DIGESTION 257 



containing a trace of an antiseptic. After a period of ten 

 or twelve hours the extract should be strained and subse- 

 quently filtered, when the enzyme may be precipitated from 

 the filtrate by adding strong alcohol. It is very evident 

 that this process will not yield it pure, for the solvents 

 employed will dissolve many constituents of the tissue 

 besides the enzymes, particularly proteins and sugars. 

 The former will be thrown down with the enzyme by the 

 alcohol. 



Any description of the process of digestion should 

 naturally be followed by an account of the subsequent one 

 of true assimilation or the construction of protoplasm from 

 the food which is supplied to it as the result of digestion. 

 Unfortunately but little can be said upon this subject, as such 

 problems remain almost entirely unsolved. If we study the 

 changes which take place in the growing points of plants, 

 where such assimilation must necessarily be most active, we 

 can find very little evidence of what is taking place. We can 

 trace, for instance, the progress of sugar along the stem for 

 a considerable distance, but just where it is assimilated our 

 methods fail us. Sugar can no longer be detected, but in 

 what way it has been incorporated into the living substance 

 is still a mystery. Similar acknowledgment must be made 

 in respect of the proteins. Amido-acids can be detected 

 along the translocatory paths almost up* to the locality of 

 growth, but beyond that nothing can at present be said. 

 We are unable also to explain the manner in which the 

 food originally constructed ministers to the nutrition of the 

 protoplasts or cells in which it is formed. 



17 



