RESPIRATION 299 
In making the estimation of the respiratory interchanges 
we are apt to lose sight of a fact to which attention has 
already been called, viz. that carbon dioxide is not the only 
respiratory exhalation. The watery vapour which accom- 
panies if must also be accounted for. On the hypothesis 
of the direct oxidation of carbon and hydrogen, if the volume 
of carbon dioxide is equivalent to that of the oxygen, there 
cannot have been the absorption of sufficient of the latter to 
unite with hydrogen to form the water. Even when the 
respiratory quotient is less than unity, the same considera- 
tion has a certain value. The idea of such direct oxidation 
cannot, therefore, be accepted. 
It is evident from the foregoing considerations that the 
vital activity of the protoplasts is somehow associated 
with the two factors in the gaseous interchange. In the 
absence of oxygen this vital activity gradually ceases, the 
living substance being in fact slowly stifled or asphyxiated. 
During its life one of the manifestations of its vitality is 
the formation and exhalation of two fairly simple com- 
pounds, carbon dioxide and water. To ascertain what is 
the true relation of the two processes, it is necessary to 
look closely at the nature of the chemical changes going 
on in the’ protoplasm itself, or what is usually spoken of as 
its metabolism. 
Respiration in the strict sense is therefore a process 
going on in the living substance itself. The gaseous inter- 
change observed is the expression of the beginning and the 
end of a series of complex changes in which the molecules 
of the living substance are involved. The details of the 
absorption of the oxygen of the plant from its environment, 
and its presentation to the protoplasm, together with those 
of the ultimate exhalation of the carbon dioxide and water 
from the plant-body, should be regarded rather as belonging 
to the mechanism of respiration than to respiration itself, 
which is a function of the living substance only. The former 
‘corresponds to the entry of the oxygen into the lungs of 
an air-breathing mammal, and its transport to the tissues, 
