104 EXPERIMENTS ON THE DESCENT OF THE SAP. 



the wood Repeating the experiment, I left a much greater length of 

 bark between the intersections ; but no more wood than in the former 

 instance was generated. I therefore concluded that a small quantity 

 of sap must have found its way through the wood from the leaves above; 

 and I found that when the upper incisions were made at ten or twelve 

 lines distance, instead of one or two, and the bark between them, 

 as in former experiments, was removed, no wood was generated by the 

 insulated bark. 



I shall conclude my paper with a few remarks on the formation of 

 buds in tuberous-rooted plants, beneath the ground. They must, if my 

 theory be well founded, be formed of matter which has descended from 

 the leaves through the bark. I shall confine my observations to the 

 potato. Having raised some plants of this kind in a situation well 

 adapted to my purpose, I waited till the tubers were about half grown ; 

 and I then commenced my experiment, by carefully intersecting with a 

 sharp knife the runners which connect the tubers with the parent plant, 

 and immersing each end of the runners thus intersected in a decoction 

 of logwood. At the end of twenty-four hours I examined the state 

 of the experiment; and I found that the decoction had passed along 

 the runners in each direction ; but I could not discover that it had 

 entered into any of the vessels of the parent plant. This result I had 

 anticipated ; because I concluded that the matter by which the growing 

 tuber is fed must descend from the leaves through the bark ; and 

 experience had long before taught me that the bark would not absorb 

 coloured infusions. I now endeavoured to trace the progress of the 

 infusion in the opposite direction, and my success here much exceeded 

 my hopes. 



A section of potato presents four distinct substances ; the internal 

 part, which, from the mode of its formation and subsequent office, I con- 

 ceive allied to the alburnum of ligneous plants ; the bark which surrounds 

 this substance ; the true skin of the plant ; and the epidermis. Making 

 transverse sections of the tubers which had been the subjects of experi- 

 ments, I found that the coloured infusion had passed through an elabo- 

 rate series of vessels between the cortical and alburnous substances, 

 and that many minute ramifications of these vessels approached the 

 external skin at the base of the buds, to which, as to every other part 

 of the growing tuber, I conclude they convey nourishment. 



