Cultural Characteristics 297 



Morphology of the Colonies 299 



Morphology of the Bacteria 302 



Staining 303 



Bacteroids 303 



Mode of Entrance into the Host 304 



Growth of the Nodule 304 



Relationship to Host . 305 



Mechanism of Fixation (Metabolism) 306 



Chemical 308 



Source of Energy 312 



Aeration 312 



Moisture 312 



Temperature 313 



Influence of Fertilizers 313 



Legumes Associated with Non-legumes 314 



Soil Gains in Nitrogen 315 



Soil Inoculation 316 



Method Involving the Use of One Commercial Culture . . . .317 



Alternative Method 317 



Commercial Cultures 318 



References 318 



CHAPTER XXV. 



CROP ROTATION 



Essential Elements 319 



Element Added by Legumes 319 



Nitrogen 320 



Rothamsted Rotation 321 



Nitrogen Obtained from Atmosphere by Legumes ..... 322 



Distribution of Nitrogen in Legumes 323 



Legumes Feed on Nitrates 323 



Nitrification in Soils 324 



How to Maintain Soil Nitrogen 325 



References 326 



CHAPTERS XXVI. 



CELLULOSE-DECOMPOSING ORGANISMS 



Cellulose 327 



Early Observations '. . 327 



Work of Omelianski ... 330 



Morphology and Physiology 331 



Later Work on Cellulose Fermentation 333 



Function 333 



References 335 



CHAPTER XXVII. 

 BACTERIA IN AIR 



How Bacteria Enter Air 336 



Number and Kind 336 



Factors Governing Number and Kind 337 



Bacteria in Inspired and Expired Air 339 



Air-borne Infection 339 



