22 PLANT PHYSIOLOGY 



effected to secure oxygen for oxidation of other substances, 

 but the decompositions themselves release as kinetic the 

 potential energy which was needed to hold the complex 

 substances together. 



The mutual attraction of one atom of carbon and two of 

 oxygen is so great that the molecule of carbon-dioxide 

 is very stable as well as very simple, for the "affinities" 

 of the carbon and oxygen are "satisfied." In such complex 

 compounds of carbon, hydrogen, and oxygen as the 

 starches, sugars, etc., the affinities of the component ele- 

 ments are not satisfied ; the compounds are much less stable, 

 as is shown by their ability to take up more oxygen. At 

 ordinary temperatures, however, and under ordinary condi- 

 tions, these compounds are stable. They owe their stability 

 to the mutual affinities of their component atoms, which 

 exert an attraction upon one another sufficiently powerful 

 to hold them together in definite form. When the atoms 

 are separated from one another and arrange themselves 

 closer together in simpler forms in space, their "bonds" or 

 "affinities" are more completely "satisfied," they unite more 

 perfectly, oxidation takes place in the rearrangement, and 

 energy is accordingly liberated and made available for other 

 purposes. Energy is "stored" in the starch, or sugar, or 

 oil molecule; the kinetic energy (solar or other) employed 

 in the construction of the molecule remains in it as poten- 

 tial energy, holding the atoms together. The destruction of 

 the molecule results in the liberation of so much kinetid 

 energy as was employed in constructing it from the simple 

 compounds worked upon. 



The complete oxidation or combustion of a gram of 

 dextrose (sugar), resulting in the formation of carbon- 

 dioxide and water as represented in the following reaction 



C 6 H,, O fl + 6 O a = 6 C0 2 + 6 H, O 



liberates 3,939 small or ordinary calories, or mechanical 

 units of energy in the form of heat. * ( A calorie ( c ) is the 



* Rechenberg, C. von. Uber die Verbrennungswarme organischer Ver- 

 bindungen. Inaug. Diss., Leipzig, 1880. See also Pembry, M. on 

 Animal Heat in Schafer's Text-book of Physiology, vol. I., Edinburg, 

 London, New York, 1898. Here the literature is fully given. 



