292 VEGETABLE PHYSIOLOGY 



different conditions. If its value is determined in the case 

 of germinating seeds, these differences are soon evident, 

 With starchy seeds the quotient is unity ; with oily seeds 

 it is much lower. That is, in the former case the seeds 

 absorb a volume of oxygen equal to that of the carbon 

 dioxide they exhale ; in the latter case they take up more. 



Various observers have shown that in certain cases 

 succulent leaves, such as those of the Agave or of particular- 

 plants belonging to the SaxifragacecB and the Crassulacece, 

 or again the phylloclades of Opuntia, one of the Cactacece, 

 are capable of absorbing oxygen without the simultaneous 

 exhalation of carbon dioxide. Nor is the oxygen absorbed 

 in these cases any more than it is in others without enter- 

 ing into some form of chemical combination, for it cannot 

 be extracted by the air-pump. The latter also fails to 

 extract any carbon dioxide from the plants. The oxygen 

 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 given 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 liberated. 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 

 dependent upon each other. 



In making the estimation of the respiratory inter- 

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



