CAMBtUM. 



77 



operation of girdling, by which the vessels of their al- 

 burnum are divided, not only arrests the vegetation, 

 of trees but destroys their life. 



This opinion is also confirmed by experiments with 

 coloured infusions, which tinge the vessels through 

 which they pass, and give us a better opportunity of 

 tracing their progress. In this way Mirbel ascertained 

 that the small tubes are the channels through which the 

 sap ascends, because they and they only are tinged with 

 the infusion, and he adds in confirmation of his 

 own correctness, that they are filled with a limpid fluid 

 during the bleeding season. 



Mr. Knight has pursued this enquiry still further, 

 and finds that the sap enters the leaf-stalk, through the 

 centre of those fibres by which it is attached to the 

 branch, and that it passes on to the surface of the leaf, 

 through vessels which appear to be a continuation of 

 the small tubes of Mirbel. In the cellular tissue of the 

 leaf, the sap is renovated, its superfluous moisture es- 

 capes through the pores of the epidermis, and the pe- 

 culiar juice of the vegetable, sometimes denominated 

 Cambium, is formed. It is green in the Periwincle, 

 orange in the Bloodroot, and white in the Poppy and 

 Milkweed, being the great principle from which the 

 various organs of the plant are developed, and ofttimes 

 the seat of its medical virtues. Like the sap it passes 

 in an appropriate set of tubes, which arise from the 

 expanded surface of the leaf, pass through its footstalk 

 exterior to the central tubes, and penetrate the stem or 

 branch to which the leaf is attached. From the ex- 

 periments of Mr. Knight, we learn through what por- 

 tion of the stem, and through what system of vessels, 

 it descends to the root. Having detached a ring of 

 bark from the trunk of a young tree, he found that the 



