179 yntiMIUAL. AC; 1 1^!^ Ult' 



There can be no question of the general purpose an- 

 swered to the vegetable constitution by these functions 

 of leaves. They confirm Mr. Knight's theory of vege- 

 tation, 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 essential oils may 

 be produced, which are known, as well as sugar, to be 

 composed of oxygen, hydrogen and carbon in different 

 proportions. We can now have a general idea how 

 the nutritious sap, acted upon by all the agents above 

 mentioned during its stay in the cellular substance of the 

 leaf, and returned from thence impregnated with them 

 into the bark, may prove the source of increase, and of 

 peculiar secretions, in the vegetable frame. That por- 

 tion of sap sent to the flower and fruit undergoes no less 

 remarkable changes, for purposes to whicii those curi- 

 ous 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 particular 

 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 machined a most 

 virulent poison ; how sweet and nutritious herbage 

 should grow among the acrid crowfoot and aconite, we 

 find ourselves totally unable to comprehend the exist- 

 ence of such wonderful powers in so small and seeming- 



