286 Mr. KnighTs Account oj some Experiments 
from the bark, and to terminate at the line of its first union with 
the stock. 
An examination of the manner in which wounds in trees be- 
come covered, (for, properly speaking, they never can be said 
to heal,) affords further proof, were it wanted, that the medul- 
lary processes, (as they are improperly named,) like every other 
part of the wood, are generated by the bark. 
Whenever the surface of the alburnum is exposed but fora 
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 incontestibly 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 
connection whatever with the latter substance. And surely no- 
thing 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 
