C 220 ] 



of the silver grain, the phenomenon is in perfect uni- 

 son with the views advanced in the preceeding part 

 of this Lecture. 



The ilex, or evergreen oak, preserves its leaves 

 through the winter, even when grafted upon the com- 

 mon oak; and in consequence of the operation of the 

 leaves there is a certain motion of the sap from the oak 

 towards the ilex, which, as in the last case, seems to 

 be inconsistent with the theory of irritable action. 



It is impossible to peruse any considerable part 

 of the Vegetable Statics of Hales, without receiving a 

 deep impression of the dependence of the motion of the 

 sap upon common physical agencies. In the same tree 

 this sagacious person observed, that in a cold cloudy 

 morning when no sap ascended, a sudden change was 

 produced by a gleam of sunshine 9 of half an hour; and a 

 vigorous motion of the fluid. The alteration of the 

 wind from south to the north immediately checked 

 the effect. On the coming on of a cold afternoon after 

 a hot day, the sap that had been rising began to fall. 

 A warm shower and a sleet storm produced opposite 

 effects. 



Many of his observations likewise shew, that the 

 different powers which act on the adult tree, produce 

 different effects at different seasons. 



Thus in the early spring, before the buds ex- 

 pand, the variations of the temperature, and changes 

 of the state of the atmosphere with regard to moisture 

 and dryness, exert their great effects upon the expan- 

 sions and contractions of the vessels; and then the tree 

 is in what is called by gardeners its bleeding season. 



