238 INTRODUCTION. [CH. IX 



but it is not often that extracts can be prepared which 

 will contain only those compounds to which the attention 

 is directed, consideration may therefore necessarily be 

 given to substances occurring in the extracts, although in 

 themselves comparatively unimportant. 



The necessity for quantitative results in experiments 

 on metabolism is obvious, and even for qualitative work it 

 is sometimes necessary to employ rather complicated 

 chemical methods. 



The arrangement followed in these sections is based 

 on practical convenience, those substances which are com- 

 monly extracted together being placed in the same section. 

 Each chapter contains a short general explanation of the 

 methods to be used, followed by instructions for performing 

 the qualitative and quantitative experiments selected. 



Attention is chiefly directed to substances which are 

 either themselves 'plastic' or are believed to have im- 

 portant significance in metabolic processes the study of 

 other compounds is only introduced in so far as these are 

 liable to interfere with the examination of the above. 



It is seldom necessary to attempt a complete analysis 

 of all the constituents of a tissue, but it is essential to have 

 due regard to those which may interfere with the recogni- 

 tion or estimation of a particular compound (e.g. tannins 

 in the detection and estimation of sugars). 



Before beginning the chemical examination of a vege- 

 table tissue, it is of the greatest assistance to consider 

 carefully what substances are likely to be present: a 

 knowledge of the general distribution of the commoner 

 plastic substances should suggest the method of pro- 



