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part. Vessels on the contrary, being a species of cell, designed to 

 elaborate materials, like as do the ordinary cells, but, for the pro- 

 duction of remote parts, and afterwards being intended to convey 

 their elaborated contents to the place of destination, require that free 

 communications should exist between them, in order that the fluid 

 elaborated and accumulated more particularly in one part of the vas- 

 cular system, may, by the force of distension, be propelled into all the 

 other parts communicating with it. Hence, when the process of 

 elaboration is most active in the vessels of the leaves, and a quantity 

 of fluid, proportionate to the activity of this process [i. e. to the quan- 

 tity of soluble matter which is being formed) is, by endosmose, passing 

 into them from the adjacent intercellular tissue, this fluid cannot fail 

 to be forced into those vessels which are receiving less fluid from the 

 surrounding paits ; for instance, into those of the petioles of the leaves, 

 and from these vessels some of the elaborated sap must pass into the 

 vessels around the pith to furnish the material necessary for the pro- 

 duction of starch, or other nutrient matter for the leaf-buds of the en- 

 suing year, but the greater quantity will pass into the ducts of the 

 wood (these being continuous with the vessels descending from the 

 petioles), for the production of new wood and new bark. It is possible 

 that some part of the contents of the vessels, during their passage 

 through the cellular spaces in the bark, may escape, and that to this 

 is due the granular matter, or the clusters of cambium cells found in 

 these spaces. But the most probable cause of the passage of the 

 elaborated sap from the vessels and ducts must be looked for in the 

 endosmic action of the surrounding cells. These cells in vegetables 

 performing probably a function similar to that which epithelial cells 

 perform in animal structures, especially in those structures which are 

 but sparingly supplied with blood, such for instance as cartilage. In 

 animals a thin membrane called hasement, or limitary membrane, is 

 generally situated between the vessels containing the nutrient fluid, — 

 the blood-vessels, — and the epithelial cells, to which physiologists 

 ascribe more or less of importance. In vegetables no such membrane 

 is present ; but in the ligneous parts of a plant the walls of the ducts 

 are provided with minute nucleated cellules generally of an hexagonal 

 figure, and disposed in the form of a tessellated epithelium. Where 

 the vessels of the petiole become continuous with these ducts, the 

 spiral fibre gets more and more imperfect in proportion as this dis- 

 position of cellules makes its appearance: hence, as the vessels of 

 wood must be confessed to have a more active function, in reference 

 to the production of new parts, than those of the petiole of leaves (the 



