RESPIRATION AND FERMENTATION 259 



of energy, but likewise from self-poisoning. In this respect the 

 experiments by Nabokich are very instructive. He placed ger- 

 minating pea and sunflower seeds in a large vessel containing 

 water, and by continued evacuation removed the last traces of 

 air. The seeds not only remained alive for several weeks, but 

 even showed some growth. This is explained by the fact that the 

 poisonous products of anaerobic respiration were leached out by the 

 water. In a gaseous oxygen-free medium the same seeds exhibit 

 no signs of growth. They die after 3 to 5 days. 



The usual material for anaerobic as well as normal respiration 

 is sugar. Other substances, as for example, glycerine, mannite, 

 tartaric and quinic acids, peptone, etc., may also be utilized. It 

 appears, however, that the breaking down of these substances is 

 preceded by their conversion into sugar, or at least into closely 

 related substances. 



Anaerobic respiration, which in the higher plants is but a 

 temporary substitute for normal respiration, is in certain micro- 

 organisms the fundamental process of securing the necessary 

 energy. Yeast is a most conspicuous example of such micro- 

 organisms. Its anaerobic respiration is called "alcoholic fermen- 

 tation." 



Alcoholic fermentation has been known to mankind since time 

 immemorable. It is applied not only in the preparation of bever- 

 ages, where the alcohol produced by yeast is used, but also in bread 

 making, where the counterpart of fermentation, the elimination 

 of carbon dioxide which raises the dough, is utilized. For a long 

 time, the nature of this process remained mysterious. It was 

 often ascribed to a purely chemical " autodecomposition " of sugar. 

 By his classical investigations, Pasteur (1860) succeeded in ascer- 

 taining correctly the biological nature of alcoholic fermentation 

 and in proving that fermentation is the manifestation of vital 

 activities of yeast. 



Yeast belongs to the genus Saccharomyces, related to the Asco- 

 mycetes. These are single-celled organisms which multiply rapidly 

 by budding, and sometimes form fragile chains. Under special 

 conditions there are produced within each cell four spores (Fig. 

 104). A great number of various kinds of yeast are known. They 

 differ from each other physiologically as well as morphologically; 

 for instance, by the form and size of their cells. All forms produce 

 typical alcoholic fermentation and give rise to different byproducts. 



