I] INTRODUCTION 9 



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 ? 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 expres- 

 sion is at present beyond our knowledge. 



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

 lines of metabolic syntheses in different parts of plants are regulated 

 and correlated with each other. Some of the phenomena involved are 

 shortly outlined as follows. There is undoubtedly, under suitable con- 

 ditions, a constant synthesis of sugars in the leaves. In all probability 

 aromatic substances are also synthesized in the same organs, for there 

 is evidence that there is an increase of these compounds in the leaf if 

 translocation through the petiole is prevented. It is possible that amino- 

 acids also are formed in the leaf The above products are constantly 

 translocated to the growing organs as material for growth. They may, 

 nevertheless, be temporarily stored in the tissues where they have been 

 synthesized, and of this there is evidence in at least one case, e.g. starch 

 in the leaf. But, apart from the immediate use for growth, there is 

 in practically every plant, some tissue where, owing to some unknown 

 stimulus (causing probably changes in permeability of the cell-mem- 

 branes), accumulation of compounds occurs. This accumulation is 

 characteristic of organs from which growth will take place when it is 

 impossible for the plant to obtain fresh supplies by carbon assimilation, 

 as, for example, of bulbs, rhizomes, tubers, buds, seeds, fruits and woody 

 tissues. In these cases, in due time, the products stored supply the 

 growing shoots. 



