ACTION OP THE SILVER GRAIN. 53 



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 accom- 

 plished, 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- 



