404 PHYSIOLOGY 
tion of plants, or of green plants at least, is exactly the reverse of 
that of animals. This misconception is due to confusing the effect 
produced upon a limited volume of air by the respiration of animals 
and by the photosynthesis of plants, two processes which are as little 
comparable in their results as are walking and eating. 
Neither gaseous exchange nor combustion. The striking change 
that most organisms produce in the air of a limited space is the reduction 
in the amount of oxygen and the increase in the amount of carbon dioxid. 
This can readily be demonstrated by putting a considerable quantity 
of germinating seeds or opening flowers into a fruit jar and sealing it for 
a few hours. On then lowering a lighted taper into the jar, the flame 
will be extinguished; and a cup of baryta water will be covered quickly 
with a film of barium carbonate. This has led to a superficial concep- 
tion of respiration, current in text-books and encyclopedias, as an ex- 
change of the gases, oxygen and carbon dioxid, between the air and 
the organism. Because in the burning of wood and other carbon com- 
pounds oxygen is consumed and carbon dioxid is produced, respiration 
has been assumed to be a process of oxidation, in which foods undergo 
" combustion " in the same sense as the fuel in a furnace, the energy 
being liberated as heat and in other forms, when the carbon of the com- 
pounds is combined with the oxygen of the air. One striking difference 
between " combustion " inside an organism and outside is that the former 
occurs at low temperatures, while the latter takes place commonly at 
high temperatures. To escape this difficulty the term " physiological 
combustion " was invented. But the conception of respiration as an 
exchange of gases accompanying oxidation of carbonaceous foods is 
inadequate, and comparing it to any sort of combustion is more mislead- 
ing than helpful. 
Aerobic and anaerobic respiration. In the first place, though or- 
dinarily oxygen is fixed, oxygen is not indispensable to respiration; 
and in the second place, though ordinarily CO 2 is evolved, carbon dioxid 
is not a necessary product and probably in no case does the O 2 combined 
with the C come directly from the air. That being so, it is obvious that 
the above-mentioned conceptions as to respiration cannot be valid. 
That respiration sometimes goes on in the absence of free oxygen, makes 
it necessary to distinguish normal or aerobic respiration and intramo- 
lecular or anaerobic respiration. 1 Aerobic respiration proceeds only 
1 Inasmuch as under the conditions one is as really normal as the other, and as the 
term intramolecular expresses an interpretation of anaerobic respiration which is no 
