PREFACE xi 



by residence at a distance, and, in recent years, by ill health. 

 Let our critics take note, and be lenient : they have been severe 

 enough at times in former years. 



It remains for us to express our thanks to two of the staff 

 at the Rothamsted Experiment Station for the kindly assist- 

 ance which they have given us in preparing this volume : to 

 Miss Adam for verifying the references contained in it, and to 

 Dr. Winifred Brenchley for writing a short account of the nature 

 and functions of plants ; to Mr. J. Scott Taylor our thanks are 

 likewise due for his kindness in reading and correcting the proof 

 sheets. 



B. 

 S. P. 



