213 ] 



I referred in the beginning of the Third Lecture 

 to Mr. Knight's opinion, that the contractions and 

 expansions of the silver grain in the alburnum, are 

 the most efficient cause of the ascent of the fluids con- 

 tained in its pores and vessels. The views of this ex- 

 cellent physiologist are rendered extremely probable 

 by the facts he has brought forwards in support of 

 them. Mr. Knight found that a very small increase 

 of temperature was sufficient to cause the fibres of the 

 silver grain to separate from each other, and that a 

 very slight diminution of heat produced their contrac- 

 tion. The sap rises most vigorously in spring and 

 autumn, at the time the temperature is variable ; and 

 if it be supposed, that in expanding and contracting, 

 the elastic fibres of the silver grain exercise a pressure 

 upon the cells and tubes containing the fluid absorbed 

 by the capillary attraction of the roots, this fluid must 

 constantly move upwards towards the points where a 

 supply is needed. 



The experiments of Montgolfier, the celebrated 

 inventor of the balloon, have shewn that water may 

 be raised almost to an indefinite height by a very 

 small force, provided its pressure be taken off by con- 

 tinued divisions in the column of fluid. This princi- 

 ple, there is great reason to suppose, must operate in 

 assisting the ascent of the sap in the cells and vessels 

 of plants which have no rectilineal communication, 

 and which every where oppose obstacles to the per* 

 pendicular pressure of the sap. 



The changes taking place in the leaves and buds 7 

 and the degree of their power of transpiration, must 



