RESPIRATION 121 
Respiration 
The general features of respiration were discussed in connec- 
tion with seed germination where respiration is not only promi- 
nent but also must be reckoned with in understanding the ger- 
minative process. There it was stated that respiration takes 
place only within the cell and that it is comparable to ordinary 
combustion in that it is an oxidation process resulting in the 
breaking down of substances into simpler elements with the re- 
lease of potential energy. 
It is a well known fact that whenever carbon and oxygen are 
united energy is released. This is the principle employed in heat- 
ing plants, steam engines, etc. where energy in the form of heat is 
obtained through the union of oxygen with the carbon in the coal, 
wood, or some other combustible substance. If sugars, starches 
or other substances containing carbon were used for fuel, the same 
results would be obtained. In the cell, however, since most of 
the energy released is used in protoplasmic movements, and in 
chemical changes involved in enlarging cell walls, making more 
protoplasm, etc., not much is exhibited as heat, although enough 
that all living plant parts are generally a little warmer than their 
surroundings, sometimes 2 or 3 degrees in case of large flowers 
and often much more in germinating seeds. Again, although 
the rate of respiration increases with the temperature up to a 
certain point, respiration proceeds in a lower temperature than 
does ordinary combustion. In fact, a temperature high enough 
to start the combustion of most substances is entirely too high 
for respiration, which in most plants ceases before 60° C. is 
reached. Also in respiration the process of oxidation is initiated 
and kept going by enzymes or directly by the protoplasm, while 
there are no such agents involved in combustion. Thus, although 
similar in results, in operation respiration is very different from 
combustion. 
In combustion there is a constant ratio between the oxygen 
used and the carbon dioxide produced. Thus in the combustion 
of Grape sugar, as illustrated by the formula CsHOs + 6 Op = 
2 
6 CO, + 6 H,0, the ratio so is 1. In respiration, however, 
2 
although the ratio is often unity, it varies much, sometimes being 
greater and sometimes much less than unity. In germinating 
