8 INTRODUCTION [ch. 



can generally be extracted from the plant by water, especially if the 

 tissues arc thoroughly disintegrated. Their chemical constitution is 

 at present unknown, and they are usually destroyed by temperatures 

 greater than 60°C. Moreover, many of the processes which they control 

 in the plant can be brought about by them in vitro under suitable 

 conditions, and it is by means of such experiments that information as 

 to their role in plant metabolism has been ascertained. The majority of 

 known enzymes control both hydrolysis and its converse, synthesis by 

 condensation with elimination of water, but under artificial conditions 

 hydrolysis most frequently occurs. The enzyme, diastase, for instance, 

 found in all starch-containing plants hydrolyzes in vitro starch to dextrin 

 and maltose. Similarly the enzyme, maltase, hydrolyzes maltose into 

 glucose. Other enzymes hydrolyze proteins into amino-acids, and others, 

 again, hydrolyze fats into fatty acids and glycerol. 



Until fairly recently the fact escaped notice that such reactions are 

 reversible, and that these enzymes in situ in the plant may, according 

 to the conditions, control not only the hydrolytic but also the cor- 

 responding synthetic process. The latter may also be brought about, 

 though not readily, in vitro. This, and other evidence, leads us to 

 believe that enzymes in the plant control the reactions in both directions. 



Hydrolysis, and synthesis with elimination of water are not however 

 the only processes catalyzed by enzymes. There is another type of these 

 catalysts, the oxidizing enzymes, which bring about oxidation of sub- 

 stances in the plant, notably of aromatics. In addition, there is the 

 enzyme, zymase, which decomposes sugar with the production of alcohol 

 and carbon dioxide. 



The question which now arises is — How many reactions in the plant 

 are catalyzed by enzymes i It is conceivable that a greater number of 

 enzymes may exist than are at present known, but that they are unable 

 to be extracted by our present methods of isolation. A certain number 

 of reactions probably take place in the cell-sap between the substances 

 in solution ; others are catalyzed by enzymes which are supposed to be 

 intimately connected with the protoplasm, but there are an enormous 

 number to which there is at present no clue as to how they are brought 

 about, such, for instance, as the synthesis of carbohydrates from carbon 

 dioxide and water, and the formation of the benzene ring from the open 

 carbon chain. Such processes are usually said to be controlled by the 

 "living protoplasm," but what exactly is the significance of this 

 expression is at present beyond our knowledge. 



Finally, also, little is known of the question as to how the various 



