ARTIFICIAL OBSTACLES TO SAP. 



49 



and alburnum ; and the tree upon which the operation was 

 performed continued to live for two years, after which it was 

 not observed. In this instance, no doubt can be entertaiaed 



Fig. XIII. — Crucial incisions- in the trunk of a tree. 



that the whole of the sap was directed into the four pillars, 

 after passing through which it was conveyed laterally both in 

 ascent and descent until the whole system was again filled. 



The cause .of the _/?ow 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 conse- 

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

 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 



