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it may be useful to bring together and contrast the 
peculiarities which distinguish, respectively, the con- 
sumption and the production of oxygen gas by living 
plants ; for whether we regard the agents employed, 
the mode and circumstances in which . they act, or 
the results which are afforded, there are no two pro- 
cesses, which, in many respects, present phenomena 
more dissimilar. Both in the shade and in sunshine, 
however, oxygen gas is essential to vegetation ; but 
in one situation, it is converted into carbonic acid ; 
in the other, this acid is re-converted into oxygen 
gas. Without the presence of oxygen, the living 
plant is unable to survive ; but an atmosphere of car- 
bonic acid speedily destroys its life. When oxygen 
is consumed, the union of that gas with carbon ap- 
pears to take place exterior to the vascular system 
of the plant ; when the same gas is produced, the 
decomposition of carbonic acid is effected in the cel- 
lular structure of the leaf. To the continuance of 
living action the formation of carbonic acid seems to 
be essential ; but its decompo'-ition is, in no respect, 
necessary to the life of the plant. The one operation 
proceeds continually, by day and by night, in sun- 
shine and in the shade ; the other takes place only 
at intervals, and while the plant is exposed to the 
sun. Where, without light, oxygen is consumed, 
the plant lives and grows, but its colour and proper- 
ties are impaired ; where light is present, and oxy- 
gen is produced, the colour and other properties of 
the vegetable attain the greatest perfection. Lastly, for 
the conversion of oxygen gas into carbonic acid by 
plants, a certain temperature is required 5 but the 
