ORGANIC SUBSTANCES AS PLANT FOOD 235 



Some organisms, especially bacteria, are not able to use either 

 the NH 2 , or N0 3 groups. They require organic nitrogenous sub- 

 stances, and are usually grown by adding peptone to the nutrient 

 medium. The method of nutrition of such organisms, to which 

 belong the lactic-acid bacteria, and also some facultative parasites 

 related to the bacteria of anthrax, is somewhat similar to the nutri- 

 tion of animals. 



The digestion of nutritive substances by saprophytes is similar 

 to the digestion of the initial products of assimilation or the reserve 

 substances by autotrophic green plants. The most important 

 agencies in this work are the enzymes, the energetic action of which 

 in the saprophytes often surpasses that in higher plants. Another 

 peculiarity of the enzymatic action of saprophytes is the secretion 

 by them of enzymes into the surrounding medium. This has an 

 especially important significance in the utilization of insoluble 

 substances, such as starch or proteins, since they require a pre- 

 liminary hydrolysis before they can be absorbed by the cell of the 

 saprophyte. Such enzymes are called "ectoenzymes," to distin- 

 guish them from endoenzymes, which work only inside the cell. 

 Of the enzymes formed by molds, one very well known, and even 

 used in medicine, is Takadiastase. It is secreted by the fungus, 

 Aspergillus oryzae, found on rice grains. A preparation of this 

 enzyme will convert starch into sugar with great rapidity, but it 

 likewise will develop a proteolytic activity, liquefying gelatin, 

 which indicates the admixture of a considerable quantity of proteo- 

 lytic enzymes with this diastase. 



An interesting phenomenon is the regulation of the secretion of 

 enzymes in saprophytes. Aspergillus or Penicillium, commonly 

 used in laboratory work, for instance, are capable of producing the 

 most varied enzymes. They do not always produce them in any 

 definite proportion, but will form predominantly or even exclusively 

 those enzymes which will act upon the substratum on which these 

 organisms are developing at a particular time. Thus, when grown 

 on starch, they will form mainly diastase, but on sugars diastase 

 is not formed at all. On fats they form lipase in large quantities, 

 while on protein substances an energetic secretion of protease takes 

 place. Even when unusual nutrient substances are supplied to 

 them, for example tannin, special enzymes, as tannase, are pro- 

 duced, while normally not a trace of these enzymes seems to be 

 present. 



