ACTION OP LEAVES. 41 



* 



tions are duly performed. Consequently, whatever 

 tends to impede the free action of leaves, tends also 

 to diminish the healthiness of a plant. 



65. These functions are performed by means of the 

 vital forces of vegetation, -which we cannot estimate 

 or comprehend, assisted by the influence of an exter- 

 nal agent, the nature of whose action may be under- 

 stood from its effects. That agent is solar light. 



66. It is the property of solar light, when striking 

 upon the leaf of a plant, to cause : 1. A decomposi- 

 tion of carbonic acid ; 2. An extrication of nitrogen ; 

 and, 3. Insensible perspiration. By their vital forces 

 plants appear to decompose water, independently of 

 the action of light. 



67. Carbonic acid is originally introduced into the 

 interior of a plant, either dissolved in the water it 

 imbibes by its roots, or by attraction from the atmo- 

 sphere, or by the combination of the oxygen obtained 

 by a decomposition of water or otherwise, with the 

 carbon in its interior. When a leaf is exposed to the 

 direct influence of the sun, it gives off oxygen, by 

 decomposing the carbonic acid ; whereupon the car- 

 bon remains behind in the interior of the leaf in a 

 solid state. Although the nature of the air thus ex- 

 tricated can only be determined by a chemist, yet the 

 extrication itself can be easily seen by any one who 

 will plunge a leaf in water and expose it to the sun ; 

 for bubbles of oxygen will be seen to form themselves 

 upon the surface of the leaf But, if the same leaf 

 be observed in the total absence of solar light, there 

 will be little or no extrication of air, and what little 



