RESPIRATION 31 



Although at present unable to determine the respective 

 parts taken by respiration and nutrition in fermentation, 

 decay, and disease, let us briefly consider some examples. 

 The fermentation most studied because the most useful is 

 alcoholic fermentation, by which is understood the produc- 

 tion of the ethyl alcohol of commerce. The organisms which 

 produce most alcohol from a given quantity of sugar are 

 certain species of yeast (Saccharomyces cerevisise, S. eltip- 

 soidenfi ) , but certain species of Mucor* ( M. erectus, M. race- 

 niosus, even M. mucedo) and bacteria also form ethyl alco- 

 hol in considerable quantities. We have already seen that 

 in the forced anaerobic respiration of higher plants (fruits 

 and germinating seeds) alcohol is one of the products. It 

 may therefore be inferred that, like anaerobic respiration, 

 the production of alcohol in fermentation is not peculiar to 

 the plants most used for the purpose, but that the power to 

 produce it is possessed by most, if not all, plants. In the 

 majority, however, this power is undeveloped. 



Not only the amount of alcohol but also the nature and 

 the amounts of the other substances produced, determine 

 the availability of a plant for the commercial production of 

 ethyl alcohol. The best yeasts, cultivated under the most 

 favorable conditions, convert about 6% of the sugar used 

 into other compounds than alcohol, e. g\ glycerine, succinic 

 acid, higher alcohols, ethereal compounds (to which the 

 ''bouquet" of wines is chiefly due), etc. Under less favor- 

 able conditions, with poorer yeasts, and especially with 

 mixtures of yeasts and bacteria, the amounts and kinds of 

 undesirable by-products increase. The amount of alcohol 

 produced by the bacteria commonly associated with beer, 

 bread, wine, and other yeasts used in the arts in this coun- 

 try, is slight in comparison with their other products ; and 

 it is quite as much the kinds and amounts of these other 



* See E. Chr. Hansen. Meddelelsor fra Carlsberg Laboratoriet, Bd. I., 

 etc. Untersuchungen a. d. Praxis der Gahrungsmdustrie. Jorgensen. Die 

 mikroorganismen der Gahnmgsmdustrie, 3te Aufl., Berlin, 1892. E. Got- 

 schlich. Gahrungserregung ; in Fliigge's Mikroorganiemen, 3te Aufl., Leip- 

 zig, 1896, Bd. I., pp. 219-270. Lalar. Technical Mycology, tranel. by 

 Salter. London and Philadelphia, 1898. 



