32 



PROTOPLASM AND THE CELL 



cellent media are artificially prepared from the dissolved pro- 

 teids sugar and salts extracted from the young cells of sprouting 

 barley. This medium, known as sweet wort supplies the neces- 

 sary elements for the living protoplasm, and the vital processes 

 go on at a rapid rate. With them go on the extra vital activi- 

 ties for which yeast is commercially valuable, viz., fermentation. 

 Sweet wort, however, and the sugary juices of fruits in general, 

 are too complex to give any more adequate notion of the food 

 value of specific elements, than would the proteid 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 up 



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



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 able 

 by omitting one substance after another to determine what 

 elements are necessary for the vital activities of yeast. The 

 fluid, known as Pasteur's fluid, has the following percentage 

 composition: 



