DIGESTION 263 



The former will be thrown down with the enzyme by the 

 alcohol. 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. 



