99 
conditions essential to the exercise of that function, 
it cannot be regarded as an operation necessary to 
the existence of the plant, or dependent on its living 
powers, but must be considered as a secondary and 
subordinate office, the performance of which is entire- 
ly governed by the accidental circumstances in which 
the vegetable may happen to be placed. 
339. But if it be thus granted, that the operation, 
by which plants afford oxygen, may go on in circum- 
stances where the powers of vegetation are not only 
suspended, but where they are unable to exist, then 
it also follows, that the entrance of gaseous fluids 
into plants, by which that operation is supported, is 
not necessarily to be considered as the result of a vi- 
tal function. These gases have been shewn (318.), 
in certain cases, to enter plants with the fluids which 
they absorb from the soil ; and if this, as is probable, 
be the ordinary way in which they gain admission, 
then we may say generally, that gases are absorbed 
by plants. But as these fluids may likewise be -re- 
ceived into plants, although they hold no gases in so- 
lution, and vegetation may then also go on, the ab- 
sorption of gases cannot, in this sense, be deemed a 
proper and necessary function of the plant. 
340. Farther, gaseous bodies enter into plants, in 
an elastic form, and, as we have seen (300.), under 
circumstances fatal to the exercise of their living 
powers ; in which case, their admission must depend 
on the operation of a chemical, or mechanical cause. 
Do we then possess evidence that such an affinity 
subsists between vegetables and certain gases, as is 
sufficient to explain the phenomena which have been 
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