276 PLANT PHYSIOLOGY 



have been reported also from wheat embryos, green peas, many 

 fruit juices, the various cereals, and from the leaves of lettuce, 

 cabbage, etc. The erepsin of cabbage has been extensively studied 

 by Blood (1910), who found it to be active in acid, neutral, and 

 alkaline conditions. 



It has been hard to isolate proteases from resting seeds, and 

 Palladin has stated that no enzymes are present there although 

 they are easily found in germinating seeds. Dean (1905), however, 

 has found enzymes in resting seeds like the bean, and it is reason- 

 able to suppose that seeds such as the bean, pea, and corn, which 

 contain protein reserves, would also contain the enzymes for their 

 digestion unless they are in the form of zymogens and not yet 

 transformed into active enzymes. 



In testing for the presence of an enzyme, an extract made of 

 the organ under consideration is placed upon a suitable (carbohy- 

 drate, fat, protein, etc.) substrate. In protein studies, gelatin is 

 often used as the substrate or digestive medium. 



QUESTIONS 



1. Why are parsnips sweeter in the spring than in the previous fall? 



2. How is it possible for the delicate filaments of wood-destroying fungi 

 to penetrate the hard wood? 



3. Why is digestion a "katabolic" process? 



4. Why are substances rich in water more easily digested than those which 

 are not? 



5. Give the derivation of the names amylose, amylodextrin, and achro- 

 odextrin. 



REFERENCES 



Fisher, E. — The vegetable proteases. Biochem. Jour., 13:124, 1919. 

 Hepburn, J. S. — Biochemical studies of the pitcher liquor of Nepenthes. 



Proc. Am. Phil. Soc, 57:112, 1918. 

 McGinty, R. A. — Diastase activity in tubers of Solanum tuberosum. Ann. 



Mo. Bot. Gard., 6:223, 1919. 

 Rhine, J. B. — Translocation of fats as such in germinating fatty seeds. Bot. 



Gaz., 82:154, 1926. 



