102 ORGANS AND FUNCTIONS OF PLANTS. 



leaves of plants perform some function similar to that 

 of the lungs of animals. 



In animals, the air taken into the lungs in breathing 

 through the nose and mouth becomes decomposed (in 

 the dark it may be remarked) by giving up part of its 

 oxygen, which combines with the blood, and receiving 

 in turn from the blood a portion of carbonic acid gas 

 and watery vapour. 



In plants this process is reversed ; for the sap, which 

 has mounted into the leaves and young green shoots, 

 and which is composed of water, carbonic acid gas, 

 potass, and a few other ingredients, either derived from 

 the soil, or taken up on passing up through the plant, 

 becomes partly decomposed in the light ; a portion of 

 the oxygen being set free from the carbon, which 

 remains in the leaf while the oxygen is given off into 

 the air, at the same time that the large portion of the 

 water is given off undecomposed, in the form of 

 vapour. 



The quantity of water thus exhaled by a cabbage 

 has been proved to be seventeen times greater than 

 that transpired by a man in what is termed insensible 

 perspiration. The exhalation of water takes place 

 through the pores already described of the green parts ; 

 but not the decomposition of air, which is effected, as 

 De Candolle remarks, where there are no pores, 



It is important to remark that light is indispensable 

 in affecting what may be called the aeration of plants, 

 that is, in decomposing the sap in the leaves, and con- 

 densing the carbon, potass, and other matters, indis- 

 pensable to nutrition, while watery vapour is at the 



