474 COMPOUND PRODUCTS. CHAP. I. 



It has been observed by Saussure that the propor- 

 tion of the oxides of iron and of manganese augments 

 in the ashes of plants as their vegetation advances. 

 The leaves of trees furnish more of these principles 

 in autumn than in spring. It is so also with annual 

 plants. Seeds contain metals in less abundance 

 than the stem ; and if plants are washed with water 

 the proportion of their metallic oxides is aug- 

 mented. 



SECTION XXIX. 



General Remarks. 



THE substances that have been above described 

 constitute, as is evident, the principal ingredients 

 that enter into the vegetable composition. They 

 are indeed numerous, though some of them, such 

 as the metallic oxides, occur in such small propor- 

 tions as to render it doubtful whether they are in 

 reality vegetable productions or no. The same 

 thing may be said of some of the other ingredients 

 that have been found in the ashes of plants, which 

 it is probable they have absorbed ready formed by 

 the root, and deposited unaltered, so that they can 

 scarcely be at all regarded as being the genuine 

 products of vegetation. 



But besides the substances above enumerated 

 there are also several others that have been supposed 

 to constitute distinct and peculiar genera of vege- 

 table productions, and which might have been 



