SECT. II PHYSIOLOGY 247 



This feature of apparently great waste of material is found in 

 other fermentations, in which by a rapid breaking down of carbo- 

 hydrates and proteids the fermenting organisms obtain a supply of 

 energy. It is impossible to draw a sharp line between those decom- 

 positions which go on without the assistance of atmospheric oxygen 

 and those in which oxygen plays a part. We are obliged to class as 

 fermentations all those metabolic processes by which energy is obtained, 

 which diiTer from typical oxygen respiration. In this sense the oxida- 

 tion of alcohol to acetic acid effected by the acetic acid bacteria and 

 also the production of acids in the higher plants, especially in succulent 

 plants (p. 243), would be fermentations. Lastly, the processes of de- 

 nitrification and of reduction of sulphates, in which anaerobic bacteria — 

 probably in order to obtain oxygen — reduce nitrates to free nitrogen 

 and sulphates to sulphuretted hydrogen, cannot be excluded from 

 fermentations. 



Many fermentations have another significance besides that of 

 obtaining energy. The products of fermentation such as alcohol, 

 acids, etc., are poisons ; they are, as a rule, more injurious to other 

 organisms than they are to those Avhich produce them. On this 

 account they are suited to exclude other organisms from the supply 

 of food material. It is true that a fermentation organism in a pure 

 culture on a definite substratum renders, by the products of its meta- 

 bolism, the latter not only unsuitable to concurrent organisms but 

 sooner or later for itself. When organic material, as is the case in 

 nature with the remains of dead organisms, is the prey of various 

 micro-organisms these co-operate in their action ; metabolic products 

 of one kind of micro-organism are further decomposed by others until 

 the organic compounds are converted into inorganic or mineral sub- 

 stances. The final products are water, hydrogen, methane, ammonia, 

 nitrogen, and sulphuretted hydrogen, and all of these can be oxidised 

 or assimilated by some other organisms. It is only by this co-operation 

 of all organisms that life is maintained on the earth and the elements 

 again brought into circulation. If only one type of organism existed, 

 it would in a short time have destroyed the possibility of its own 

 existence by its one-sided metabolism. 



D. Production of Heat and Light in Respiration 

 AND Fermentation 



Heat. — Since typical Respiration is a process of oxidation, it is easy 

 to understand that it is accompanied by an evolution of heat. That 

 this evolution of heat by plants is not perceptible is due to the fact 

 that it is not sufficiently great, and that considerable quantities of heat 

 are rendered latent by transpiration, so that transpiring plants are 

 usually cooler than their environment. 



The spontaneous evolution of heat is easily shown experimental!}-, if transpira- 



