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 Saxzfragacee and the Crassulacee, 
or again the phylloclades of Opuntia, one of the Cactacea, 
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. Ripe 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- 
