266 RESPIRATION, FERMENTATION AND DIGESTION 



the direct action of living matter, or to any of the above enzymes 

 have been observed, and it is probable that the above list will be 

 greatly extended by further investigations. 



337. Origin and Distribution of Enzymes. Some enzymes may 

 be formed in almost any cell of a plant, in which their origin may 

 not be traced to any special plastid. In other instances special- 

 ized cells are differentiated for the chief purpose of secreting 

 these substances. Such glandular cells may be seen as forming 

 the aleurone layer in the seeds of grasses and other monocoty- 

 ledons, and also the epithelial layer of the cotyledon or scutel- 

 lum. The secretion of an enzyme seems to be preceded by the 

 formation of a granular substance known as zymogen, as a result 

 of the joint action of the nucleus and cytoplasm in such glandular 

 organs, although it is not possible to observe all stages of the 

 process in every instance. Extracted enzymes are dialyzable 

 with difficulty, and may not pass out of the glandular cells in 

 which they are formed by diosmose. Their discharge must be 

 effected by a passage along interprotoplastic threads, or they 

 may pass through the walls and plastic membranes by filtration 

 pressure in an emulsified condition in the same manner that oils 

 and waxy substances accomplish translocation. The last named 

 method is the only one by which the enzymes of bacteria and 

 other unicellular organisms could be excreted. In seeds the en- 

 zyme must pass many membranes to reach all parts of the 

 storage tissue containing food which must be digested. It is 

 possible that zymogen might pass a wall or membrane and then 

 become converted into an enzyme. 1 



338. Localization of Digestion. Digestion occurs in all proto- 

 plasts in which reserve food accumulates. The products of syn- 

 thetic processes may be of such nature as to be capable of assimi- 

 lation without change, which would be a desirable and economical 

 arrangement. The preponderating constructive capacity of the 

 plant however, furnishes it with a surplus which is changed to 



'Laurent, J. Sur 1'exosmose de diastase par les plantules. Compt. Rend. 131 : 

 848. 1900. 



