LECT. III.] GENERAL FLUID COMPONENTS. 121 



in the perfected plant, contain nourishment al- 

 ready properly adapted to be immediately assi- 

 milated into the substance of the plant. But this 

 preparation takes place, either during the time, or 

 after, the sap has been exposed to the action of the 

 light and air in the leaf; as no woody fibre is 

 found in the ascending sap, although the prin- 

 ciples of it are undoubtedly contained in that fluid. 

 A new chemical combination of these principles 

 takes" place ; but how this is effected, or by what 

 means the change is produced, we know not ; and 

 it is one of those mysteries of nature from which 

 human ingenuity will never perhaps be able to 

 remove the veil. In the same manner the blood 

 of animals contains the components of the mus- 

 cular fibres already formed ; and an assimilation 

 of it is constantly going on, without our being able 

 to perceive it, or even to form the most distant 

 conception of the manner in which it is performed. 

 The elementary principles of the proper juice 

 of plants and of the sap are the same ; but differ 

 in the relative proportions. These principles are 

 carbon, hydrogen, and oxygen. The same prin- 

 ciples, differently modified, form all the secretions 

 and the solid materials of the plant itself. The 

 extraneous ingredients which some plants are 

 found to contain, as part of their substance, such 

 as the alkaline and neutral salts, metallic oxyds, 

 silex and other earths, are probably obtained ready 



