152 TEXTBOOK OF BOTANY 



sidered because it does not occur in plants. Body temperature ordinarily 

 cannot be used because the temperature of a plant is usually near that 

 of the surrounding air. The parts of a plant exposed to the sun and 

 very rapidly growing parts may be warmer than the surrounding air. 

 Manure heaps and other masses of decaying organic matter may become 

 hot as a result of the energy liberated by the respiration of the numerous 

 bacteria in them. Stacks of green hay and freshly harvested grain may 

 likewise become hot, partly as the result of the respiration in the living 

 cells of the hay or grain, but mainly because of the respiration of the 

 bacteria that are present. For a clear demonstration of the liberation of 

 heat energy by respiration in plants it is necessary only to enclose them 

 in a thermos bottle into which a thermometer extends. 



Since the oxidation of food results in a loss in drv weight, the changes 

 in dry weight of comparable examples of plants during a given period 

 of time mav also be used to determine the amount of respiration that 

 occurs in them. From preceding facts and from the equation below 

 it should be obvious that this method can be used for green tissues only 

 in the dark, but isolated non-green tissues may be thus tested in either 

 light or darkness. The rate of respiration in different samples of germi- 

 nating seeds is easilv measured by this method. Since sugar is the most 

 frequently oxidized food in respiration, we may illustrate the loss of dry 

 weight by a simple equation : 



Sugar (bound energy) + Oxygen > j- -j + Water -f 



^>yA r> ^ ■ dioxide energy 



674 Calories ^"^ 



180 gm. CeHizOc + 192 gm. O2 > 264 gm. C0« -f 108 gm. HoO + 674 Cal. 



Note that while 192 grams of oxygen pass into the plant, 372 grams 

 of CO2 and H2O are liberated, a net loss of dry weight of 180 grams, 

 which is just the weight of the sugar oxidized. 



This generalized equation is the converse of that of photosyntliesis, 

 except that the free energy in respiration is heat energy instead of the 

 radiant energy of light. It should always be remembered, however, that 

 such equations indicate only the initial substances and the ultimate prod- 

 ucts. In most biological processes there are complicated intermediate 

 steps involving enzymes, intennediate products, and intermediate trans- 

 formations of energy that may be the real essence of the process. The 

 intermediate processes and products in respiration and photosynthesis 

 are not exact opposites. 



