air in other parts or vessels, surrounding and compressing these; and lastly the action, so ingeniously supposed by Mr. Knight, of those thin shining plates called the silver grain, visible in oak wood, which pressing upon the sap-vessels, and being apparently susceptible of quick changes from variations in heat or other causes, may have a powerful effect. "Their restless temper," says Mr. Knight, "after the tree has ceased to live, inclines me to believe that they are not made to be idle whilst it continues alive." Phil. Trans, for 1801, p. 344. These plates are presumed by the author just quoted to be peculiarly useful in assisting the ascent of the sap through the alburnum of the trunk or chief branches, where indeed the spiral coats of the vessels are either wanting, or less elastic than in the leaf-stalks and summits of the more tender shoots.
However its conveyance may be accomplished, it is certain that the sap does reach the parts above mentioned, and there can surely be now as little doubt of the vessels in which it runs. That these vessels have been thought to contain air only, is well ac-
