536 VITAL ACTIONS OF THE LOWER FUNGI. 



temperature of about 60 C., and when the reaction is. 

 acid and comparatively large quantities of peroxide 

 of hydrogen, carbolic acid, oil of turpentine, &c., 

 hardly weaken their action, while under the same 

 conditions we constantly observe a complete cessation of 

 the life, or, at any rate, of the fermentative activity of 

 all micro -organisms. 



7. Fermentation. 



Definition of Under certain circumstances we find a deviation in 

 on- the biological behaviour of micro-organisms, which is 

 accompanied by a very thorough decomposition and con- 

 sumption of the nutrient material, and the formation of 

 special products, characterised by their quality and 

 quantity. Among these products, volatile gases usually 

 take a prominent place ; and in addition bodies are con- 

 stantly formed of less heat-producing power than those 

 materials from which they are derived, and thus in the 

 process of decomposition there is constantly a liberation 

 of vital energy. The sum total of these phenomena is 

 usually designated by the term " fermentation." 



Fermentation must also, at any rate ultimately, be 

 referred to decompositions in the protoplasm, to the 

 intramolecular respiration. We may suppose that the 

 act of fermentation was evolved from this respiration, and 

 that it originally only represented an act of self-preserva- 

 tion due to the continued absence of oxygen. Without 

 active combustion by the aid of oxygen, the internal 

 respiration alone is not able to furnish sufficient energy 

 for the purposes of the organism. When oxygen is 

 absent, the nutrient material is split up superficially, it 

 is true, but so much the more extensively, and thus 

 the energy which is requisite for the complete life of the 

 micro-organisms is obtained. In the course of their further 

 evolution the organisms become divided into two great 

 groups : in the one, the fermentative action only occurs when 

 there is necessity for it owing to the absence of oxygen ; 

 in the other, the fermentative action which corresponds 

 to those special conditions has been gradually developed 



