86 BIOLOGY OF PLANTS. 



to vegetate in the dark. In the light the leaves decompose 

 the acid, assimilate the carbon and a part of the oxygen, while 

 the remainder is yielded to the atmosphere. In the dark, 

 the reverse takes place ; the leaves absorb oxygen, and the 

 carbonic acid is not decomposed, but is thrown out into the 

 air ; hence, as little carbon is assimilated, plants which grow 

 in the shade, or in a cellar, are soft, spongy, pale and sickly.* 

 The quantity of acid, absorbed during the day, and decom- 

 posed, is greater than that given out during the night, and 

 the quantity of oxygen emitted by day exceeds that absorbed 

 at night ; hence, the atmosphere furnishes carbonic acid to 

 plants, and they in turn furnish oxygen to the air. Carbonic 

 acid thus performs a similar office to vegetables which oxy- 

 gen does to animals, the former purifies the juices of the veg- 

 etable, the latter the blood of the animal ; hence, the animal 

 and vegetable kingdoms contribute to each other's support. 

 Animals absorb oxygen, and convert it into carbonic acid ; 

 vegetables absorb the acid thus formed, and give back to the 

 air an equal volume of oxygen, necessary to support animals. 

 Thus the equilibrium of the atmosphere is maintained, and 

 both kingdoms flourish together. 



Although an equal volume of gas, is given back to the at- 

 mosphere, when carbonic acid is decomposed by the leaves 

 under the influence of solar light, it is not all oxygen, but a 

 part of it is nitrogen. Saussure found that of 2.1 .75 cubic inch- 

 es of carbonic acid absorbed, only 14.72 inches of oxygen was 

 given back, together with seven inches of nitrogen ; part of 

 the oxygen is thus assimilated to the plant. 



2. The quantity of carbonic acid, thus absorbed, has been 

 determined with some degree of certainty. In Saussiire's ex- 

 periments, plants absorb daily, more than their bulk, and, as 

 this acid is composed of 6.12 parts by weight of carbon, and 

 16 of oxygen in 22.12, it is possible to calculate the probable 

 amount of carbon, which is derived from this source. The 



* See Chaptal, p. 81. 



