WOOD AND BARK OF TREES. $3 
plants continually, as I do, would know these little facts 
that never present themselves to the eye, but require to be 
sought with a diligence few will bestow, though rewarded 
with a delight none but those who have felt it can conceive. 
Numbers of shrubs and semishrubs also lose their bark, and 
retain the inner bark-vessels only. If, then, we suppose the The bark go- 
returning sap- vessels to die away in the winter, what is to be- w * n “ 
come of the sap in the intermediate time ? for it is a certain 
thing that the sap flows all the year ; and though, at the time 
it rises to form the alburnum, it is in greater quantity, and 
more visible to us, because it then shews itself between the 
bark and wood ; when the bark retires back, the sap fills all 
the intervening place, and is only prevented from running into 
the bark by the curtain or division I have before mentioned. 
It is at this time that trees will bleed their sap, and the quantity 
that they will yield in that case, plainly shews that it is not 
their own juices, especially as custom will increase them to an 
amazing quantity. But the second reason is still more forcible. 
The sap-vessels, as they mount, are most apparent in every 
part of fhe wood j their large open mouths prove at once for The large 
what they are designed, and the quantity of sap always found °P en mouths 
in these vessels, shew most plainly their office. If, then, there vessels.^* 
were returning vessels, would they not also be visible ? But, Would not the 
in the bark of more than twenty trees which I have taken to returning yes- 
* sels be visible? 
pieces, fibre from fibre, tracing their direction, examining them 
in the microscope one by one, so that no vessel could escape 
me, not a sap-vessel can I find. The inner bark-vessels, iden- 
tified by the dark liquid that fills them, and their peculiar shape 
and form, are not to be mistaken for any others ; and I believe 
I may say, that I am now so well acquainted with all the dif- 
ferent vessels of a tree , that I can no longer fail from ignorance. 
But here, except the inner bark-vessels, all proceed in a diffe- 
rent direction either round the tree, or from the centre to the 
circumference, how is it possible that such large and powerful 
! parts should be invisible ?' The use of dissection is to correct Theuseofdis- 
the work of imagination, or those experiments which have section, 
that defect, forcing the juices into channels foreign to that 
, which nature has appointed for them. I have before said, that 
I ever found nature disposed to seek resources in case of any 
unnatural impediment. I have myself (in several instances 
I could 
