EESPIEATION 125 



enters the plant, and is in some way fixed or combined ; 

 the other process which usually accompanies this absorp- 

 tion does not take place, the carbon dioxide not only not 

 being exhaled, but apparently not even formed. 



Conversely, carbon dioxide may be thrown off from a 

 plant without any simultaneous or even antecedent absorp- 

 tion of oxygen. When a seed is made to germinate in a 

 vacuum over a column of mercury, carbon dioxide is 

 found to be evolved. Kipe fruits have been found to give 

 off this gas in an atmosphere quite devoid of oxygen. 

 Too much stress must not, however, be laid upon these 

 latter observations, as we have certain evidence which 

 points to a different mode of formation of the carbon 

 dioxide in the presence and in the absence of oxygen 

 respectively. 



Again, it is found that the respiratory quotient varies 

 according to the temperature at which the observations are 

 made. Evidently the two processes are not directly depen- 

 dent upon each other. 



In making the estimation of the respiratory inter- 

 changes we are apt to lose sight of a fact to which atten- 

 tion has already been called, viz. that carbon dioxide is 

 not the only respiratory exhalation. The watery vapour 

 which accompanies it 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 consideration 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 



