aTmospiikkk; aiu as tiii: food <ik i'lants. 4vi 



can floiirish 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 })lant itself, wriile the oxygen 

 is exhaled into the atmo-^phere in the free state. 



Relative volumes of absorbed (arbouic Acid and ex- 

 baled Oxygen. — From the numeri)us 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 cai'bonic acid absorbed. 

 Since free oxygen occupies the same bulk as the carbonic 

 acid produced Ly 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 jilant requires 

 free oxygen, and that this gas is absorbed by tliose parts 

 of vegetation which are in the act of growth. As a con- 

 sequence of this entrance of oxygen into the plant, a cor- 

 resj)onding amount of carbonic acid is produced within 

 and exhales from it. There go on accordingly, in the ex- 

 panding plant, tAvo opposite processes, viz., the absorption 

 of oxygen and exhalation of carbonic acid, and the ab- 

 sorption of carbonic aciil 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. 



