298 VEGETABLE PHYSIOLOGY 



conditions. If its value is determined in the case of ger- 

 minating 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 suc- 

 culent leaves, such as those of the Agave or of particular 

 plants belonging to the Saxifragacece 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 entering 

 into some form of chemical combination, for it cannot in 

 any case 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 com- 

 bined ; the other process which usually accompanies this 

 absorption 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. Eipe 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 observa- 

 tions, 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. This is explained by the fact that both the absorption 

 of oxygen and the exhalation of carbon dioxide vary with 

 differences of temperature, but they vary in different 

 degrees. Evidently the two processes are not directly 

 dependent upon each other. 



