PREFACE TO THE SECOND EDITION 



IN the present edition, some account, accompanied in most cases by 

 illustrative experiments, has been given of a number of substances, or 

 groups of substances, involved in plant metabolism, which were not in- 

 cluded in the first edition. These are notably the "vegetable acids," 

 waxes, sterols, lecithins, inositol, phytin, the "essential oils" and nucleic 

 acid. Corrections have also been made in order to include more recent 

 additions to our knowledge on certain problems, as, for instance, those 

 connected with oxidizing enzymes. 



Since it is advisable to keep the book as short as possible, a few of 

 the original experiments have been omitted to make space for others 

 considered to be of greater value to the student. 



The chapter on the colloidal state is intended to give the student a 

 preliminary conception, only, of the importance of such phenomena. 

 Additional information, both as to theory and experiment, is to be found 

 in text-books which deal more exclusively with this subject. 



Sufficient experience has not yet been gained to admit of the in- 

 clusion, in the present edition, of quantitative class-work in Plant Bio- 

 chemistry. 



I am much indebted to Dr F. F. Blackman, F.R.S., for kindly assist- 

 ing with the proofs. 



M. W. O. 



Cambridge, 

 Deeemher, 1922. 



