110 TERRESTRIAL ADAPTATIONS. 



The part of the process of vegetable life for 

 which light is especially essential, appears to be 

 the functions of the leaves ; these are affected by 

 this agent in a very remarkable manner. The 

 moisture which plants imbibe is, by their vital 

 energies, carried to their leaves ; and is there 

 brought in contact with the atmosphere, which, 

 besides other ingredients, contains, in general, a 

 portion of carbonic acid. So long as light is 

 present, the leaf decomposes the carbonic acid, 

 appropriates the carbon to the formation of its 

 own proper juices, and returns the disengaged 

 oxygen into the atmosphere ; thus restoring the 

 atmospheric air to a condition in which it is more 

 fitted than it was before for the support of animal 

 life. The plant thus prepares the support of life 

 for other creatures at the same time that it ab- 

 sorbs its own. The greenness of those members 

 which affect that colour, and the disengagement 

 of oxygen, are the indications that its vital powers 

 are in healthful action : as soon as we remove 

 light from the plant, these indications cease : it 

 has no longer power to imbibe carbon and dis- 

 engage oxygen, but on the contrary, it gives back 

 some of the carbon already obtained, and robs 

 the atmosphere of oxygen for the purpose of re- 

 converting this into carbonic acid. 



It cannot well be conceived that such effects 

 of light on vegetables, as we have described, 

 should occur, if that agent, of whatever nature it 



