PREFACE TO THE SECOND EDITION. 



THE great advances made in the chemistry of Plant 

 Pigments since the issue of the first edition have neces- 

 sitated the re-writing of the section dealing with this 

 subject. For the rest, we have confined ourselves to 

 making a few minor additions and corrections and 

 adding a number of further references to the literature. 



P. H. 

 T. G. H. 



July, 1916. 



