CHAP. III. 



FUNCTION OF NUTRITION'. 



205 



supposed by some persons that an important advantage 

 might be taken of this circumstance ; and that by 

 stripping a tree of its bark some time before it was 

 felled, the sap would be forced to descend along the 

 newly formed wood and thus ripen or harden it more 

 speedily than would have been the case in the natural 

 course of things. But experience has shown that such 

 timber is very brittle and unfit for the purposes of 

 building. 



(191.) Progression of the Sap. Although the proper 

 juice appears to descend more especially by the bark 

 and those portions of the tree which are towards the 

 surface, and which are in fact the parts where the 

 vitality of the trunk resides, there still appears to be 

 a very general diffusion of the nutritious juice con- 

 tinually taking place throughout all parts of the tree, 

 sometimes in one direction and sometimes in another. 

 This may be shown by a 

 contrivance of M. Biot (fig. 

 157.)- A wooden wedge boiled 

 in wax and oil to render it 

 impervious to moisture, has a 

 groove cut in the upper part, 

 and is then driven into a ca- 

 vity which it exactly fits in the 

 trunk of a tree ; a space is 

 hollowed out both above and 

 below this wedge ; the roof of 

 the cavity above it shelves 

 towards the middle, so that 

 the descending sap collects there and drops into the 

 open extremity of a pipe placed in the groove to re- 

 ceive it. The ascending sap rises into the lower cavity 

 which is also cut into a groove, and it is there re- 

 ceived into another pipe placed in the bottom. In 

 this manner a flow of sap is obtained either simul- 

 taneously from both pipes, or at separate times and in 

 different proportions according to the' state of the at- 

 mosphere, season of the year, and other circumstances 



