loo The Living Plant 



CqR,A = 2 CO, + 2 CsH.O 



And this equation expresses exactly the known facts of the 

 process. 



Wliat now is the meaning of fermentation, and why does the 

 Yeast do it? Nowhere in Nature, so far as I can find, excepting 

 in the case of humanity, is there even the least evidence that any 

 kind of organism ever does anything whatever for the sake of 

 service to any other kind. We should not expect to find, accord- 

 ingly, that the Yeast makes the carbon dioxide and alcohol for 

 any disinterested or philanthropic purposes, — not for providing 

 thrifty housewives with light bread or their shiftless husbands 

 with strong drink, — and we turn to seek some desirable object of 

 its own to which the use by mankind is purely incidental. But 

 of course, the reader has inferred the explanation before this, — ■ 

 fermentation is simply the Yeast's respiration, the source of its 

 power for growth and other work that it does. And the explana- 

 tion of so peculiar a form of respiration is well known. Living im- 

 mersed in a liquid, the Yeast cannot obtain respiratory oxygen 

 from the air, and must take it from some other source. Only one 

 source is available. Locked up in the molecule of sugar is some 

 oxygen brought into it with the hydrogen, which holds it away 

 from the carbon, as the formula C0H12O6 suggests. But the Yeast 

 plant, absorbing the sugar into its body, shatters the molecules 

 (by means of a peculiar agency called an enzyme soon to be 

 described), and allows the carbon and oxygen in the fragments 

 to unite with one another; this produces the usual result, — a 

 copious release of energy which the Yeast at once utilizes for its 

 growth, while of course the resulting carbon dioxide is thrown off 

 into the liquid. This is the object, or meaning, of fermentation; — 

 to secure a union of carbon and oxj'^gen for the sake of the energy 

 which is always thus released. As to the alcohol, that is simply 

 the remains of the shattered molecule; it is a chemical fact that 

 the number of atoms of carbon, hydrogen and oxygen which hap- 

 pen to be left after the carbon dioxide is formed, fall naturally 



