ATMOSPHERIC AIR AS THE FOOD OF PLANTS, 43 
ean flourish only when its foliage is bathed by an atmos- 
phere which contains a certain small amount of carbonic 
acid; 2d, that this gas is absorbed by the leaves, and, un- 
der the influence of sunlight, is decomposed within the 
plant, its carbon being retained, and in an unknown man- 
ner becoming a part of the plant itself, wile the oxygen 
is exhaled into the atmosphere in the free state. 
Relative volumes of absorbed Carbonic Acid and ex- 
haled Oxygen.—From the numerous experiments of De 
Saussure, and from similar ones made recently with greatly 
improved means of research by Unger and Knop, it is es- 
tablished that in sunlight the volume of oxygen exhaled 
is nearly equal to the volume of carbonic acid absorbed. 
Since free oxygen occupies the same bulk as the carbonic 
acid produced by uniting it with carbon, it is evident that 
carbon mainly and not oxygen to much extent, is retained 
by the plant from this source. 
Respiration and Fixation of Carbon by Plants,—In 
1851 Garreau, and in 1858 Corenwinder, reviewed experi- 
mentally the whole subject of the relations of plants to 
carbonic acid. Their researches fully confirm the conclu- 
sions derived from older investigations, and furnish some 
additional facts. 
We have already seen (p. 22) that the plant requires 
free oxygen, and that this gas is absorbed by those parts 
of vegetation which are in the act of growth. As a con- 
sequence of this entrance of oxygen into the plant, a cor- 
responding amount of carbonic acid is produced within 
and exhales from it. There go on accordingly, in the ex- 
panding plant, two opposite processes, viz., the absorption 
of oxygen and exhalation of carbonic acid, and the ab- 
sorption of carbonic acid and evolution of oxygen. The 
first process is chemically analogous with the breathing 
of animals, and may hence be designated as respiration. 
We may speak of the other process as the fixation of 
carbon. 
