NATURAL HISTORY. 



831 



winch I must refer you,) I have 

 called the returning vessels of the 

 !eaf-shilk ; and to be deposited oa 

 the external sides of what I have 

 there named the central vessels, 

 and on the medulla. The latter 

 substance appeared wholij inactive, 

 and I could not discover any thing 

 like the processes supposed to ex- 

 tend from it, ia all cases, into the 

 wood. 



The organization of the young 

 shoot is extremely similar to ''hat 

 of the leaf-stalk, previous to the 

 formation of wood within it. The 

 same vessels extend through both ; 

 and therefore it appeared extremely 

 probable, that the wood in each 

 would be generated in the same 

 manner: and subsequent observa- 

 tion soon removed all grounds of 

 doubt. 



It is well known that, in the ope- 

 ration of budding, the bark of trees 

 being taken otf, readily unites itself 

 to another of the same or kindred 

 species. An cxaniination of the 

 manner in which this union takes 

 place promised some further infor- 

 mation : in the last summer, there- 

 fore, I inserted a great number of 

 buds, which I subsequently examin- 

 ed in every ])rogressive stage of their 

 union with the stock. A line of 

 confused organizafion marks the 

 place where the inserted bud first 

 conies into contact with the wood 

 of the stock ; between which line 

 and the bark of the inserted bud, 

 new wood regularly org;; lized is 

 gcneraled. This wood possesses 

 all the characteristics of that from 

 which the bud was taken, without 

 any a|)parent mixture whatever 

 with the character of the stock in 

 which it is inserted. The substance 

 which is called the medullary pro- 

 cess iti clearly sccu (o spriiij; from 



the bark, and to terminate at the 

 line of its first union with the stock. 



An examination of the manner ia 

 which wounds in trees become 

 covered, (for, properly speaking, 

 they never can be said to heal,) 

 afl'ords further proof, were it wanted, 

 that the medullary processes, (a» 

 they are improperly named,) lik« 

 every other part of the wood, are 

 generated by the bark. 



Whenever the surface of the al- 

 burnum is exposed but for a few 

 hours to the air, though no portion 

 of it be destroyed, vegetation oa 

 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 termi- 

 nate on the lifeless surface of the 

 old wood within the wound. These 

 facts incoutestibly 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 con- 

 nection 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, tho 

 most solid, and most durable part of 

 tin! wood, to be composed of the 

 soft cellulas and j)crishable sub- 

 stance of the medulla. 



In my last paper I have supposed 

 that the sap acquired the power to 

 generate wood in the leal", an(l I 

 have subsequently found no reasou 

 to retr..( t that opinion. 15ut tha 

 experiment in which wood was ge- 

 nerated in the leaf-stalk, apparently 

 by tJiu sa]) descended from the 



bark. 



