42 AGRICULTURE. 



the atmosphere is consumed, and a portion of oxy- 

 gen left in its place. If new supplies of carbonic 

 acid be given to the air, the same result follows ; 

 whence it has been concluded, that air furnishes 

 carbonic acid to the plant, and that the plant fur- 

 nishes oxygen to the air. This double function of 

 absorption and respiration is performed by the green 

 leaves of plants.* 8. Carbon is to vegetation what 

 oxygen is to animal life ; it gives support by puri- 

 fying the liquids and rendering the solids more 

 compact. 



IV. Of light, heat, and electricity, and their agen- 

 cy in vegetation. 



When deprived of light, plants are pale, lax, and 

 dropsical ; restored to it, they recover their colour, 

 consistency, and odour. If a plant be placed in a 

 cellar, into which is admitted a small portion of 

 light through a window or cranny, thither the 

 plant directs its growth, and even acquires an un- 

 natural length in its attempt to reach it.f These 

 facts admitted, no one can doubt the agency of 

 light in vegetation ; but, in relation to this agency, 

 various opinions exist ; one, that light enters vege- 

 table matter and combines with it ; another, that it 

 makes no part either of the vegetable or of its ali- 

 ment, but directly influences substances which are 

 alimentary ;J and a third, that, besides the last ef- 

 fect, it stimulates the organs of plants to the exer- 

 cise of their natural functions. 



Without doing more than state these opinions, 

 we proceed to offer the results of many experi- 

 ments on this subject. 1st. That in the dark no ox- 

 ygen is produced, nor any carbonic acid observed ; 

 on the contrary, oxygen is absorbed and carbonic 

 acid produced. 3d. That plants exposed to light 



* This was a discovery of Sennebier. 



t It is by a knowledge of this fact that gardeners bleach chic- 

 ory, cellery, &c. 

 t See Fourcroy, vol. viii. See Chaptal on Vegetation. 



