FERMENTATION. 49 



sugar. This claim has been somewhat vigorously disputed 

 and cannot be regarded at present as wholly demonstrated. 



While a number of organized ferments have been thus found 

 to produce enzymes, this is not yet true of them all. The lac- 

 tic bacteria have the power of producing lactic acid from milk 

 sugar, thus souring the milk if growing in it. This is a fer- 

 mentation and there is as yet no evidence that the phenom- 

 enon is produced by an enzyme secreted by the bacteria. The 

 lack of evidence is very likely due to the lack of proper 

 methods of study, and it is not improbable that in future years 

 a similar chemical ferment may be isolated in this case. At 

 present we must still recognize that the organized ferments 

 may probably act by two different methods. The one is an 

 active metabolism, by which the living cell breaks to pieces the 

 molecule of the fermented material, and the other is by excret- 

 ing an active enzyme which itself induces the direct chemical 

 change. They may, eventually, all be reduced to the latter 

 mode of action, but even though this should be done it would 

 not detract from the importance of the microorganism in pro- 

 ducing fermentations. 



There is one phase of this subject which puts the agency of 

 bacteria in a different light, and is leading to a feeling that in 

 the past the importance of bacteria has been in some cases over- 

 drawn. As already noticed the living cells of higher plants 

 may, and commonly do, produce enzymes which are excreted 

 as the result of their activity. We have already noticed what 

 a long list of such bodies are already known. Now, after the 

 death of the plant these enzymes remain in a condition for 

 activity for some time and may produce post-mortem changes. 

 It very commonly happens that after the death of the plant it 

 undergoes some kind of fermentative change. When piled 

 into a compost heap, or stored in a silo, the plant tissues cer- 

 tainly show unquestionable evidence of marked fermentative 

 changes. These phenomena are accompanied by a rise in 



