PREFACE TO THE THIRD EDITION. 



The necessity for a third edition has afforded an oppor- 

 tunity for making certain changes in the arrangement of 

 the subject-matter. In order to give the more purely 

 physiological aspect of the subject fuller treatment, with- 

 out at the same time unduly increasing the size of the 

 volume, the work now appears in two parts. Volume I. 

 is essentially the same in scope as the earlier editions 

 and deals primarily with the more chemical side of the 

 subject : a sufficiency of plant physiology has, however, 

 been retained to make the account reasonably complete 

 and to preserve the character of the work. Volume II., 

 which is in preparation, will be devoted to more purely 

 physiological problems, and will contain some of the 

 matter previously found in the original volume. 



The present volume has been brought up to date as 

 far as is possible ; some portions have been rewritten. 

 Section VIII. for example, and in other sections a 

 certain amount of rearrangement has been deemed 

 advisable. 



P. H. 

 T. G. H. 



October, ig20. 



vn 



