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. Wecan 
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 
