INTRODUCTION 5 



or, being highly reactive, may undergo change during the 

 process of extraction. 



Often the presumed role of a compound can only be 

 established after it has been isolated from the organism 

 and its behaviour studied in conditions which never occur 

 in the plant. There is sometimes a tendency to discard such 

 evidence as having no bearing on physiological processes ; 

 this is not necessarily sound judgment, for a change which 

 can take place in the unnatural conditions of the test tube 

 may be effected with ease in the living cell. 



