C 217 3 



by the transpiring functions of the leaves, and the 

 cilaginous, inflammable, and astringent constituents, 

 increased by the agency of heat, light, and air, the 

 continued impulse upwards from the alburnum, forces 

 the remaining inspissated fluid into the cortical vessels? 

 which receive no other supply. In these, from its 

 weight, its natural tendency must be to descend; 

 and the rapidity of the descent must depend upon 

 the general consumption of the fluids of the bark 

 in the living processes of vegetation; for there is every 

 reason to believe, that no fluid passes into the soil 

 through the roots; and it is impossible to conceive a 

 free lateral communication between the absorbent ves- 

 sels of the alburnum in the roots, and the transport- 

 ing or carrying vessels of the bark; for if such a com- 

 munication existed, there is no reason why the sap 

 should not rise through the bark as well as through 

 the alburnum; for the same physical powers would 

 then operate upon both. 



Some authors have supposed that the sap rises in 

 the alburnum, and descends through the bark in con- 

 sequence of a power similar to that which produces the 

 circulation of the blood in animals; a force analagous 

 to the muscular force in the sides of the vessels. 



Dr. Thomson, in his System of Chemistry, ha3 

 stated a fact which he considers as demonstrating the 

 irritability of living vegetable systems. When a stalk 

 of spurge (Euphorbia peplis) is separated by two in- 

 cisions frm its leaves and roots, the milky fluid flows 

 through both sections. Now, says the ingenious au- 

 thor, it is impossible that this could happen without 



