32 



PROTOPLASM AND THE CELL 



analyzed than can vital processes in higher types of cells. 

 This belief, indeed, has been realized to a certain extent, 

 although the secrets of constructive and destructive meta- 

 bolism are still unrecognized. The sweet fluids of fruits 

 offer an excellent medium in which yeast cells grow and 

 multiply. An even more excellent medium is prepared from 

 the proteins, sugar and salts extracted from the young cells 

 of sprouting barley. This medium, known as sweet wort, sup- 

 plies the necessary elements for the living protoplasm of yeast, 

 and the vital processes go on at a rapid rate. Sweet wort, 

 however, and the sugary juices of fruits are too complex to 

 give any more adequate notion of the food value of specific 

 elements, than would the protein food of higher forms of life. 

 Fortunately, however, the yeast processes are so primitive that 

 more direct and exact knowledge is possible. 



If a quantity of pure yeast is burned, the mass first chars by 

 the deposit of carbon, then, with continued heat, this is used 



FIG. 15. Endogenous spore formation in yeast. (From Sedgwick and Wilson.) 



up in forming carbon dioxide by union with oxygen, while at 

 the same time the nitrogen of the yeast is given off in the form 

 of nitrogen gas, hydrogen as water vapor, and sulphur as sul- 

 phurous acid or sulphur dioxide. Finally nothing remains but 

 a white ash composed of potassium, lime, magnesium, and 

 phosphoric acid. 



Pasteur made up a fluid composed of the ingredients thus 

 obtained by analysis, and found that yeast cells would grow 

 and multiply in it as in sweet wort. With such a fluid he was 



