RESPIRATION 17 



be large or small according to the environment. Submerged 

 aquatics will vary least, floating aquatics more, and ter- 

 restrial plants most ; but as the temperature of small, still 

 bodies of water (pools, etc.) varies considerably, so the 

 body-temperature of the organisms living therein will vary, 

 warmed by the sun and cooled during the night. The body- 

 temperature of the larger terrestrial plants is likely to be 

 higher at night ( except in the exposed surfaces ) and lower 

 in the day, than that of the surrounding air. Owing to the 

 very great external surface of the larger plants in propor- 

 tion to their mass, radiation from them is rapid, and a 

 body-temperature independent of their environment could be 

 maintained only at great expense of material laboriously 

 collected and elaborated. Plants work economically, must 

 do so, and such extravagance is avoided. 



Heat is the form in which the energy set free by respira- 

 tion usually makes itself evident, but it does not necessarily 

 follow that only so much energy is liberated as is recogniz- 

 able as heat, or that this is the only form in which energy 

 is liberated. Only that energy becomes evident as such 

 which is not at once used. To determine the amount of 

 energy liberated in respiration, it is necessary to know and 

 to measure the material products of respiration. 



The substances ordinarily engaged in the process of physi- 

 ological oxidation are the highly complex nitrogenous and 

 non-nitrogenous compounds elaborated by the organism. 

 The ordinary products are carbon-dioxide, water, and 

 various small amounts of several other substances, e. g. 

 oxalic acid. Since the production of energy rather than of 

 any particular compounds is what is striven for in respira- 

 tion, and since the substances acted upon by free oxygen 

 are different in different plants and cells, the products will 

 differ accordingly. 



Although the oxidation of nitrogenous matters also takes 

 place, it is mainly the non-nitrogenous contents of the living 

 cell which are involved in physiological oxidation. In the 

 animal body, the oxidation of organic nitrogenous com- 

 pounds ( proteids ) results in the production of urea and of 

 other similar substances no longer usable and presently 

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