BESPIRATION 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 maintained 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, being marked at 

 or near the original rate only in the regions of the active 

 meris terns. 



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 C0 3 : Oa 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 



