50 HOFFMANN ON THE CIRCULATION 



passes into the latter when it is brought into the plant by the 

 leaves ; a circumstance which is doubtless to be explained by the 

 intimate anatomical connexion between the vessels of the me- 

 dullary sheath and those of the petioles in young shoots. 



4. The Descending Sap after direct absorption by CUT SURFACES 



OF THE WOOD. 



Salix vitellina. The end of a pendent young leafy shoot was 

 cut off, and the lower peripherical portion stripped of bark for 

 2 inches ; the part thus laid bare was immersed 1 inch in the 

 solution. At 2 inches further up in the same shoot, the bark 

 was slit up at the side, and the cylinder of wood cut out for a 

 length of 2 lines ; the bark being returned to its place, and the 

 wound wrapped in fresh leaves, the whole shoot was supported by 

 a splint, to keep it in a fixed position. After four days the shoot 

 had absorbed the fluid as far as it dipped in it. In this case 

 the saline solution had passed the bridge of bark, had advanced 

 4 \ inches beyond the vacancy in the wood, and into all parts ; 

 furthest, however, in the bark and wood, principally in the me- 

 dullary sheath and the peripherical part of the wood. Repeated 

 experiments gave the same result, but sometimes the liber, some- 

 times the wood, had conducted a little farther. Consequently, 

 here, where a forced entrance of the fluid had accomplished the 

 passage through the bridge of bark, the solution had again 

 penetrated in the horizontal direction through the wood above 

 this bridge, and sometimes even advanced further in it than in 

 the bark itself. 



S. alba. A portion 1^ foot long of a leafy young shoot 

 was stripped of its bark for 2 inches at the upper end, and 

 dipped, with its wood wrong end upward, 1 inch deep in the 

 solution. Then 1 inch of bark was peeled from the other free 

 end, and a closed glass tube turned down over it to prevent de- 

 siccation. After seven days the fluid had ascended through all 

 parts; whence, comparing this with the cases mentioned in 

 section A 2, it results, that in absorption by exposed layers of 

 wood, it makes no difference in the conduction of the sap whether 

 the shoot is immersed in the fluid upright or in a reversed posi- 

 tion. In this case also, some salt had crystallized out upon the 

 leaves. When the free end of the wood was enveloped in blot- 



