ANAEROBIC RESPIRATION AND FERMENTATION 307 



by processes which do not require the addition of oxygen from 

 the outside, is easily demonstrated by placing seeds of peas, horse 

 beans, etc., in an atmosphere devoid of oxygen. This may be 

 accomplished either by placing in an atmosphere of nitrogen or 

 hydrogen or merely by placing in a vacuum (Fig. 19). Carbon 

 dioxide will continue to be evolved for some time even though no 

 oxygen is present but only so long as 

 there is an abundant food supply. Vicia 

 faba leaves were found to give off 

 782.4 mg. of carbon dioxide when sup- 

 plied with food and only 256.8 mg. 

 during the same period (30 hours) when 

 not so supplied, in one of Palladin's 

 experiments (1894). In the light of 

 the importance of food in respiration 

 (Chap. XXIV) this is to be anticipated. 



In normal respiration, the food is ox- FiQ 19 ._ Ana e robic respi . 

 idized (or rather hydrolyzed) and ulti- ration. At the beginning of 

 mately water and carbon dioxide are the experiment the test tube 

 produced as end products. In anaerobic ^'iS^uX £ 

 respiration, if sugars are present, one i OW er rim. At the end of 24 

 of the products commonly formed is hours the mercury will be 

 alcohol, which can be detected in the g^ ^ y? of the tube as 

 anaerobic respiration of seeds, yeast, 



etc. In the case of the anaerobic respiration of the cultivated 

 mushroom (Agaricus), carbon dioxide is found but no alcohol, 

 which agrees with the fact that no sugar is found in this 

 plant. 



It was formerly believed that alcohol was formed when sugars 

 were oxidized, whether the process occurred anaerobically or 

 aerobically. If under the former conditions, i. e., in the absence 

 of free oxygen, then the process was considered to stop at that 

 point; but if oxygen was present, then the process was thought 

 to continue until the alcohol was broken down into carbon dioxide 

 and water. This viewpoint has been abandoned for various reasons, 

 but one good argument against this theory is the fact that alcohol 

 is much harder to oxidize than sugar. There is thus no reason to 

 assume that alcohol is an intermediary step in normal respira- 

 tion. The present tendency is to represent the various steps as 

 follows : 



