THE PLANT AND THE ANIMAL 279 



It is probably this very heat that furnishes the plant 

 with the energy necessary for its development. Fer- 

 mentation appears to be a sort of succedaneum of 

 respiration. But this process is not as useful to the 

 plant as respiration, because for the same expenditure of 

 sugar much less energy in the form of heat is generated. 

 Formerly the process of fermentation was considered 

 essentially peculiar to the yeast fungus, but now it 

 appears that any plant or vegetable organ placed in 

 an atmosphere devoid of oxygen begins to give off car- 

 bonic acid, without absorbing oxygen, and forms 

 alcohol, i.e. begins to decompose its stores of sugars 

 and to ferment. The yeast fungus, which ferments 

 the sugars in the liquid where it develops, evidently 

 cannot suffer from this process in the same way as 

 the higher plants, which destroy their own substance 

 unproductively during fermentation. This circum- 

 stance, together with the accumulation of alcohol in 

 their cells- given off by yeast into the surrounding 

 liquid probably explains why it is that higher plants 

 cannot maintain their life by means of the process of 

 fermentation ; in the absence of oxygen all their 

 movements as well as growth come to an end, and if 

 they are kept a long time in such an atmosphere they 

 finally die. 



Therefore fermentation can only maintain the exist- 

 ence of lower organisms, and even these but for a short 

 time, because they also appear to need respiration from 

 time to time ; whereas higher organisms cannot endure 

 fermentation even for a very short time. Happily 

 natural conditions in Nature do not expose them to this 

 danger. They begin to ferment only when exposed 

 to an artificially confined atmosphere, e.g. when they are 

 enclosed under a glass bell, from which oxygen has been 

 removed in a word, when they suffocate. We cannot 

 simply say to an organism : Cease to live. It is true 

 it either lives or dies ; but while it lives it clings to 



