Mr. Knight on the Origin and Formation of Roots. 171 
rating alburnous matter ; and a transverse section of the root 
then shews the alburnum arranged in the form of wedges 
round the medulla, as it is subsequently deposited on the cen- 
tral vessels of the succulent annual shoot, and on the surface 
of the alburnum of the stems and branches of older trees.* 
If a leaf-stalk be deeply wounded, a cellular substance, 
similar to that of the bark and young root is protruded from 
the upper lip of the wound, but never from the lower ; and 
the leaf-stalks of many plants possess the power of emitting 
roots, which power can not have resided in alburnum, for the 
leaf-stalk does not contain any; but vessels, similar to those 
of the bark and radicle, abound in it, and apparently convey 
the returning sap ; and from these vessels, or perhaps more 
properly from the fluid they convey, the roots emitted by the 
leaf-stalk derive their existence. -f- 
If a portion of the bark of a vine, or other tree, which 
readily emits roots, be taken off in a circle extending round 
its stem, so as to intercept entirely the passage of any fluid 
through the bark ; and any body which contains much mois- 
ture be applied, numerous roots will soon be emitted into it 
immediately above the decorticated space, but never imme- 
diately beneath it : and when the alburnum in the decorticated 
spaces has become lifeless to a considerable depth, buds are 
usually protruded beneath, but never immediately above it, 
apparently owing to the obstruction of the ascending sap. 
The roots, which are emitted in the preceding case, do not 
appear in any degree to differ from those which descend from 
the radicles of generating seeds, and both apparently derive 
* Phil. Trans, for 1801, Plate 27. 
f Phil. Trans, for i8ox. 
