FERMENTATION. 505 



is nothing left for them except to obtain it from the orgcanic compounds of their 

 own bodies. This cannot be carried on permanently, and if the frog is kept for 

 a long time in an atmosphere without oxygen, it will at length die. For a short 

 period, however, it is able to proloug its life in the way indicated. Exactly the 

 same thing is seen in plants. When placed in a chainlier from which free oxygen 

 is absent, they do not immediately die, but endeavour for a short time to retain 

 their life by utilizing combined oxygen, by withdrawing it from nitrates which 

 have been absorbed with food, or from the organic compounds of their own 

 bodies, richly fui'nished with oxygen. The oxygen obtained in tliis way is able 

 to replace that usually derived from the environment, and can also bring about a 

 combustion of carbon compounds; it can therefore provide the kinetic energy neces- 

 sary for the continuance of life. Carbon dioxide is then exhaled from plants, even 

 in an atmosphere without oxygen, and heat is liberated just as in normal respira- 

 tion. But this abnormal source of energy does not last very long. If free atmos- 

 pheric oxygen continues lacking, the plants exposed to such unaccustomed 

 conditions at length perish from exhaustion and suffocation. 



But it is also possible that living plants may exist in a region which is indeed 

 devoid of free oxygen, but in which combined oxygen is present. Let us suppose 

 that a plant, hitherto surrounded by atmospheric air from which it obtained free 

 oxygen for use in respiration, has been plunged into a sugar solution, in wliich, 

 of course, free oxygen is absent, but which contains a large quantity in combina- 

 tion with carbon and hydrogen in the form of sugar. Would such a plant be able 

 to wrest the oxygen from the sugar and to utilize it for itself ? In most cases 

 certainly not. But in a few instances the living protoplasm has the power of 

 splitting up the fluid oxygen -containing compounds with which it comes into 

 contact, and can so obtain the oxygen necessary for the continuance of its life. 

 It can also make use of other materials liberated from combination in the decom- 

 position. This process has the greatest resemblance to respiration, carbon com- 

 pounds are actually burnt with the help of the derived oxygen; carbon dioxide is 

 exhaled, and heat is liberated. The plant, the living protoplasm of which accom- 

 plishes all this, maintains itself alive, prosper.s, and even grows and multiplies in 

 a surprising manner. This process, however, is not called respiration, but is known 

 as fermentation. 



Of course the plants producing fermentation must not be supposed to include 

 large leafy structures. On the contrary, they are all very insignificant and belong 

 exclusively to spore-plants which are devoid of chlorophyll, and which are gener- 

 ally classed together under the name of fungi. In particular there are the four 

 allied families. Bacteria, Yeasts, Moulds, and Basidiomycetes, of which many species 

 in certain stages of development are capable of inducing fermentation. 



Bacteria, which are also called Fission-Fungi or Schizomycetes, are the smallest 

 of all living organisms, and the question has repeatedly arisen as to whether they 

 are to be regarded as independent organisms, or as organized portions of dead, de- 

 composing protoplasm. The discussion of this question will be left to the second 



