FUNCTION OF THE LEAVES 5 



exceptions to these general rules will be noticed 

 presently. 



Function of the Leaves.— 1. Assimilation. — The 

 source of vegetable carbon is the carbonic acid gas 

 present in the atmosphere. Carbonic acid and the 

 other gases of the atmosphere pass into the plant 

 chiefly through the stomata of the leaves, and are 

 dissolved by the cell sap ; the carbonic acid is retained 

 in greatest proportion, as it is much more soluble in 

 water than the nitrogen and oxygen which make up 

 the bulk of the atmosphere. The dissolved carbonic 

 acid is decomposed within the chlorophyll cells of the 

 plant under the influence of light, oxygen being evolved, 

 and the carbon retained by the plant. The carbonic 

 acid being thus removed from the cell sap, it becomes 

 capable of dissolving a fresh supply. All green parts 

 of a plant share in this power of decomposing carbonic 

 acid, but it is pre-eminently the function of the leaves. 

 The decomposition of carbonic acid does not proceed 

 in darkness, or at a very low temperature. The rays 

 of light most active in effecting the decomposition are 

 the orange-red rays ; the green, violet, and dark red 

 rays of the spectrum have scarcely any influence. 

 The rays of Hght absorbed by the green chlorophyll 

 are, in fact, the ones which accomplish the chemical 

 work. 



The decomposition of carbonic acid by green plants 

 during dayhght is of the utmost importance in main- 

 taining an atmosphere suitable for the respiration of 

 animals. An animal in breathing inspires atmospheric 

 air ; it expires air in which a part of the oxygen has 

 been replaced by carbonic acid; the result of animal 

 }ife is thus to accumulate carbonic acid in the atmo- 



