]32 PROCESS OF NUTRITION. CHAP. III. 



eharged its contents into the second, and is no 

 longer acted upon by the stimulus of the sap, it- 

 begins again to be dilated to its original capacity, 

 and prepared for the introsusception of a new 

 portion of fluid. Thus a supply is constantly kept 

 up, and the sap continues to flow. 



Knight. The above is by far the simplest as well as most 

 satisfactory of all theories accounting for the ascent 

 of the sap. But Mr. Knight has presented us with 

 another which, whatever may be its real value, merits 

 at least our particular notice, as coming from an 

 author who stands deservedly high in the list of 

 phytological writers. This theory rests upon the 

 principle of the contraction and dilatation, not of 

 the sap vessels themselves, as in the theory of Saus- 

 sure, but of what Mr. Knight denominates the 

 silver grain, assisted perhaps by heat and humidity, 

 expanding or condensing the fluids. The appella- 

 tion of the silver grain seems to be synonymous 

 with that of the medullary rays already explained. 

 On the transverse section of the trunk of woody 

 plants, particularly the Oak, they appear in the 

 form of the radii of a circle extending from the 

 pith to the bark, and on the longitudinal cleft or 

 fissure of the trunk of most trees, but particularly 

 the Elm ; they appear in the form of fragments of 

 thin and vertical lamina or plates, interlacing the 

 ascending tubes in a transverse direction, and touch- 

 ing them at short intervals, so as to form with them 

 a sort of irregular wicker work, or to exhibit the 

 7 



