production of this gas, by the decomposition of car- 
bonic acid, occurs in temperatures unequal to the 
support of vegetation. These numerous and striking 
dissimilarities must be allowed to create an essential 
difference between that operation of plants, by which 
oxygen gas is converted into carbonic acid, and that 
by which this acid is re-converted into oxygen gas. 
383. Now the former process, by which oxygen 
is consumed, must be regarded as necessary to vege- 
tation, because without air plants do not grow ; and 
no other known change is produced in this air .than 
the conversion of its oxygenous portion into carbonic 
acid gas. This conversion, also, is connected with 
the living powers of the plant ; for when these powers 
are suspended by cold, the plant no longer continues 
to vegetate, and the oxygen of the air is then no 
longer changed. It proceeds, likewise, at all times, 
and in all situations ; so that if a circumstance which 
universally accompanies vegetation, and without 
which it cannot continue, be entitled to be consider- 
ed a necessary condition of that function, then the 
conversion of oxygen gas into carbonic acid by li- 
ving vegetables must be so considered. 
334. But, on the other hand, the production of 
oxygen gas, by plants exposed to the sun, is not, ne- 
cessarily, accompanied by any signs of vegetation $ 
neither does it require the conditions, which are es- 
sential to the exercise of that function. Thus, al- 
though air is indispensable to vegetation, yet various 
experiments prove that oxygen gas is afforded by the 
leaves of plants, when they are immersed in water. 
It may be said, that the air, existing in the water, is, 
