NUTRITION 401 
in peach, almond, and apple seeds, into hydrocyanic acid, glucose, and benzoic 
aldehyde, thus : 
H 2 O ^ C 7 H 4 O + HCN + 2(C 6 Hi 2 O 6 ) 
amygdalin water benzoic hydro- glucose 
aldehyde cyanic acid 
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: first, 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 enzymes 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. 
Inasmuch 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, 1 which grows thereby. This final step in the chemical progress 
of foods, by which they become a part of the living protoplasm, is known 
1 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 burnt. 
