2i6 METABOLISM 



these organisms live during anaerobiosis on stored oxygen ; at all events, 

 organisms are known which possess such a power (Ewart, 1897). There are 

 certain Bacteria which render themselves to a certain extent independent of 

 the presence of oxygen by forming a pigment which has the capacity, even 

 when dissolved out of the cells, of absorbing oxygen, just as the haemoglobin of 

 the blood does, and of combining it in such a way that the oxygen is set free 

 again in any situation not provided with free oxygen, and, in the course of 

 a few hours, diffuses out of the cells. These organisms can, for a long time, 

 maintain normal respiration in a space free from oxygen, at the cost of reserves 

 of oxygen which they have accumulated. How far they make use of this power 

 has not as yet been determined. On the other hand, this power is of great 

 service to anaerobes, whether they are able to live always or only for a certain 

 time without oxygen ; for they can live in places where, owing to the activity of 

 aerobes, the oxygen has been used up, and thus nutritive material from which 

 other organisms are debarred will be accessible to them. 



Having now learned from a study of the formation of ethyl- and butyl- 

 alcohol, and, earlier, of oxalic and other organic acids, that there are various 

 tjTpesof fermentation, which differ very essentially in their oxygen requirements, 

 we may now glance at some of the numberless other types of fermentation, and 

 chiefly at those by which the products of the animal and plant worlds are 

 transformed, through the fermentative agency of lower organisms, into simple 

 bodies which may serve once more as nutrients to the higher plants. 



If we begin with the alcohol which arises from the fermentative activity of 

 yeast, we have first of all to notice that this substance is produced artificially 

 not only in brewing, wine-making, and distilling, but that it is generally to be 

 found in nature wherever sugar-containing sap occurs. Thus yeasts or other 

 Fungi always locate themselves on the outer surface of many fruits or in sap 

 expressed from plants by osmotic pressure, and in such situations they are able 

 to induce alcoholic fermentation. Among the chief products so formed there 

 is one, carbon-dioxide, a completely oxidized body, for whose further alteration 

 in the plant an expenditure of energy is necessary (p. 130), and which on that 

 account is of no value to any organism as a material for the support of metabolic 

 changes ; the other product, however, alcohol, is relatively poorer in oxygen 

 than sugar, and hence can serve as a source of energy to some types of organisms. 

 An earlier opportunitj^ was taken of noting that alcohol may serve as a source 

 of carbon to many Fungi, and it may be concluded that it may serve to help 

 not only in the construction of the organism but also in respiration. Acetic acid 

 Bacteria (compare Hoyer, 1898, Henneberg, 1898) oxidize alcohol completely 

 into acetic acid, approximately according to the following formula : — 



For this process oxygen in large quantity is obviously necessary. Since carbon- 

 dioxide (in the first instance at all events) is not formed, normal respiration 

 does not take place ; only when the alcohol is all decomposed, is acetic acid 

 further decomposed into carbon-dioxide, but doubtfully in the case of all acetic 

 acid Bacteria. At present we cannot say with certainty whether an acidifying 

 of the substratum is the primary object of acetic acid fermentation so as to 

 shut out associated organisms, or whether the object is merely to make use of 

 the chemical energy of alcohol. The latter seems to us to be somewhat improb- 

 able because there would appear to be no reason why the formation of acetic 

 acid should cease, and, in support of the former hypothesis, we must remember 

 that the acetic acid Bacteria are more resistant to that acid than other organisms. 

 Acetic acid Bacteria are furthermore not exclusively confined to alcohol as 

 a medium ; they can exist in other substances as well, all of which they are cap- 

 able of oxidizing. Thus they can transform higher alcohols into fatty acids, e. g. 

 propyl-alcohol into propionic acid, and butyl-alcohol into but5n:ic acid. Some of 



