286 Plant Physiology 



many physiologists respiration has long been considered 

 to be a combustion process. If by combustion one under- 

 stands a direct union of the O 2 with C in such a manner 

 that CO 2 is a direct result, this comparison is unfortunate. 

 The combustion of any product results in a perfectly 

 definite amount of energy as heat; and this heat-energy 

 may be very simply determined, whether it involves the 

 combustion of coal, of proteins, of starch, or of cane- 

 sugar. There is, furthermore, in combustion (however 

 produced) a definite relation between the amount of oxy- 

 gen needed and the amount of carbon dioxid given off. 

 There is therefore a definite CO 2 / O 2 ratio ; thus the 

 combustion of glucose would require 6 molecules of O 2 , 

 and 6 molecules of CO 2 would be produced, by the follow- 

 ing formula : 



C.H u Oa + 6 2 = 6 C0 2 '+ 6 H 2 0. 



The combustion quotient in this case is unity. In respi- 

 ration the transformations are not necessarily complete, 

 and the respiratory quotient is seldom exactly unity, and 

 as a consequence there are by-products and products 

 less stable then CO 2 . The quotient is affected by tem- 

 perature and other environmental conditions; thus it is 

 possible to picture a more complex and less definite se- 

 quence of changes in which protoplasm is involved. The 

 series of transformations may be of the same type in the 

 two cases, but they are not properly regarded as compa- 

 rable processes so far as may be determined at present. 

 166. Respiratory activity. Respiratory activity is 

 greatest during periods of rapid growth and differentiation. 

 So soon as adequate water is absorbed by seeds previously 



