154 TEXTBOOK OF BOTANY 



apparent anomaly of course does not occur in the green tissues in the 

 dark. 



Twice during the day, once in the morning and again in the evening, 

 the intensity of hght must be such that the rate of photosynthesis just 

 equals that of respiration. The rates of photosynthesis and respiration 

 may also be equal temporarily under certain other environmental condi- 

 tions such as dark cloudy weather and high temperatures. These condi- 

 tions are frequently present in greenhouses during the winter months; 

 during the darkest days photosynthesis may be less than respiration. 



The relative amount of carbon dioxide formed during respiration is 

 related to the kind of food that is being oxidized. For instance, when 

 fats are being oxidized in plants the first steps in the process may result 

 in the oxidation of fats only to sugar, with little or no release of carbon 

 dioxide. The sugar may then be oxidized to CO2 and H2O. This fact may 

 be experimentally demonstrated during the germination of seeds con- 

 taining considerable fat. When fat is completely oxidized the volume of 

 oxygen consumed is much greater than the volume of carbon dioxide 

 released, whereas when only sugar is oxidized the volume of oxygen con- 

 sumed is just equal to the volume of carbon dioxide released. Conse- 

 quentlv the kind of food being oxidized in living organisms may often 

 be detected by placing them in air-tight enclosures and measuring the 

 amount of oxygen consumed in proportion to the amount of carbon 

 dioxide released. If the ratio is about one, the oxidation of sugar is indi- 

 cated; if it is much less than one, the oxidation of fats or even proteins 

 is indicated; however, proteins are seldom oxidized appreciably so long 

 as sugar or fat is available.^ 



The discovery that this ratio varies with the kind of food available in 

 the cells was one of the first definite contradictions of the oft-quoted 

 traditional belief that protoplasm is the substance that is oxidized in 

 respiration and must therefore be constantly repaired. There is no evi- 

 dence that protoplasm is destroyed by respiration or that it needs repair 

 in a healthy cell well supplied with food. 



^ While the ratio described above may be used as a general rule, it should not be used 

 as an infallible one. Certain well-known exceptions to it occur in plants. These are mostly 

 cases in which the sugars and fats are not at once completely oxidized to CO, and H„0. 

 The lowest ratios are obtained when substances like the fatty acids, which have relati%ely 

 little oxygen in them, are oxidized in respiration; for example, the oxidation of palmitic 

 acid as shown in the following equation: 



C16H32O2 + 23 O2 > 16 CO2 -h 16 H2O 



