DESTRUCTIVE METABOLISM 405 
when O 2 is present in sufficient quantities, and among the end products 
two, CO 2 and H 2 O, are characteristic, though formed in very variable 
quantities in proportion to the O 2 taken up. Anaerobic may replace 
aerobic respiration in any organism when O 2 is cut off, and may proceed 
for a long time; but the end products are various and quite different 
from those of aerobic respiration. Among them are commonly ethyl 
alcohol and hydrogen, and less CO 2 . Certain minute organisms may 
pass their whole existence without oxygen, which indeed hinders or alto- 
gether stops their development, and they are thus restricted to anaerobic 
respiration. In most organisms, however, anaerobic respiration can 
be considered only as a makeshift. 
Nature. What then is the fundamental feature of a process that 
goes on under such different conditions and results in such diverse prod- 
ucts ? So far as now appears, respiration consists in the decomposition 
of the protoplasm or some of its constituent proteins, either directly, 
or as a result of the action of an enzyme or of some internal force (stim- 
ulus) upon it. Inasmuch as the inciting cause is rarely apparent, spon- 
taneous or self-decomposition is often spoken of, but this merely means 
that the reason is unknown. 
The view here presented is not the one most generally held at present, but appeals 
to the author as most consistent with the known facts. Many physiologists consider 
respiration to consist primarily in the decomposition of foods by the protoplasm 
or by enzymes, without their assimilation into the living substance. In this case 
foods are a kind of fuel for the body (see p. 406). It is not denied that some de- 
composition of protoplasm occurs, but this is slight; as it were, a sort of natural 
wear and tear in consequence of work. 
Advantage. The advantage of respiration is not certainly known, 
but as the plant in order to do work must expend energy, the inference 
is that respiration sets free energy by which that work is performed. 
Now complex and unstable compounds contain much available potential 
energy, the store of which is diminished when they decompose, and the 
essence of nutritive processes is the building up of those compounds 
which disappear in respiration. Furthermore, heat, one easily observed 
form of energy, is generated by respiration, though it is not known that 
this is of any service to the plant. But the most definite reason for con- 
necting the release of energy with respiration is that those tissues in 
which growth or other work is proceeding rapidly are also characterized 
longer tenable, the words aerobic and anaerobic (aer, air; bios, life; an, not), applied 
first to organisms that live in air or flourish only when it is excluded, are preferable. 
