the tme Sap of Trees is deposited during Winter. 101 
vessels, which contain this fluid in the alburnum, are in contact 
with those which carry up the aqueous sap ; and it does not 
appear probable that, in a body so porous as wood, fluids so 
near each other should remain wholly unmixed. I must there- 
fore conclude that when the true sap has been delivered from 
the cotyledon or leaf into the returning, or true sap vessels of 
the bark, one portion of it secretes through the external cellular, 
or more probably glandular substance of the bark, and gene- 
rates a new epidermis, where that is to be formed ; and that 
the other portion of it secretes through the internal glandular 
substance of the bark, where one part of it produces the new 
layer of wood, and the remainder enters the pores of the wood 
already formed, and subsequently mingles with the ascending 
aqueous sap; which thus becomes capable of affording the 
matter necessary to form new buds and leaves. 
It has been proved in the preceding experiments on the 
ascending sap of the sycamore and birch, that that fluid does 
not approach the buds and unfolding leaves in the spring, in the 
state in which it is absorbed from the earth : and therefore we 
may conclude that the fluid, winch enters into, and circulates- 
through the leaves of plants, as the blood through the lungs of 
animals, consists of a mixture of the true sap or blood of the 
plant with matter more recently absorbed, and less perfectly 
assimilated. 
It appears probable that the true sap undergoes a considerable 
change on its mixture with the ascending aqueous sap ; for this 
fluid in the sycamore has been proved to become more sensibly 
sweet in its progress from the roots in the spring, and the liquid 
which flows from the wounded bark of the same tree is also 
sweet; butlhave never been able to detect the slightest degree of 
