48 AETIPICIAL OBSTACLES TO SAP. 



Some curious experiments upon this subject were contrived 

 by Mr. N. Niven (Gardeners' Magazine,\o\. xiv.). In one case, 

 he divested the stem of a tree of a deep ring of bark, and of the 

 first twelve layers of wood below it (Fig. XII.) ; nevertheless the 



Fig. XII. — Einged tree, having wood removed as well as bark, 



tree continued to live and be healthy. From the exposed 

 surface of the wood no sap made its appearance, except from a 

 cut which had been inadvertently made with the saw on one 

 side, to the depth of, perhaps, five or six layers of wood beyond 

 the twelve actually removed. From that cut a flow of sap took 

 place, and continued to run during the whole of the season in 

 which the operation was performed. In this case, the sap 

 cannot have ascended exclusively by the alburnum, but must 

 have chiefly passed through the central wood. 



In another case, by making four deep and wide incisions into 

 the trunk of a tree (Fig. XIII.), and removing the centre, the upper 

 part of the trunk was placed upon four separate pillars of bark 



