DESCENT OF THE SAP IN TREES. 103 



Whenever the surface of the alburnum is exposed but for a few hours 

 to the air, though no portion of it be destroyed, vegetation on that 

 surface for ever ceases : but new bark is gradually protruded from the 

 sides of the wound, and by this, new wood is generated. In this wood 

 the medullary processes are distinctly seen to take their origin from the 

 bark, and to terminate on the lifeless surface of the old wood within the 

 wound. These facts incontestably prove, that the medullary processes, 

 which in my former paper I call the silver grain, do not diverge from the 

 medulla, but that they are formed in lines converging from the bark to 

 the medulla, and that they have no connexion whatever with the latter 

 substance. And surely nothing but the fascinating love of a favourite 

 system, could have induced any naturalist to believe the hardest, the 

 most solid, and most durable part of the wood, to be composed of the 

 soft, cellular, and perishable substance of the medulla. 



In my last paper, I have supposed that the sap acquired the power to 

 generate wood in the leaf; and I have subsequently found no reason to 

 retract that opinion. But the experiment in which wood was generated 

 in the leaf-stalk, apparently by the sap descended from the bark of the 

 graft, induces me to believe that the descending fluid undergoes some 

 further changes in the bark, possibly by. discharging some of its com- 

 ponent parts through the pores described and figured by Malpighi. 



I also suspected, since my former paper was written, that the young 

 bark, in common with the leaf, possessed a power, in proportion to the 

 surface it exposes to the air and light, of preparing the sap to generate 

 new wood ; for I found that a very minute quantity of wood was deposited 

 by the bark, where it had not any apparent connexion with the leaves. 

 Having made two incisions through the bark round annual shoots of the 

 apple-tree, I entirely removed the bark between the incisions, and I 

 repeated the same operation at a little distance below, leaving a small 

 portion of bark unconnected with that above and beneath it. By this 

 bark a very minute quantity of wood in many instances appeared to be 

 generated at its lower extremity. The buds in the insulated bark were 

 sometimes suffered to remain, and in other instances were taken away ; 

 but these, unless they vegetated, did not at all affect the result of the 

 experiment. I could therefore account for the formation of wood in this 

 case, only by supposing the bark to possess in some degree, in common 

 with the leaf, the power to produce the necessary changes in the descend- 

 ing sap ; or, that some matter, originally derived from the leaves, was 

 previously deposited in the bark ; or that a portion of sap had passed 

 the narrow space above, from which the bark had been removed, through 



