164 CHEMICAL ACTION OF 



answered to the vegetable^constitution by these func- 

 tions of leaves. They confirm Mr. Knight's theory of 

 vegetation, who has proved that very little alburnum 

 or new wood is secreted when light is kept from the 

 leaves. They also help us to understand how essen- 

 tial oils may be produced, which are known, as well as 

 sugar, to be composed of oxygen, hydrogen and car- 

 bon in different proportions. We can now have a ge- 

 neral idea how the nutritious sap, acted upon b} all 

 the agents above mentioned during its stay in the cel- 

 lular substance of the leaf, and returned from thence 

 impregnated with then? into the bark, may prove the 

 source of increase, and of peculiar secretions, in the 

 vegetable frame. That portion of sap sent to the flower 

 and fruit undergoes no less remarkable changes, for 

 purposes to which those curious organs are devoted ; 

 nor is it returned from thence, as from the leaves, to 

 answer any further end. The existence of those organs 

 is still more temporary, and more absolutely limited 

 to their own purposes, than even that of the leaves, 

 from whose secretions theirs are very distinct. 



But when we attempt to consider how the particu- 

 lar secretions of different species and tribes of plants 

 are formed ; how the same soil, the same atmosphere, 

 should in a leaf of the vine or sorrel produce a whole- 

 some acid, and in that of a spurge or manchineel a 

 most virulent poison ; how sweet and nutritious her- 

 bage should grow among the acrid crowfoot and aco- 

 nite, we find ourselves totally unable to comprehend 



