DIGESTION 263 
The former will be thrown down with the enzyme by the 
aleohol. Moreover, some enzymes are slightly soluble in 
mixtures of alcohol and water of varying degrees of con- 
centration, while alcohol destroys others. 
Any description of the process of digestion should natu- 
rally 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 
with certainty. It may be that sugar can be made to combine 
with the molecule of protoplasm, and that amino- or amido- 
acids and not protein are the nitrogenous materials that 
are actually incorporated into the living substance. 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. 
