EESPIKATION 291 



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 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 2 : is usually spoken of as the 

 respiratory 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 



