DIGESTION 273 



breaks it up into glucose, hydrocyanic acid, and benzaldehyde 

 (Chap. XV) : 



C 2 oH27NOi 1 +2H20 = 2C 6 H 12 06+HCN+C 6 H 5 CHO. 



In the ordinary almond, emulsin is present and may be extracted 

 for digestive studies, but the glucoside is absent. Because of the 

 HCN formed in this hydrolysis, cattle and sheep which eat wild 

 cherry leaves are commonly poisoned. The same result may be 

 expected when bitter almonds are eaten. Since the HCN is not 

 found until the parts are crushed, it has been concluded that 

 the glucoside and the enzyme are kept apart in the cell by im- 

 permeable membranes of some kind, and only when crushed do 

 the two come in contact. In a similar way, the acid of the Oxalis 

 leaf and similar leaves with a high acid content may be kept away 

 from the protoplasm. 



Salicin from the willow and poplar, and sinigrin from the Cru- 

 ciferae are other glucosides whose digestion is discussed in Chap- 

 ter XV. 



Esterases. — Fats cannot be translocated as such but must first 

 be split into acid and glycerin by esterases, which include the lip- 

 ases or fat-hydrolyzing enzymes. It will be recalled that the fats 

 are esters of glycerin and fatty acids, so that the term esterase 

 is a more general and hence more appropriate term than lipase, 

 which applies particularly to the fat-splitting enzymes. Lipases 

 have been known in animals for many years, but not until 1890 

 was their presence observed in plants. At this time Green noted 

 that germinating seeds which were rich in fats or oils acquired an 

 acid reaction. This acid, which results from the breaking down of 

 the fat, catalyzes the enzyme reaction and hence acts like a co- 

 enzyme. Tanaka believes the lipase in the seed is in the form of a 

 zymogen and that the acid acts as a transformer. This seems 

 hardly possible, however, since the acid is not absolutely neces- 

 sary to start the reaction. A small amount of acid added at the 

 start will cause complete hydrolysis to take place within a few 

 hours, while about four days are required if the acid has to be 

 supplied by the hydrolysis itself as the reaction progresses. In 

 castor beans, where lipases have been studied more than in any 

 other seed, lactic acid is the acid produced. Mineral acids may be 

 used to replace the organic acid but much less is required, i. e., 

 the optimum amount is reached much sooner. In Germany, castor 



