42 HOFFMANN ON THE CIRCULATION 



The isolating power of the epidermis against moisture is worthy 

 of notice. 



B. THE DESCENDING SAP. 



It seemed advisable in this case also to examine the different 

 conditions separately, since it must be influential whether the 

 fluids ascend from the uninjured roots before descending from 

 the peripherical parts of the stem, or make their way down di- 

 rectly from the leaves, or from the points of cut shoots, &c. 



1. The Descending Sap when absorbed through the LEAVES. 



To warrant this experiment on physiological grounds, it suf- 

 fices to refer to the fact of such a condition occurring in nature 

 in every fall of dew or rain, wherein it in fact constitutes a 

 condition essential to the well-being of plants. 



Salix fragilis, L. On June 7th a large uninjured leaf was 

 immersed in the solution, and on the 10th the shoot which bore 

 it was cut off. Analysis. In the outer part of the shoot all the 

 systems reacted ; nearer the base at length only isolated tracheae 

 of the wood, and to the greatest distance on the side of the twig 

 on which the leaf arose. 



S.fragilis, L. Experiment as before, with the modification 

 that the larger shoot which bore the absorbing twig was notched 

 deeply on the corresponding side. In three days the solution 

 could be traced in the absorbing twig, farthest in the dotted 

 vessels of the woody layer, and above all in the unreliable spirals 

 of the medullary sheath. In the main shoot the fluid had de- 

 scended over the boundary between the inner and outer layers 

 of wood, but not beyond the notch. 



S.fragilis, L. Fresh leaves of a young twig were immersed 

 one after another for several days in the solution, until a large 

 quantity of it had been absorbed. The main shoot which bore 

 the absorbing twig was ringed 3 lines broad down to the wood, 

 and the ring of bark removed. Analysis after twelve days. In the 

 peripherical part of the absorbing twig all systems had again 

 absorbed, but only the medullary sheath conducted far down ; 

 from this the fluid had passed out to the cicatrices of the partly 

 fallen leaves, while the buds in the axils of these, at that time 

 without any vascular connexion with the medullary sheath, 

 were wholly passed over. It had descended a good way in the 



