238 PHYSIOLOGY OF NUTRITION 



the latter being reduced to ethyl alcohol. The hydrogen acceptor is not used up in the 

 process. Reductase can not act unless both an oxidizer and a reducer are present. 



3. Other Kinds of Fermentation. — Lactic acid fermentation (the ordinary souring of 

 milk) is caused by the lactic acid bacillus, which grows in the presence of oxygen, and 

 the same process is carried on by many other forms. It results in the hydrolytic 

 splitting of lactose (C12H22O11) into four times as many molecules of lactic acid 

 (C3H6O3), one molecule of water being consumed for each molecule of lactose decom- 

 posed. Other sugars (such as saccharose and maltose) are similarly decomposed with 

 the formation of lactic acid, by other microorganisms. 



Butyric acid fermentation occurs in the absence of oxygen and results in the forma- 

 tion of carbon dioxide and hydrogen, as well as of butyric acid. It is caused by certain 

 forms of bacteria, especially species of Clostridium. Either glucose (C 6 Hi 2 6 ) or 

 lactic acid (C 3 H 6 3 ) may be fermented in this way. 



Very many other fermentation processes are known, due to numerous different 

 forms of bacteria, etc., each organism being limited to certain kinds of fermentation. 



4. Conditions Influencing Aerobic Respiration in Plants. — Ingen-Housz (1779) 

 discovered that oridnary green plants respire like animals, taking in oxygen and giving 

 off carbon dioxide, and DeSaussure made the first quantitative study of this process. 

 The rate of gaseous exchange is greater for higher temperatures, up to about 40°C, 

 above which the rate remains about the same until death occurs. Any change of 

 temperature accelerates respiratory activity for a time. The value of the respiratory 

 ratio (the amount of carbon dioxide given off divided by the amount of oxygen 

 absorbed in a given time period) is low (about .35 to .40) for temperatures about io° or 

 i5°C, and is progressively higher for either progressively higher or progressively lower 

 temperatures. For temperatures about 35°C. this value was found to be .95. 



Aerobic respiration in chlorophyll-bearing cells is apparently related to carbo- 

 hydrate photosynthesis (which may be considered as the reverse of respiration), 

 especially on account of the dependence of respiration on water-soluble carbohydrates. 

 Of course the carbon dioxide produced by respiration in green leaves in sunlight is 

 regularly fixed by the photosynthetic process. During periods of sunlight (other 

 conditions being suitable) no carbon dioxide passes from green leaves to the surround- 

 ing air and no free oxygen enters the leaves; photosynthesis is then more active than 

 respiration, and the net result of both processes is absorption of carbon dioxide and 

 elimination of oxygen. The direct influence of light on the respiration rate may be 

 readily studied in organisms and tissues without chlorophyll. 



The partial pressure of oxygen in the surroundings influences the rate of respiration, 

 since this pressure affects the rate of oxygen supply to the respiring cells. The supply 

 of water and the osmotic value of the cell sap and of the environmental solutions, 

 influence the respiration rate, as do also the rates of supply or partial concentrations 

 of many other substances. Here may be mentioned phosphates, and many alkaloids, 

 ethers, alcohols, aldehydes, etc. Respiration is accelerated by the presence of poisons 

 (such as alcohols, ether) if these substances are supplied in the right concentration, 

 which must be very weak. Wounding accelerates aerobic respiration. 



The aerobic respiration rate in ordinary plants is closely related to the rate of en- 

 largement; as the latter rate increases and then decreases, during the grand period of 

 growth, the respiration rate alters in a similar manner. Actively enlarging tissues 

 generally absorb somewhat more oxygen than they give off in the carbon dioxide 



eliminated; the value of their respiration ratio (^7) is less tnan unity. 



