202 PLANT PHYSIOLOGY 



all except the invertase. In the secretions of pollen tubes have 

 been found amylase, invertase, catalase, reductase, pectinase, 

 proteases, lipase, and zymase. Between fifteen and twenty en- 

 zymes have been isolated from molds, which can produce differ- 

 ent enzymes according to the kind of food they are using. Other 

 observers find, however, that many enzymes are produced re- 

 gardless of the nature of the substrate. 



Many enzymes may exist in the plant which have not yet been 

 found, and probably all or most reactions in the organism will 

 ultimately be found to be catalyzed by the action of enzymes. 

 These processes are generally spoken of as " controlled by the 

 action of the living protoplasm," but this phrase explains nothing, 

 and all such reactions are probably catalytic. 



Analysis and Synthesis. — It was formerly thought that enzymes 

 caused only decompositions, and the syntheses were explained by 

 the " action of the living protoplasm" just mentioned. There is 

 now ample evidence that the enzymes cause both synthesis and 

 analysis depending upon the environment, — acidity, presence 

 of the reacting substances, etc. The surrounding conditions de- 

 termine, for example, whether dextrose is to be built up into mal- 

 tose and starch or whether starch will be torn down into maltose 

 and dextrose. The same enzyme can cause the reaction to pro- 

 ceed in either direction, depending upon the conditions in the 

 cell where the work is taking place. This idea, that the enzyme 

 which accelerates the decomposition may accelerate the synthesis, 

 has been proved in the case of maltase, which has been used for 

 synthesizing maltose from glucose. In a similar manner arti- 

 ficial fats have been synthesized by the aid of lipase from the 

 pancreas, simple proteins from amino acids by the aid of pro- 

 teases, and glucosides from their components by glucosidases. 



Mode of Action. — Much discussion has been held over the 

 action of the enzymes. How do they bring about the acceleration 

 of the reactions? The prevalent theories may be classified as 

 follows : 



1. Mere adsorption — -physical. 



2. Some kind of chemical combination. 



a. colloidal. 



b. molecular. 



Those who hold to the adsorption theory consider that the 

 enzyme draws the two reacting substances so close together that 



