36 VITAL ACTIONS. 



upon four separate pillars of bark and alburnum ; 

 and the tree upon which the operation was performed 

 continued to live for two years, after whict it was 

 not observed. In the latter instance, no doubt can 

 be entertained that the whole of the ascending sap 

 was directed into the four pillars of alburnum, which 

 were allowed to remain. 



58. The cause of ih&flow of the sap appears to be 

 the attraction of it by the leaves, which continually 

 diminish its quantity ; and the necessity that the sap 

 abstracted should be replaced by a further supply 

 sent upwards from the roots. The consequence of 

 this is, that sap always begins to flow at the ends of 

 the branches, a circumstance which has led to the 

 erroneous idea that it proceeds from above downwards 

 through the alburnum. The flow of the sap must 

 not, however, be confounded with the motion of the 

 sap, which takes place in the winter as well as in the 

 summer, and is a mere impletion of the system, caused 

 by the attraction of the roots, unaffected by the ex- 

 halation of the leaves. 



