RESPIRATION OF PLANTS 97 



to possess toxic properties. Consequently, in the absence of 

 oxygen, a plant perishes not merely from an excessively low 

 supply of energy but likewise from self-poisoning. In this 

 respect, the experiments by Nabokich (1905) are very instructive. 

 He placed germinating 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 toxic 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, e.g., glycerin, mannite, tartaric and 

 quinic acids, or peptone, may also be utiUzed. 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 necessary energy. 

 Yeast is a most conspicuous example of such microorganisms. 

 Its anaerobic respiration is called '^ alcoholic fermentation." 



Alcoholic fermentation has been known to mankind since 

 time immemorial. It is used not only in the preparation of 

 beverages, where the alcohol is produced by yeast, but also in 

 breadmaking, where the counterpart of fermentation, the elimi- 

 nation of carbon dioxide, is used to raise the dough. For a long 

 time, the nature of this process remained unknown. It was often 

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

 By his classical investigations, Pasteur (1860) succeeded in 

 ascertaining correctly the biological nature of alcoholic fermenta- 

 tion and in proving that fermentation is due to the vital activities 

 of yeast cells and that it is indispensable to their existence in an 

 oxygen-deficient medium. The significance of fermentation has 

 been briefly formulated by Pasteur in the following statement: 

 Fermentation is life without oxygen (''oxygen" meaning, of 

 course, oxygen from the air). 



For a long time, fermentation was thought to be the result of 

 the general types of metabolism of substances in the yeast cells. 

 It was assumed that yeast first assimilates sugar and then, after a 



