[Chap. XVII RESPIRATION 153 



We shall see later that the heat energy liberated by respiration in green 

 plants is usually insignificant in the life of the plant. The intermediate 

 transformations of energy and substances must be the essential ones in 

 respiration. 



Sometimes the free energy at the end of the process of respiration is 

 not entirely heat energy. For instance, at definite periods in the firefly 

 it is largely light. Likewise the phosphorescent light of certain bacteria, 

 fungi (Fig. 263), and deep-sea fishes is a result of energy transforma- 

 tions during respiration. 



Both the liberation of energy and the decrease of dry weight have been 

 used in numerous experiments as a means of determining the occurrence 

 and rate of respiration in plants. It is well known among botanists that 

 respiration occurs continuously in all active living cells of plants just as 

 it does in the cells of animals, and that when deprived of a source of 

 oxygen green plants will eventuallv suffocate just as animals do. This 

 fact might have been deduced from the similarity of protoplasm in plants 

 and animals, for after all it is the processes in protoplasm that are de- 

 pendent upon respiration. 



Perhaps dry dormant seeds and spores may remain alive for a time 

 without respiration — at least without detectable respiration. Experi- 

 ments have shown that dry seeds may remain alive in a vacuum for 

 months. It is often assumed that some respiration is occurring in these 

 seeds, but satisfactory evidence is lacking. 



Respiration and the escape of carbon dioxide. The escape of carbon 

 dioxide from plants is also used as external evidence of respiration in 

 them. The amount that escapes within a given time may be measured 

 and considered to be a measure of the rate of respiration, though this 

 method has certain limitations. 



The first of these limitations should be obvious from our knowledge 

 of the utilization of carbon dioxide in photosynthesis. During certain 

 periods of the day carbon dioxide may be utilized in photosynthesis 

 in green cells 15 to 20 times as rapidly as it is liberated by respiration. 

 Carbon dioxide that is released by respiration within or near chloroplasts 

 exposed to light is probably immediately used in the making of sugar, 

 and little, if any, escapes to the atmosphere. Even in the green cell as a 

 whole, carbon dioxide enters far more rapidly than it escapes; hence 

 its concentration in the surrounding air decreases and that of oxygen 

 increases even though respiration is occurring in the green tissues. This 



