464 THE AMERICAN NATURALIST. (VoL: XXXV. 
large or small according to the environment. Submersed 
aquatics will vary least, floating aquatics more, and terrestrial 
plants most in body temperature, other things being equal. But 
as the temperature of small, still bodies of water (pools, etc.) may 
vary considerably, so the body temperature of the organisms 
living therein will vary, being warmed by the sun and cooling 
during the night. The body temperature of the larger terres- 
trial plants is likely to be higher at night (except on the exposed 
surfaces), and lower in the day, than that of the surrounding 
air. Owing to the great extent of their surface as compared 
with their mass, radiation from the larger plants 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, are com- 
pelled to do so, and this extravagance is avoided. 
Heat is the form in which the energy set free by respiration 
usually makes itself evident, but it does not necessarily follow 
that only so much energy is liberated as is recognizable 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. In order 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 taking part and produced in the 
process of physiological oxidation are the highly complex 
nitrogenous and non-nitrogenous compounds elaborated by the 
organism and carbon-dioxide, water, and various small amounts 
of several ‘other. substances. Of these last, oxalic acid is the 
commonest and most important. Since the production of 
energy and not of any particular compounds is what is striven 
for in respiration, and since the substances acted upon by free 
oxygen are different in different plants and even in different 
cells of the same plants, the products differ accordingly. 
Although the oxidation of nitrogenous matter 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 compounds 
(proteids, etc.) results in the production of urea and of other 
