122 VEGETABLE SAP. 



diffusibility through the cellular and vascular struc- 

 ture in all directions ; but that the sap should retreat 

 in a body from one set of vessels, which, notwith- 

 standing, are not emptied, and proceed to others that 

 are already full, is extraordinary ! If we consider the 

 motion of fluids in general, and in all ordinary circum- 

 stances, we invariably observe that their motions are 

 caused by their fluidity, ponderosity, or temperature ; 

 if they flow from a place which is full, it is because 

 there is a vacant space to receive them : a rarer fluid 

 will give place to one that is heavier ; and conse- 

 quently the warmest parts of a fluid will rise above 

 those which are colder. But in all these cases an 

 outlet or vacuum must exist, or the removal of some 

 pre-occupying fluid must take place before any mo- 

 tion can be generated. Now, we presume no prac- 

 tical eye has ever discovered that roots are more 

 charged with sap in winter than they are in summer, 

 nor that the topmost shoots are sapless during the 

 former season. If it be urged that the lower tem- 

 perature of the sap in the branches is the cause of 

 its subsidence to the roots, it may, with equal reason, 

 be asserted, that the warmer portion in the roots will 

 be propelled aloft by such subsidence, and that an 

 equilibrium will be always maintained. But it is 

 further quite manifest, that if there be any descent 

 at all, it not only takes place in the autumn, but is 

 in operation all the summer ; because one of the 

 principal proofs of the descent of the sap, namely, 

 the swelling of a stem above a ligature, becomes 



