272 PLANT PHYSIOLOGY 



before it can be translocated is an excellent example of the in- 

 completeness of the ordinary definition of digestion, viz., liquefac- 

 tion. Cane sugar, we have just seen, is soluble and yet it too must 

 be digested. 



Inulase hydrolyzes inulin to levulose in the same manner that 

 diastase hydrolyzes starch to glucose. Doubtless here, as in the 

 case of the diastase, there is a series of enzymes which act con- 

 secutively. 



The pectic enzymes, pectosinase and pectinase, are also im- 

 portant carbohydrases that should not be forgotten in our study of 

 the carbohydrate-splitting enzymes (Chap. XII). 



Cytase is an enyzme which acts upon cellulose breaking it down 

 into monosaccharides such as galactose and mannose. Fungi 

 which penetrate the cell wall secrete cytase which permits them 

 to enter. That some soil and water bacteria secrete cytase is 

 evidenced by the gradual disappearance of cellulose (paper) in 

 the soil, and Winogradski has found at least a dozen different kinds 

 of such bacteria. These forms are, for the most part, anaerobic, 

 i. e., they function best in the absence of oxygen, according to 

 Waksman (1926), while fungi play the chief role in cellulose de- 

 composition under aerobic conditions. The large quantities of 

 paper which enter the streams and oceans annually in sewage 

 would soon accumulate were it not for the action of these cytase- 

 secreting forms. Cytase is also found in the intestinal tract of 

 herbivorous animals, which are able to use a certain percentage 

 of cellulose as food, so that even straw is to a certain degree 

 digestible. 



Gummosis of plants, which was once thought to be due to 

 wounds, is now considered to be due to an overproduction of 

 cytase so that the cell walls are partly digested and broken down 

 into a gummy mass. 



These are the principal carbohydrases which are of importance 

 in the digestion and transformation of carbohydrates. It will be 

 recalled that these same enzymes are considered to be responsi- 

 ble for the synthesis of the compounds as well as for their 

 hydrolysis. 



Glucosidases. — The glucosidases are those enzymes which 

 break down glucosides into their constituents. One of the best 

 known of these enzymes is emulsin found in the almond, peach, 

 plum, cherry, etc., which acts upon the glucoside, amygdalin, and 



