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 Sawxifragacee and the Crassulacee, 
or again the phylloclades of Opuntia, one of the Cactacee, 
are capable of absorbing oxygen without the simultaneous 
exhalation of carbon dioxide. Nor is the oxygen absorbed 
in these cages 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. 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 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. 
