RESPIRATION 297 
supplied to them, which, as we have seen in an earlier chapter, 
is a condition necessary to set up changes in the protoplasm, 
respiration commences, and increases as the proportion of 
water present rises up to a certain limit. 
When the respiratory processes are carefully measured 
and compared with the weight of the organism, it is found 
that under appropriate conditions they are more intense in 
plants than even in warm-blooded animals. The respiratory 
activity is as great in many seedlings as it is in the human 
body, provided that both are maintaimed at the same tem- 
perature. There is, however, a very great variability in this 
respect, and the maximum activity is never maintained very 
long in any particular plant. As maturity succeeds to 
development its amount falls materially, bemg marked at 
or near the original rate only in the regions of the active 
meristems. 
All seedlings, again, are not alike in the vigour with 
which they carry on their respiratory processes. 
We may pass on to inquire what is the relation between 
the absorption of oxygen and the formation and elimination 
of carbon dioxide and water. It is conceivable that the 
oxygen may unite in the plant with carbon and with hydro- 
gen to produce at once the exhaled compounds. A study 
of the living organism at work, however, soon shows us 
that the process is not of this simple nature. We have 
said, in the course of what has already been advanced, that 
the amount of the carbon dioxide exhaled and that of the 
oxygen absorbed are approximately equal. This, however, 
is only true within certain limits; if each is measured 
accurately, they are not found to show an exact correspon- 
dence. The ratio CO, : O; is usually spoken of as the respira- 
tory quotient. When the two processes are equal the value 
of the respiratory quotient is unity; when the carbon 
dioxide is in excess it is greater, and when the oxygen 
is in largest amount it is less, than unity. The respiratory 
quotient has been found to vary to a greater or less extent 
in different plants, and in the same plant under different 
