RESPIRATION 291 
supplied to them, which, as we have seen in an earlier 
chapter, ig 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. Therespiratory 
activity is a8 great in many seedlings as it is in the human 
body, provided that both are maintained at the same 
temperature. 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, being 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 
hydrogen 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 
respiratory quotient. When the two processes are equa] 
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 
