NUTRITION 401 



in peach, almond, and apple seeds, into hydrocyanic acid, glucose, and benzoic 

 aldehyde, thus : 



The so-called "mustard oil " is produced, along with glucose and two other com- 

 pounds (see p. 398) from sinigrin, a glucoside characteristic of the mustard family. 

 These actions are very rapid, as shown by the formation of the peculiar flavor or 

 pungency almost as soon as the parts are crushed by the teeth and the enzyme 

 thus brought into contact with the glucoside. 



Protein enzymes. — Several enzymes are known which digest proteins. 

 In animals their digestion proceeds by two prominent stages; lirst, the 

 peptic enzymes (i.e. those like pepsin of the stomach) convert proteins 

 into peptones, which are soluble and diffusible; second, the trypsin of 

 the intestine converts proteins and peptones alike into amino-acids and 

 other compounds, still more freely soluble and diffusible. At first 

 protein digestion in plants was ascribed to peptic enzymes; later, be- 

 cause of its completeness, it was referred to tryptic enz3Tnes and the 

 presence of peptic enzymes was denied. Now, however, it is possible 

 to distinguish the two classes of enzymes, though they act together and 

 carry forward the processes to completion without a pause at any par- 

 ticular stage of simplification. 



Inafmuch as the proteins are not prominent among surplus foods, it might seem 

 at first sight that protein digestion was unimportant in plants. But aside from the 

 stored food, many instances where such digestion must occur may be cited. Thus, 

 the exhaustion of proteins to a large extent from the foliage of annuals as the seeds 

 ripen (e.g. as shown in cereals), and the partial recovery of proteins from leaves 

 of trees before their fall, presuppose protein digestion. So, also, the action of a 

 plant parasite or saprophyte on animal bodies, and of the curious pitchers and 

 traps of carnivorous or insectivorous plants involve protein digestion. 



Assimilation. — All the digestive changes are preliminary to the trans- 

 location of foods from places of manufacture to places of storage or use, 

 or from places of storage to places of use. And before foods are of 

 real use they must be incorporated into the living substances of the 

 body,' which grows thereby. This final step in the chemical progress 

 of foods, by which they become a part of the living protoplasm, is known 



^ This view is only partly shared by those physiologists who believe that food can be 

 "oxidized" directly to serve as a source of energy. See the section on Respiration (p. 403). 

 For them the food so oxidized is no more incorporated into the body than fuel is into the 

 furnace in which it is biu'nt. 



