PREFACE 



THE present volume is the outcome of the keen 

 personal interest that I have taken for some eight 

 years in the work on nitrogen-fixing organisms that 

 has been in progress in Professor Bottomley's 

 Laboratory at King's College. It would have been 

 difficult for me to write of the subject without en- 

 thusiasm. To many botanists the beauty of plant 

 life to some extent masks the supreme mysteries of 

 the vegetable world. Few conceptions are grander 

 than the wonderful storage system which from the 

 beginning of biological time has enabled plant life 

 to seize and hold in the tissues some of the steady 

 stream of energy that flows continually from sun to 

 earth, avoiding waste, steadily purifying the air, and 

 rendering its composition constant. That is an old 

 conception well established and commonly known. 

 To-day, however, we are watching the growth of a 

 new conception. Just as the plants have been 

 steadily storing the energy that is required for the 

 support of animal life, and making existence possible 

 for them, so the bacteria in the soil have been 

 supplying the plants with the essential substances 

 that they require for their more limited activities. 

 Nitrogen as such is valueless to plants. Were it not 

 for the work of the bacteria in the soil no plant could 



