SECT. II. ASCENT OF THE SAP. 115 



indeed exhibited some traces of ink, but the bark 

 and buds none. In some other examples the ex- 

 ternal layers of the wood only were tinged. In 

 the Honeysuckle the deepest shade was about the 

 middle of the woody layers ; and in the Filbert 

 there was also observed a coloured circle surround- 

 ing the pith ; but none in the pith itself, nor in the 

 bark. 



Such were the experiments of the earlier phyto- 

 logists with a view to discover the vessels conduct- 

 ing the sap in its ascent, which, though they do not 

 exactly determine the point in question, do yet very 

 much circumscribe the limits of inquiry, showing 

 that it ascends through the vessels of the longitu- 

 dinal fibre composing the alburnum of woody 

 plants, and through the vessels of the several 

 bundles of longitudinal fibre constituting the woody 

 part of herbaceous plants. But it has been already 

 shown that the vessels composing the woody fibre 

 are not all of the same species. There are simple 

 tubes, porous tubes, spiral tubes, mixed tubes, and 

 interrupted tubes. Through which of these, there- 

 fore, does the sap pass in its ascent ? 



The best reply to this inquiry has been furnished Mirbeland 

 by Mr. Knight and M. Mirbel, whose experiments night * 

 on the subject are considerably more luminous than 

 the preceding. Mr. Knight prepared some annual 

 shoots of the Apple and Horse-chesnut, by means 

 of circular incisions, so as to leave detached rings 

 of bark with insulated leaves remaining on the 



i 2 



