ORGANIC SUBSTANCES AS PLANT FOOD 211 



substances than proteins, resins, or other colloids. By repeated 

 dissolving and absorption, it is possible in this manner to concen- 

 trate certain enzymes and to purify them from various admixtures. 

 The best results were secured with invertase from yeast, which may 

 be extracted in a quite pure condition by water. Willstatter was 

 able to increase the concentration of this enzyme 1,600 times. He 

 has shown that such an almost pure preparation of invertase does 

 not contain phosphorus and does not give a reaction for proteins 

 or carbohydrates. But after all, a preparation of this type is not 

 absolutely pure, and an analysis of it does not give a true idea of 

 the chemical nature of enzymes. 



The chemical composition of enzymes is not known, therefore 

 a strict definition of them cannot be given. The presence of 

 enzymes is known only by their action. One may judge of their 

 quantity only by the amount of substance which they transform 

 per unit of time. Even in using crude unpurified preparations 

 of enzymes, however, one of their remarkable properties can 

 be noticed; namely, that an insignificant amount of enzyme is 

 able to induce the transformation of an enormous amount of 

 organic substance. This property is still more marked when puri- 

 fied preparations of enzymes are used. Thus, for instance, 1 gm. 

 of invertin may induce the hydrolysis of 1,000,000 gm. of sucrose. 

 It does not follow, however, that the quantity of enzyme plays no 

 essential role. On the contrary, if the rate at which hydrolysis 

 proceeds is determined, it will be found that it is proportional to 

 the amount of enzyme used. If a rapid enzymatic disintegration of 

 a substance is required, therefore, it is necessary to apply a large 

 amount of enzyme. 



Of the external conditions which exert a great influence on the 

 rate of enzymatic reactions, the temperature and the actual 

 acidity of the medium must be mentioned. The influence of the 

 temperature on the activity of the enzyme has much in common 

 with the influence of the temperature on vital processes. With an 

 increase of temperature the reaction first accelerates, then a certain 

 optimum temperature is reached at which the reaction proceeds 

 at its highest rate. With a further increase of temperature the 

 reaction begins to decrease and, finally, the destruction of the 

 enzyme occurs. In short, the three cardinal points on the tem- 

 perature curve of enzymatic action are found : the minimum point 

 at which the reaction only begins to show a perceptible activity, 



