48 HOFFMANN ON THE CIRCULATION 



Salix arhuacula. — Branch 1^ inch in diameter. Repeatedly 

 wetted, as in the preceding case, during eight days: result 

 almost the same. The solution had only advanced 1 line beyond 

 the exposed spot, and quite uniformly upwards, downwards, and 

 to the side ; the sap-wood reacted ^ of a line deep. When the ex- 

 periment was continued for eighteen days the result was the same. 



Consequently there is no layer in the whole tree less favour- 

 able than the cambium for the conduction of the sap. In oppo- 

 sition to the views of many inquirers, this part, being in the 

 most active condition of development, most energetically arrests 

 the fluids. 



It is clear how unfitted the bark is for the transport of fluids ; 

 they occupy a longer time there than in most other parts, in 

 changing their place. And in this experiment, we must not be 

 led away by the results of what are called the " magic rings*" 

 on trees. For if a thickened border is formed on them at the 

 upper cut edge, this only proves that the sap in general has a 

 descending motion ; not, however, that this does not take place 

 far better and more easily in the totally uninjured woody layer. 

 When we reflect that even in the oldest trees a continual in- 

 crustation of the cells, a continual increase of that transforma- 

 tion of the saps, goes on deep in the interior of the wood, the 

 result of which is the concentric growth of the heart-wood, at the 

 expense of what is at first sap-wood, it is seen at once that it 

 would be a great mistake to regard the wood, on account of its 

 solidity, as lifeless and unengaged in the conduction of the sap. 



3. The Descending Sap in absorption by the root. 



Salia? alba, L. — A piece l^ foot long, of a shoot ~ an inch 

 thick, was placed in the ground on the 24th of February, and 

 kept at a moderate temperature, so that roots were formed, and 

 by the 20th of April leaves had already burst out. On the 6th 

 of June the rooted portion was carefully split up the middle, 

 from below upwards, and one of the halves immersed in solution 

 of the ferrocyanide, the other in a vessel of pure water standing 

 close beside. On the 14th all the leaves were dead, the roots 

 still fresh and liealthy. On the 4th of July the height of the 

 fluids in the two vessels was not perceptibly altered, whether the 

 levels were previously alike or different, as counter-experiments 

 * Made by removing a ring of bark running all round the tree. 



