CALORIFIC VALUE 101 



From which it will be seen that hydrogen on combustion 

 yields relatively much more heat than carbon ; consequently 

 compounds rich in hydrogen have a high calorific value. 

 Moreover, since oxygen and nitrogen have no calorific value, 

 it follows that the presence of these elements in compounds 

 reduces their calorific value. Thus it comes about that fats 

 which contain only about II per cent, of oxygen have a con- 

 siderably higher heat of combustion than carbohydrates which 

 contain as much as 53 per cent, of this element. The actual 

 values of the heat obtainable by the combustion of some of 

 the more important fuel substances of the living cell are as 

 follows : — - 



1 gram of carbohydrate = 4'i calories 



1 ,, ,, alcohol =7-1 



1 ,, ,, fat = 9-1 



1 ,, ,, protein = 5-8 



In the procurement of energy the plant exhibits a wider 

 range than does the animal, and this to a larger extent than is 

 often thought ; thus Ramann and Bauer * have estimated 

 that young saplings of deciduous trees may show a loss of 20 

 to 45 per cent, of their dry weight during the burst of activity 

 which follows the winter sleep. 



The term respiration here is used to include all those pro- 

 cesses which involve a liberation of energy employable by 

 the organism in its various activities. Respiration is not 

 merely the absorption of oxygen and the excretion of carbon 

 dioxide, as is too often supposed, an idea having its origin in 

 the lungs of an animal being termed the organs of respiration. 

 Respiration is essentially a catabolic process, and any organ 

 of a plant or of an animal which is doing work is an organ of 

 respiration in that it cannot accomplish its task without the 

 energy obtained by the exertion of appropriate mechanisms. 

 The lungs and the respiratory tract, on the one hand, and the 

 stomates, lenticels, " respiratory chamber," and the intercel- 

 lular space system on the other, are strictly comparable : they 

 are organs of breathing ; structures, reservoirs, and surfaces 

 for the conveyance and initial absorption of oxygen, and for 



* Ramann and Bauer: " Jahrb. wiss. Bot.," 191 1, 5°> 67. 



